Manual do Rádio Amador-1940, Notas de estudo de Cultura

Baixe Manual do Rádio Amador-1940 e outras Notas de estudo em PDF para Cultura, somente na Docsity! THE “RADIO” HANDBOOK  e OTHER PUBLICATIONS By the Editors of “Radio” “RADIO” RADIO TECHNICAL DIGEST “RADIO” AMATEUR NEWCOMER'S HANDBOOK “RADIO” NOISE REDUCTION HANDBO OK “RADIO” ULTRA-HIGH FREQUENCY HANDBOOK (SEE ADVERTISING SECTION FOR DETAILS)  THE “RADIO” HANDBOOK | Table of Contents Foreword .....cicuccenereessenenaneaacenaranmaaseanareriancanserts 6 Chapter 1. Introduction to Amateur Radio...........c..ce..s Cereser ceranro 7 2. Introductory Electricity and Fundamental Radio Theory 19 3. Vacuum Tube Theory.......ccccicicerereraareaeeres 50 4. Radio Receiver Theory................ 66 5. Radio Receiver Tube Characteristics 9 6. Radio Receiver Construction... 7. Transmitter Theory.... 8. Radiotelephony Theory. 9. Frequency Modulation, 10. Transmitting Tubes.. 11, Transmitter Design 12. Exeiters and Low Powered Transmitters 13. Medium and High Powered Amplifiers... 14, Speech and Modulation Equipment. 15. Power Supplies........... 16. Transmitter Construction NY. U.H.F. Communication... . 18, U.H.F. Receivers and Transceivers.. 350 19. U.H.F. Transmitters. . 368 20. Antenas ....... 393 21. U.H.F. Antennas. . 447 22, Test and Measuring Equipment. 452 28. Workshop Practice.... 482 24. Broadeast Interference. . 491 25. Radio Therapy................. 498 26. Radio Mathematics and Calcutations. Appendix-Buyer's Guide—lIndex WRITTEN BY THE EDITORS OF “RADIO” i 1 sA THE “RADIO” HANDBOOK Foreword The Editors of Rapio have unquestionably become in recent years the outstanding group in radio not affiliated with a definite com- mercial interest. They are all practical radio engineers and active amateurs of many years” experience, They are the source of the reputation and prestige of RADIO, envied by publications of greater eirculation. Starting several years ago with an extensive set of “notes” com- piled for their own use, the Editors of Rapro have developed the present “Rapio”” HANDBO0OE, which is now in its seventh edition. Each edition is thoroughly revised, not merely brought up to date. To keep up with rapid developments in commercial equipment, the great majority of items shown in the constructional pages are newly built for each edition. Though a few outstanding items were selected from other publications by the same publishers, the greater portion are built especially for this handbook. All have been tried in actual practice. Taken all in all, no effort has been spared in an attempt to com- pile the most comprehensive book on the subject, both as a reference for those with wide knowledge of the field and as a practical text for those of limited knowledge and means. In closing, we wish to thank those whose year-after-year purchases have indicated their approval of such an unusual policy. This policy has only been possible, however, with the additional cooperation of our advertisers. In similar technical fields texts such as this sell from $5.00 upwards; whatever value this book may have fôr you over its purchase price is a gift to you from our advertiners.. We hope that you will reciprocate by using their products wk:n suited to the job at hand. THE PUBLISHERS Santa BARBARA, CALIFORNIA October, 1940 The Editors of BADIO in preparing this work have not only dreum upon their own knowledge and extensive experience, but also have drawn upon nearly the whole current feld of radio literature, where- fore it is impossible to give que acbnowledgment to aU whose work has been consulted to some extont. We wish to aobnowicage partic. ularty the kind permission of the RCA Manufacturing Co, Inc. to use certain of the formulas in the theoretical pages, as well as ex tensive data and specifications on vacuum tubes. CHAPTER ONE Introduction to Amateur Radio Although much of the information in this handbook is of interest to engineers, students, sound men, expérimenters, servicemen, and commercial operators, undoubiedly the larg- est single group having use for the material herein is composed of radio amateurs. Hence, to them the major portion of this book is dedicated; the material is written from their point of view. Naturally an amateur finds much use for a text that caters primarily to him. But the person interested in becoming an amatenr has still greater need for such a book. IHenee this book is not only dedicated primarily to the radio amateur, bnt is so written that previous experience with amateur radio or previous knowledge of amatcur radio is not Tequired for comprehension of its contents. Radio Amateurs. While the definition of “amateur” would seem to include shortwave listeners as radio amateurs, the term ordi- narily is used to indicate speeifically those radio hobbyists possessing a government li- eense and amateur call letters. More than 50.000 licensed amateurs in the U.S.A. are aclively engaged in this field for purposes of esperimentation, adventure, and personal enjoyradnt=: Tt is interesting to con- sider what there issabout amateur radio that, captures and holds the interest of so many people throwghout the world and from all walks of life, for unguestionably there is something about it which generates a lasting interest in its varied problems and activities. Many famous men, holding high-salaried positions of importance in the radio industry today, got their start in the radio. business by discovexing an interest in amateur radio. A large number of these executivos and engineers continne to enjoy amateur radio as am avocation even thongh commercially engaged in the radio industry, so strong is the fascination afforded by this hobby. Technical Achievement. Although “Ham- ming” generally is considered to be “only a hobby” by the general public, its history con- tains countless incidents of technical achieve- ments by its members which have served to improve radio communication and broad- casting. Many of the more important ad- vancements in the art of radio communica- tion can be chalked up to the ingenuity of xadio amateurs. Experiments conducted by inquisitive amateurs have led to important developments in the fields of electronies, tele- vision, radio therapy, sound pictures, and public address, as well ns in radio communi- eation and broadeasting. Fellowship. Amateurs are a most hospi- table and fraternal lot. Their common in- terest makes them “brothers under the' skin” and binds them together as closely as would membership in any college fraternity, lodge, or cinb. When visiting a strange town an amateur natarally first will look up any friends in that town he has made over the air. But even if he is unknown to any ama- teurs in that town, his amateur call is am “open sesame” The local amateurs will hang out the welcome sign and grect him like à long lost brother. Fk is not unusual for an amateur to boast a large cirele of friends, scattered throughout the comtry, with whom be chats nightly while seated comfortably at home. He gets to know these people intimatcly, many of whom he xill never mcet personalty. Frequently he is of servico to them, and they to him, in de- livering messages to other people. Amateur radio clubs have been formed in nearly al! of the principal cities in the United States. The first thing a newcomer should do is to attend one of these club meetings and let the members know that he is interested in joining the ranks of radio amateurs. The veteran amateurs will be glad to lend à hand  10 Introduction to Amateur Radio are in the regular military or naval service of the United States at a military post or naval station. A licensed radiotelegraph operator (other than an amateur operator who himself holds only a class C license) or a regularly employed government radiotelegraph operator, must sign the class C applican's blank in the presence of a notary publi, attesting to the applicant's ability to send and receive the continental Morse code at the required speed of 13 words per minnte, Do not send for class C blanks containing the examinations and questions until you feel you axe ready to take your exumination, as you are Not SUp- posed to hold them indefinitely after receiv- ing them. THolders of class G licenses may he required by the Commission to appear at an examin- ing point for a supervised written examina- tion and practical code test at any time dnr- ing the term of their licenses. This is seldom done except where the Commission has feason to suspeet that the applicant would háve diMeulty in passing the class B examination. For instance, an amatenr holding a class ticket who regularly is heard on the air with a bad note or modulation, or is heard sending always at 8 or 9 words a minute, or repeatedly requests QR$, should not be aí all surprised to receive a notice to appear. The class G license will be ecancelled if theholder does not appear for examination when 'called or if he fails to pass when he does appear. The privileges granted by the elass € h- cense are identical with those'of the class B. Your operator and station licenses will riih concurrently, both expiring together three The RADIO Figure 3. The recelving position of the prepossessing installation at WOPMB shows that amateur equipment may be made to blend unobtrusively with home Turmishings. gears from the date of issuance stated on the Ênce of the license. Both may be renewed without examination if an application is filed at least 60 days prior to the indicated date of expiration and the applicant offers proof thai he has communicated via amatenr radio vith three other amateur stations during the three-month period directly preceding the date of application for renewal. You may obtain just an operator license (without the station license) if you desire; this will permit you to operate any licensed amateur station. The “station” side of the license will be left blank and you will have no call letters assigned to yon. It is not pos- sible to apply for or obtain a station license singly unless you already have an operator license. Heavy penalties are provided for obtain- ing an amateur lidense by fraudulent means, such as by impersonating another person in an examination, copying from notes, books or fhe like, or misrepresenting the fact of one's U.S. citizenship. Applicants who fail to pass the examination can take it again after two months have passed from thc time of the last examination. There are so many special instances that may arise that no attempt will be made to cover every possible contingency pertaining to the application for and privileges accorded by an amateur license. If you have à special question regarding some point not covered in this book or which is not clear to you, write to the Inspector-in-Charge of your radio dis- triet. Don't guess at the proper interpreta- tion or take somebody else's word for it; you Ray get in trouble. Handbook How to Obtain Your License 11 assa KT funnanis, KO euLimues KA cum KB6 ue KG mowar KDG sonesron KE6 BANCA. HOULAND, AMER PHOENIX GA, Fê amvis, aves KG6 auenicam suo KHE CANAL ZONE K5 (Ny) Figure 4, U.S. call areas, often erronconsly referreg to as districts. U.S. radio districts (1 to 22) should not be confused with the call areas. f WI nB4 SW There is one thing you should not write to the inspector aboué and that is the necessity for a license to tramsmit. A transmitter li- cense is absolutely necessary, regardless of power, frequency, or type of emission; there are no exceptions nor special cases, Before attempting to take the amateur ex- amination, the reader should have a thorough knowledge of the regulations affecting ama- teme operators and stations, While “mem- orizing” procedure is not to be recommended vhen preparing for the technical portion of the amateur examination, the best way to pre- pare for the questions pertaining to regula- tions is to memorize the pertipent extraets ftom the communications law and also the United States amatew: regulations, given in chapter 27. They do not necessarily have to be memorized verbatim, but the appliesnt must have at his command aii of the informa- tion contained therein. Tt is important that the reader clearly understand the distinction between violations of the basic Communications Act of 1934 and violation of the rules and regulations set up under the basic act by the Federal Commumi- eations Commission. The former constitutes the more serious offense, and anyone is liable, whether he be an amateur or not. The difi- eulty some applicants experience with certain questions is in deciding whether a certain offense is a violation of the basic act or a violation of rules set up by our F.G.G. under the set, Starting Your Study. When you start your stndy to prepare yourself for the ama- teur examination, you will probably find that the circuit diagrams, tube characteristic curves, and formulas first appear confusing and difficult of comprehension. Iowever, after putting in a few evenings of study, one becomes sufficiently familiar with basic con- cepts. and fundamentals that the acquisition of further knowledge is not only easy but fascinating. As it takes considerable time to become proficient at sending and receiving code, it is a good idea to start by interspersing technical study sessions with short periods of code prac- tice. There are two reasons for this: many short code practice sessions benefit one much more than a fewer number of comparatively longer sessions, Also, it keeps one from get- ting “stale” while stndying theory and regula- tions by serving as a rest period, thus serving to maintain one's interest. Each kind oi study serves as à respite from the other. You can even start on one of the simpler receivers described in the chapter on receiver construction if you wish, though af first you will be unable to decipher many of the dots and dashes you pick up on it. However, many interesting hours can be spent listening to the conversations of amateur phone sta- tions. The numerous references to “QSA” “Rig,” “Rotary,” and other mysterions terms will begin to take on significance. When yon have practiced the code long enough, you will be able to follow the gist of the conversation of slower sending code E : mo O ativida  k i | 12 Introduction to Amateur Radio The RADIO stations, and fish for “dx.” Many stations send slower than “13 per” when working dx. Stations repeat their calls many times when calling “C(,” and one nced not have achieved * THE RADIOTELEGRAPH CODE ZrxUv-ronmoong> N<XE<LCA4ANDOTOZ NUMERALS, PUNCTUATION MARKS, ETC, 6 7 8 9 g INTERNATIONAL DISTRESS SIGNAL PERIOD COMMA INTERROGATION QUOTATION MARK COLON SEMICOLON PARENTHESIS FRACTION BAR WAIT SIGN DOUBLE DASH (BREAK) ERROR (ERASE) SIGN END OF MESSAGE END OF TRANSMISSION Figure 5. Shown above is the Continental code used for all radio communications. The more complicated Morse code is used for land Jine telegraphic communication within the U.S.A. much proficieney to make out their calls and thus determine their location, Granted that ié is advisable to start right in with learning the code, you will want to know how to go about mastering it in the shortest possible time with the least amount of cffort. Learning the Code The applicant for an amateur license must be able to send and receive the Continental code at 2 speed of 13 words per minute, with an average of 5 characters to the word, Thus 65 characters must be copied consecutively without error in one minute. Similarly, 65 consccutive characters must be transmitted without exror in that time, The applicant, however, is given sufficient opportunity to pass this code test, since sending and reeciv- ing tests are both five minutes in length. If 65 consccutive characters, at the required rate, are copied correctly, somewhere during the first five-minnte period, the applicant may then aitempt a transmission. Again, if 65 consceutive characters are sent correctly somewhere during this second period, 2 pass- ing mark is received. Failure to pass the code test results in a two-month rest period during which the ap- plieant can improve his mastery of the code; thereafter, he may again appear for another try. Approximately 30 per cent of the amateur Hcense applicants fail to pass the code ex- amination. It should be expected that nerv- ousness and exeitement—at least to some de- gree-—will hinder the applicant's code ability. The best prevention against this is to be able to master the code at a little better than the required speed, under ordinary conditions. Then a little slowing down due to nervousness will not prove “fatal” during the strain and exeitement of the examination. As to the correct method of learning the code, the fol- lowing is recommended. Unfortunately, no “trick” short cut to learning thc code has been found generally suecessful. Memorizing the Characters. To memorize the alphabet entails but a few evenings of diligent application, The time required to build up specd will be entirely dependent upon individual ability and regularity of drill, and may take any length of time from a few weeks to many months. Since code reading requires that individual letters be recognized instantly, any memoriz- ing scheme which depends upon an orderly sequence, such as learning all “dot” letters and all “dash?” letters in separate groups, is to be discouraged. Handbook Code Practice Sets 15 into a pair of earphones. The latter method makes it possible to practice without annoy- ing other people as much, though the elieking of the key will no doubt still bother someone in the same room, A buzzer-type code practice cirenit is shown in figure 7. The huzzer should be of good quality or it will change tone during keying; also the contacts on à cheap buzzer will soon wear out. The volume control, however (used only for headphone operation), may be of the least expensive type available, as it will not be subjected to constant adjustment as in a radio receiver. For maximum buzzer and battery life, use the least amount of voltage that will provide stable operation of the buzz- er and sufficient volume. Some buzzers operale stably on 114 volts, while others re- quire more. A vacuum tube audio oscillator makes the best code practice oscillator, as there is no sound except that generated in the earphones and the note more closely resembles that of a radio signal. Such a code practice oscil- lator is diagrammed schematically in figure 8. The parts are all screwed to a wood board and connections made to the phones and bat- teries by means of Fahnestock clips, as illus- trated in figure 9. A single dry cell supplies filament power and a 4-volt C battery sup- plics plate voltage. Both filament and plate current are very low, and long battery life can be expected. The vacuum tube is the biggest item from the standpoint of cost, but it can later be used in a field-strength meter with the same baiteries supplying power. Such a device is very handy to have around a station, as it can be used for neutralizing, Figure 9. THE CIRCUIT OF FIGURE 8 IS USED IN THIS BATTERY OPER- ATED CODE OSCILLATOR. A tube and audio transforme essentially comprise the oscillator. Fahnestock clips sorewed to the baseboard are used to make con- nections to batteries, key, and phones. checking the radiation characteristics of your antenna, ete. A 1H4, 30, or 144G may be used with the same results. The first two are 2-volt tubes, but will work satisfactorily on a 1.5-volt fila- ment battery because of lhe very small amount of emission required for the low value of plate current drawn. Be sure to get à socket that will accommodate the particular tube you buy. Oddly, it is important that the audio trans- former used ot be of good quality; if it is, it may have so much induetanee that it will be impossible to get a sufficiently high pitehed note. If you buy & new transformer, get the smallest, cheapest one you can buy. The old transformers used in moderately priced sets of 12 years ago arc fine for the purpose, and can oftentimes he picked up for a small fraction of a dollar at the “junk parts” stores. The tums ratio is not im- portant; it may be anything between 1,5/1 and 6/1, Correct transformer polarity is necessary for oscillation. If oscillation is not obtained, reverse the two wires going to the primary terminals of the transformer. The tone may be varied by substituting a larger (.025 píd.) or smaller (001 uíd.) condenser for the .008-p£d. capacitor shown in the diagram. A lower capacity condenser will raise the pitch of the note somewhat and vige versa. The highest pitch that can be ob- tained with a given transformer will result when the condenser is left out of the circuit altogether. Lowering the plate voltage to 3 volts will also have a noticeable effect upon the pitch of the note, If the particular trans- tb  i | l 16 Introduction to Amateur Radio former you use does not provide a note of a piteh that suits you, the pitch can be altered in this manner. Using a 1H46, a standard no. 6 dry cell for filament power, and a 4-volt € battery for plate power, the oscillator may be con- structed for about $2,00 exclusive of key and earphones. The filament battery life will be about 700 hours, the plate battery life con- siderably more. This set has an advantage over an a.e. operated practice set in that it can be used where there is no 110-volt power available; you can take it on a Sunday pienie i£ you wish, Also, there is no danger 0f elee- trical shock. The carrieroperated keying monitor de- seribed in Chapier Twenty-two also may be used for code practice, and is recommended where loud speaker operation is desired, such as for group practice, Automatic Code Machines, The two prae- tice seta just described—the buzzer and the x.t. oscillator—are of most value when you have someone with whom to-praétice. T£ yon are unable to enlist a code partner and bave to practice by yourself, the best way to get receiving practice is by use of a set of phono- graph code practice records or a tape machine (automatic code-sending machine) with sev- eral practice tapes. The records are of use only if you have a phonograph whose tumtable speed is readily adjustable. The tape machine can be rented by the month for a reasonable-fee., Once you can copy close to 10 w.pm., you can get receiving practice by listening to slow-sending amateurs on your receiver, as amateurs usually send quite slowly when The RÁDIO Figure 10. Wien. special quarters are not available for the station, the en. tire equipment often às placed in one corner of a den or bedroom, and as in the case of WOUMS, need not detract from the appearance of the room. working extreme dx. However, until you can copy around 10 w.pm, your receiver isn't of much use and either another operator or a tape machine or code records are necessary for getting receiving practice alter yon have once memorized the code, The student must observe the rule always to write down each letter as soon as it is re- ceived, never dots and dashes to be translated later. JE the alphabet has actually been mas- tered beforchand, there will be no hesitation from failure to recognize most of the char- acters unless the transmission speed is too high. ' Don't practice too long at one stretch; it does more harm than good. Twenty-five or thirty minutes should be the limit. Time must not be spent trying futilely to recall a missed letter. Dismiss it and center the attention on thc next lciter. In order to prevent guessing and to give you equal prac- tice on seldom-used letters such as X, Y, etc. the transmitted material should not be plain language except perhaps for a few minutes out of ench practice period. During the first practice period, the speed should be such that a substantially solid copy can be made of the entire transmission with- out strain. Then, in the next period, the speed should be increased slightly to a point where all of the characters can be caught only throngh eonscions effort. When the student becomes profieient at this new speed, another slight increase may be made, pro- gressing in this manner until a speed of about 16 words per minute is attained. The margin of 3 w.p.m. is recommended to over- come the possible excitement factor at ex- Handbook amination time. Then when you take the test you don't have to worry about “jitters” or an “off day” The speed must not be increased to a new level until the student finally makes solid copy for a 5-minute period at the old level. How frequently increases of speed can be made depends upon individual ability and the number of practice hours. Each in- crease is apt to prove decidedly disconcert- ing, but keep in mind the statement by Dr. G. T. Buswell, “You are never learning when you're comfortable.” Using A Key. See figure 11 for the proper position of the hand, fingers, and wrist when manipulating the telegraph key. The forcarm rests naturally on the desk. The knob of the key is grasped lightly with the thumb along the edge and the index and third fingers resting on the top towards the front edge. The hand moves with a free up and down motion, the wrist acting as a ful- crum. The power comes entirely from the arm muscles. The third and index fingers will bend slightly during sending, but not because of conscions effort to manipulate the finger muscles. Keep your fnger muscles just tight enough to act as a “cushion” for the arm motion and let the slight movement of the fingers tako cave of itself. The key spring is adjusted to the indi- vidual wrist and should be neither stiff nor “sloppy.” Use a moderafely stiff tension at first and gradually lighten it as you get more proficient. The separation between the contacts must be the proper amount for the desired specd, being about 1/16 inch for slow speeds and correspondingly closer to- gether (about 1/32 inch) for fuster speeds. Avoid extremes in either direction. If is preterable that the key be placed far enough from the cdge of the table (about 18 inches) that the elbow can rest on the table. The eharacters must be properly spaced and timed, with the dot as the yardstick. A standard dash is three times as long as a dot. The spacing between parts of the same let- ter is equal to one dot; between letters, three dots; betwcen words, five dots. This does not apply when sending slower than about 10 words per minute for the bene- fit of someone learning the code and desir- ing receiving practice. When sending at say 5 w.p.m., the individual letters should be made the same as though the sending rate were about 10 w.p.m. except that the spacing between lettors and words is greatly ex- aggerated. The reason for this is obvious. The letter L, for instanee, will sound exactly the same at 10 w.pom. as at 5 wpu, and Code Practice 17 xhen the speed is increased above 5 w.pn., the student will not have to become familiar with a new sound (faster combination of dots and dashes). He will merely have to learn the identifying of the same sounds without taking so long to do so. It has been shown that it does not aid a student to identify a letter by sending the individual components of the letter at a speed eorresponding to less than 10 words per minnte. By sending the letter moder- ately fast, a longer space can be left be- tween letters for a given code speed, giving the student more time to identify the letter. When two co-leamers have memorized the code and are ready to start sending to each other for practice, it is a good idea to en- list the aid of an experienced operator for the first practice session so that you will get an idea of what properly formed letters sound like. When you are practicing with another be- ginner, don't gloat beenuse you seem to be learning to receive faster than he. It may mean that his sending is better than yours. Remember that the quality of sending affects the maximum copying specd of a beginner by as much as 100 per cent. Yes, if the sending is bad enough, a neweomer won't be able to read it at al), even though an old timer may be able to get the general drift of what you are trying to send. A good test for any “fist” is to try it on someone who is just getting to the “13 per” stage. If You Have Trouble, Should you expe- Figure 11. PROPER POSITION OF FINGERS FOR OPERATING À TELEGRAPH KEY. The fingers hold the knob and act as à cusbion. The hand rests lightly on the key, The muscles of the forearm provide the power, the wrist acting as a fulerum. The power should not come from the wrist, but rather from the forearm muscles. Handbook electromotive force current This equation constitutes the basis for Ohm's Law, which is treated at length in the sue- ceeding text. Fundamental Electrical Units The most fundamental of the common elee- trical units are the ohm, the volt, and the ampere. The Ohm. The commonly used unit of resistance, or opposition to the fow of an electrical eurrent, is the ohm. The interna- tional ohm is the resistance offered by & column of mereury at 0º O, 144521 grams in mass, of constant eross-sectional area and 106.300 centimeters in length. The expres- sion megohm (1,000,000 ohms) is also some- times used when speaking of very large values of resistance. By definition, if a volt- age of one volt is applied across a resistance of one obm, a current of one ampere will flow. The Ampere. The fundamental unit of current, or rate of flow of electricity is the ampere. A current of one ampere will de- posit silver from & speeified solution of silver nitrate at a rate of 1.118 milligrams per second. Many persons confuse the ampere which is a unit of rate of flow with the coulomb, which is a unit of quantity of elee- trieity. A coulomb is equal to 6.28x10” elee- trons, and when this quentity of electrons flows by a given point in every second, à eur- rent of one ampere is said to be fowing. An ampere is equal to one conlomb per second; & coulomb is, conversely, equal to one ampere- second. Thus we see that coulomb indicates amount, and ampere indicates rate of flow. The Volt. The electrons are driven through the wires and components of a eir- cuit by a foree called an electromotive force, usually abbreviated em. unit that denotes this force is called the volt. This force or pressure is measured in terms of the difference in the number of electrons at one point with respect to another. This is known as the potential difference. The standard of clectromotive force is the Weston cell whieh at 20º C, has & potential of 1.0183 volts across its terminals. This cell is used only for reference purposes since only an infinitesimal amount of current may be drawn from it without disturbing its charae- teristics. The relationship between the eleetro- motivc force (voltage), the flow of current (amperes), and the resistance which impedes the flow of current (ohms), is very clesrly or EMF. The: Fundamental Electrical Units 21 expressed in a simple but highly valuable law known as Ohm's law. Ohm's Law. This law states that the cur- rent in amperes is equal to the voltage divided by tho resistance in ohms. Expressed as an equation: E R Tf the voltage (E) and resistance (R) are Imown, the current (1) can be readily found. If the voltage and current are known, and the resistanee is unknown, the resistance E (R) is equal to —. When the voltage is the 1 unknown quantity, it can be found by multi- plying 1X R. These three equation are all secured from the original by simple trans- position. The expressions are bere repeated for quick reference: E E I=— R=— E=IR R 1 ihere 1 is the current in amperes, Ris the resistance in ohms, Eis the electromotine force in volts. Applying Ohm's Lew. As a practical ex- ample suppose we take the case where it is desired to place a bleeder resistor which will draw 40 ma. aeross 8 500-volt power supply. Tn this example both the voltage and the cur- rent are Imown and the resistance value is the unknown ; hence, we use the second of the three above equations which states that the voltage in volts divided by the current in am- peres will give the desired resistance value in ohms, The current valne is given in mílli- amperes; to convert ma. into amperes the decimal point is moved three points to the left. Hence £0 ma, = 040 amperes. E 500 R=— R=-— = 12,500 ohms 1 é Thus if a 12,500-ohm resistor is placed across a 500-volt plate supply the eurrent passing through the resistor will be 40 ma. or .040 amperes. Another typical problem for the applica- tion of Obm's law would be a resistance- eoupled amplifier whose plate resistor has a value of 50,000 ohms, with a measured eur- rent through this resistor of 5 milliamperes. The, problem is to find the actusl voltage ap- plied to the plate of the tube. The resistance R is 50,000 ohms. The eur- zent 1 is given as 5 milliamperes; miliam- peres must, therefore, first be converted into amperes; .005 amperes equals 5 milliamperes. 1 cia mar 22 Fundamental Radio and Electrical Theory The RADIO Figure 1, RESISTORS IN PARALLEL. The clectromotive force or voltage, E, is the unknown quantity, Obm's law is applied as follows: Formula: E=— I 005 amperes Solution: .005 X 50,000 = 250 volts drop across the resistor. TF the power supply delivers 300 volts, the actual voltage on the plate of the tube would be only 50 volts. This means that 250 volts of the supply voltage wonld be consumed in forcing a current of .005 amperes through the 50,000-ohm plate resistor. As another example suppose that given supply voltage is 300, and that the (meas- nred) voltage on the plate of the tube is 100 volts. Find the current fowing through the plate resistor of 20,000 ohms. E From Ohm's law, I = —, and E equals the R diforence between supply and measured plate voltages. Therefore : 200 > 20,000 1 = 0,020 amperes, or 10 milliamperes. Resistançes in Series and Parallel When resistances are connected in series the total valuc of resistance is equal to the sum of each of the individual resistances. Thus a 2000- ohm resistor in series with a 3000-ochm one would make a total of 5000 ohms—and if an- other resistor of 5000 ohms were connected in serics with the other two the total value would be the sum of all three or 10,000 obms. However, when resistors are connected in paraite: (or shunt as such a conncetion is sometimes called) the resultant value of re- sistaneo is always less than the value of the lowest of the paralleled resistors. It is well to hear this simple law in mind as it will assist greatly in approximating the value of par- alleled resistors at a later time, The ealcula- tion of thc exact values of paralleled resistors will he discussed in the succecding para- graphs, Like Values of Resistance in Parallel, When two or more resistances of the same value are placed in parallel the effective re- sultant of the paralleled resistors is equal to the value of one of the resistors divided by the number of resistors in parallel, This can be expressed mathematically as: R (q resistora in paralhel) — R (each resiator) K number in parallel) Thus if (2) resistors of (5000) ohms are placed in parallel the resultant value is 5000 divided by 2, or 2500 ohms, As another ex- ample, if (4) resistors of (100,000) ohms are placed in parallel the effective resistanes of the paralleled combination is 100,000 divided by £ or 25,000 ohms. Unlike Resistances in Parallel. The re- sultant value of placing a number of unlike resistors in parallel is equal to the reciprocal of the sum of the reciprocals of the various resistors. This can he expressed as: The effective valne of placing any number of unlike resistors in parallel can be deter- mined from the above formula. Ilowever, it is commonly used only when there are three or more resistors under consideration since the simplified formula given im the following paragraph is more convenient when only two resistors are being used. Two Unlike Paralleled Resistors. When two resistances of unlikc values are to be used in parallel the following formula may be used to determine their effective resistance: RX Re Re+ Ra where R is the unknown resistance, Ba is the resistance of the first re- sistor, Re is the resistance of the second resistor, A typical example would be an ave. re. sistor of 500,000 ohms, which is to be shunted (paralleled) with another resistor of some valve, in order to bring the effective resistance value down to a value of 300,000 ohms. Sub- stituting these values in the equation for two unequal resistances in parallel; 500,000 X Ra 800,000 = —————— — 500,000 + Ro By transposition, factoring and solution, the effective value of R will be 750,000 ohms. Handbook Resistance 23 Thus a 750,000-ohm resistance must be con- neeted across the 500,000-0hm resistance in order to secure an cffective resistance of 300,- 000 ohms. In solving for values other than those given, the simplificd equation becomes : RXR a R —R where R is the resistance present, Ra is the resistance to be obtained, Re is the value of the unknown re- sistunce necessary to give Br «hem in parallel with R. Resistances in Series-Parallel. Resist- ances in serjes-parallel can be solved from the equation (sec figure 2): Re R-= 1 1 — — +———— Ri RB: Ro+R: Ra tRo+tR Power in Resistive Circuits, Ileat is generated when a souree of voltage causes à given current to flow through a resistor. If the flow of current is continually being im- peded as a result of an insufficient number of free electrons, there will be countless colli- sions between the moving clectrons and the atoms and the clectrons must, therefore, be forced throngh in order that a given number will move continnously through the conduet- ing medium. This phenomenon results in henting of the conductor, and the heating is the result of loss 0? useful poser or energy. Wattage. The power in an electrical circuit is expressed in watts and is equal to the product of the voltage and the eurrent flowing in that circuit. Hence W (watts) = EL Since it is often convenient to express power in terms of the resistance of the circuit and the current flowing through it, a sub- stitution of IR for E (E = IR) im the above formula gives: W = IR X Lor W = PR. In terms of voltage and resistance, W = FYR. Here, I = E/R and when this was substituted for I the formula became W = E X E/R or W — Eº/B. To repeat these three expressions for determining waitage in an electrical circuit: W=ELW-R and W= EYR, where W és the power in watts, E is the electromotive force àn volts, and 1 is the current in amperes. To apply the above eguations to a typical problem: The voltage drop aeross a cathode resistor in a power amplifier stage is 50 volts; the plate current flowing through the resistor is 150 milliamperes. The number of watts Ra ER Ra Re R Rz Rá Rz Figure 2. RESISTORS IN SERIES PARALLEL. the resistor will bo required to dissipate is found from the formula: W (watts) = E X or 50 X ,150 = 7.5 waits (.150 amperes is equal to 150 milliamperes). From the toresoing it is seen that a 7.5-wat resistor will safely carry the required current, yet a 10- or 20-watt resistor would ordinarily be uscd to provide à safety factor. In another problem, the conditions being similar to those above, but with resistance and current being the kxotem factors, the solu- tion is obtained as tollows: W = É X R= 02D5 X 333,38 — 7,5. E only the voltage and resistance are E 2500 R 333.33 It is seen that all three eguations give the same result; the selection of the partienlar equation depends only upon the known fae- tors. Bleeder Resistors. Resistors are often connected across the output terminals of power supplies in order to bleed off à con- staut value of current or to serve as a constant fixed load. The regulation of the power supply is thereby improved and the voltage is maintained at a more or less constant value, regardless of load conditions. When the load is entirely removed from a power supply, the voltage may rise to such a high valuc as to ruin the filter condensers. The amount of current which can be drawn from a power supply depends upon the current rating of the particular power trans- former in use, If a transformer will carry a- maximum safe current of 100 milliam- peres, and if 75 milliamperes of this current is required for operation of a radio receiver, there remains 25 milliamperes of current available which can be wasted in the bleeder resistor. An example for calculating blecder re- sistor values for safe wattage rating is as follows: The power supply delivers 300 volts. The power transformer can safely supply 75 milliamperes of current of which 60 milliamperes will be required for the re- 26 Fundamental Radio and Electrical Theory The RADIO setting will be greatly in error. Heavy bleeder currents are thus required for C-bias supplics, especially where the grid curr is changing and the bias must remain con- stant, as im certain types of phone trans- mitters. Since the grid current in a C-bias supply flows from the tap on the divider to ground, and in the same direction as the bleeder cur- rent, it is important to remember that in this case the regulation is in the opposite di- rection from the ense where power is being taken from a tap on the divider. In other words, the greater the grid current that is Aowing through the bleeder, the higher will be the voltage at this tap on the divider— and for that matter, at all other taps in the same divider, Filaments in Series and Parallel. When compntations are made for the operation of vacnum tube filaments or heaters in series conneetion, it should be remembered that each has a definite resistance and that Ohms law herc again holds true, just as it does in the casc of a conventional resistance. No particular problem is involved «when two exactly similar tubes of the same volt- age and enrrent rating ave to be operated with their filaments or heaters connected in “series in order to operate them from a source of voltage twice as high as is re- quired for cach tube. If two six-volt tubes, each requiring 0.5 ampere for heater op- cration, arc connected in series across a 12- volt power source, ench tube will have the same voltage drop (6 volts), and the total current drasn from the power supply will be the same as for one tube or 0.5 ampere. By making this connection, the resistance has actually becn donbled; yet, because the voltage is doubled, each tube automatically secures its proper voltage drop. In this example, the resistance of each tube would be 12 ohms (6 divided by 0.5). In serics, the resistance would be twice this valuc or 24 ohms. The current E would then dro=000us sL6-r 0nMs sonus zonus BESOMNS 126 VOLTS Figure 5. Obtaining the proper filament voltage drop across each of a pair of dissimilar tubes by means of a resistor across the heater of the one drawing the least ambunt of current, equal 12/24 or 0.5 ampere, from which it can be seen that the current drawn from the sup- ply is the same as for a single tube. It is important to understand that in a series connection the sum of the voltage drops across all of the tubes in the eir- enit cannot be more than the voltage of the supply. It is not possible to connect six similar 6-volt tubes in series across a 32- volt supply and expect to realize 6 volts on the filaments of each, since the sum of the various voltage drops is equal to 36 volts. The tubes can, however, he connected im such a manner that the correct voltage drop will be secured as wili be explained later. Different Tubes in Series. A 6F6 and a 6L6 are to be operated in à low-power airplane transmitter. The power supply delivers 12.6 volts. The problem is to con- nect the henters of the two tubes in such a manner that each tube will have exactly the same voltage drop across its heater terminals. The tube tables show that a type 6F6 tube draws 0.7 ampere at 6.3 volts, while the 6L6 draws 0.9 ampere at the same voltage. The resistance of the 6F6 heater is E 63 R=— = — 9 ohms. Then, the re- 1 07 63 sistance of the GL6 heater equals — = 0: 7 obms, If these tubes are connected in series without precantionary measures, the total resistance of the two will be 16 chms (9 + 7). A potential of 126 volts will pass a current of 0.787 ampere through this value of 16 ohms. The drop across cach separate resistor is found from Obm's law, as fol- lows: 9 X 0787 = 7.083 volts, and 7 X 0.787 = 54 volts. Thus, it is scen that neither tube will have the correct voltage ârop. TÉ the tubes are regarded on the basis of their respective current ratings, it will be found that the 6L6 draws 0.9 ampere and the 6F6 0.7 ampere, or a difference of 0. ampere. TE the resistance of the 6F6 is made equal to that of the 6L6, both tubes will draw the same current. Simply take the difference in current, 0.2 ampere, and divide this value into the proper voltage ârop, 6.3 volts; the answer will be 31.5 ohms, which is the value of resistor which mnst be paralleled with the 6F6 filament to make its resistance the same as that of the 6L6, Handbook Elecromagnetism 27 When tube heaters or filamenis are oper- ated in series, the current is the same throughout the entire eireuit. The ro- sistancc of all tube Slaments must then be made the same if each is to have the same voltage drop across its terminals. The re- sistance of à tube heater or filament should never be measured when cold because the resistance will be only a fraction of the re- sistance present when the tube functions at proper heater or filament temperature. The resistance can be calenlated satisfac- torily by using the current and voltage rat- ings given in the tube tables, Electromagnetism Everyone is familiar with tho common bar or horseshoe megnet. The magnetic field which surrounds it allows the magnet to attract nails, washers, or other pieces of iron to it. À peculiarity of an electric cur- rent, hence of electrons in motion in gen- erel, is that 2 magnetic ficld is set up in the xieinity of the conductor of the current for as long a period of time as the current is fowing. A field set up by an electric cur- rent is called an eleciromagnetic Seld to di tinguish it from the permanent field sur- rounding the bar magnet. Magnetic Flyr. The field, or magnetic lines of force, set up in the vicinity of the conduetor extend outwardly from the con- duetor in a plane at right angles to ifs di- reetion. Tt is these lines of magnetic force that make up the magnetic flur. The strength of this fux in the vicinity of a simple condutor is proportional to the strength of the current. However, if the conduetor is wound into a coil the fax for each turn of wire becomes additive to that of the others and the flnx becomes propor- tional to the zumber of turns as well as to tho current flow. Since the flux is linearly proportional to both the current and the numbor of turns, the magnctizing effeet of a coil may be described as a function of the ampero-tums of that coil; the mag- netizing. effeet of a coil is proportional to the produet of tho current strength and the number of turmas in the coil. The magnetic fnx inercases or doerenses in direct proportion to the change in the enrrent. The ratio of the change in flux to the change in current has a constant value known as the indueiance of a coil. Electromagnetic Effects, In drawing an analogy of voltage, current and resist- ance in terms of magnetic phenomena, magnetic fux might be termed magnetic worionoe] forecrion ELECTRO) — [F correr Y E a ER mu + Mm Q m vu qi A 4 tr N 4 ol Figure 6. tA) shows the magnetic lines of force produced around a conductor carrying an efectric current. It also indicates the difference between the motion of electrons and the flow of current. (B) indicates how the eftectiveness of the feld may be increased by winding the conductor into a coil. current, magnetomotive force or magnetic voltage. The unit of magnetomotive force (man.£.) is the gilbert. The reluctance of à magnetic circuit can be thought of às the resistance of the magnetic path. The re- lation between the three is exactly the same as that between current, voltage and resist- ance (Ohms law). The magnetic flus depends upon the ma- terial, oross section and length of the mag- netie circuit, and it varios directly as the current fowing in th eircuit. The reluotance is dependent upon the length, cross section, permeability and air gap, if any, of the mag- netie cireuit, Tn the clectrical cirenit, the current “rould equal the voltage divided by the re- sistanse, and so it is in the magnetio cireuit. Magnetic Flux (4) — magnetomotive force (mam. reluctance (r) Permeability. Permeability describes the difference in the magnetic properties oí any magnetic substanec as compared with the magnetie properties of air, Ivon, Lor example, has a permeability of around 2000 times that of air, which means that a given amount of magnetizing effect pro- duced in an iron core by a current fowiny through a coil of wire will produce 2006 times the fiur density that the same mag- netizing cífect would prodnce in air. The permeabilitics of different iron alloys vary quite widely and permeabilities up to 100,- 000 can be obtained. Core Saturation, Pormeability is sim- ilar to electric conductivity. There is, how- ever, one important diffcrence: the porme- ability of iron is not independent of the magnetic current (fux) flowing throngh it, although electrical conduetivity is nsvally ame eee 28 Fundamental Radio and Electrical Theory independent of electric current in a wite. After a certain point is reached in the flux density of s magnetic conduetor, an in- ercase in the magnetizing feld will noi pro- duce a material increase in flux density. This point ís known as the point of satura- tion. The induetance of a choke whose core às saturated declines to a very low value. Counter E.M.P. A fundamental law of electricity is: when lines of force cut noross a conduetor, a voltage is induced in that conduetor. Therefore, it can be readily seen that in the case of the coil previously mentioned the flux lines from one tum cut across the adjacent turp, and a voltage is induced in that turn. The effect of these induced voltages is to ercate a voltage seross the entire coil of opposite polarity or in the opposite direction to the original voltage. Such a voltage is called counter em.f. or back emf. If a direct current potential such ss à battery is connected across a multiturn coil or induetanee, the back cm.f. will exist at the time yhen the connection is being made or broken at which time the flux is rising to its maximum value, or falling to zero. While it is true that a current is flowing through the turns of the coil and that a magnetic field exists around and through the center of the inductanes, an induced voltage may only be produced by a changing fine. Tt is only such a changing fux that will cut across the individual tures and induce a voltage in them. By q changing fux is meant a fux that is increasing or decreas- ing as would oceur if the em. aeross the coil were alternating or changing its diree- tion periodically. The Unit of Self-Inductançe: The Henry. As the current inereases, the back em, renches a maximum; as the current decrenses, the back em.£. is maximum in the same direction ag the enrrent. This back voltage is always opposite to the exciting voltage and, hence, always acts to resist any change in current in the inductance. This property of an inductance is called its self inductance and is expressed in henrys, the henry being the unit of inductance, A coil has an induetance of one henry when a volt- age of one volt is induced by a current change Of one ampere per second. The unit, henry, while commoniy used in audio frequeney cireuits is too large for reference to induetance coils such as those used in radio-frequeney cireuits; millihenry or micro- henry are more commonly used, in the following manner: The RADIO 1 henry = 1,000 millihenrys, or 103 milli- henrys. 1 milhhenry = 1/1000 of a henry, .001 henry, or 108 henry. 1 microhenry = 1/1,000,000 of a henry, or 000002 henry, or 1 henry. One one-thousandih of a millihenry = 001 or 108 milithenrys. 1,000 microhenrys = 1 millihenry. Motnal Inductance. Jf two inductances are so placed in relation to cach other that the lines of force encireling one coil are in- terlinked with the turns of the other, a voltage will be set up or induosd in the second coil. As in the case of self. induetanee, the induced voltage will be op- posite in direction to the exciting voltage, This effect of linking two induetanees is called mutuai inductance, abbreviatod M, and is also expressed in henrys. Two cir- euits thus joined are said to be inductively coupled. The magnitude of the mutual inductance depends upon the shape and size oF the two eireuita, their positions and distances apsrt and the permeability of the medium. The extent to which two induetances are coupled is expressed by a relation known as cosfi- cient of coupling. 'This is the ratio of the mutunl inductance acinally present to the maximum possible value. Inductances in Parallel, Inductances in parallel are combined exactly as are re- sistors in parallel, provided ihat they are far enongh apart so that the mutual in- duetanee is entirely negligible, ie. if the coupling is very loose. Inductances in Series. Indactances in series are additive, just as are resistors in series, again provided that no mutual in- duetance exists. In this case, the total in- duetance L is: L=ly+L +. . Where mutual induetance does exist: L=L + lo + 2M, where M is the mutual inductance. This latter expression assumes that the coils are connected in such a way that all flux linkages arc in the same direction, ie, additive. If this is not the case and the mutual linkages subtraci from the self- linkages, the following formula holds: = Ly + Lo-2M, where M is the mutual inductance. Calcuiation of Inductance, The induct- ance of coils with magnetic cores can be determined with reasonable accuracy from the formula: «.ete. Handbook Capacitance 31 (CIRCULAR PLATE CONDENSERS. ACITY FOR A GIVEN SPAING SPACING IN INCHES VE 34 56 7689%N WIM CAPACITY IN MICRO-MICROFARADS Figure 7. material. A condenser that has a capacity of 100 nyíd. in air would have a capacity of 500 upld. when immersed in castor oil, because the dielectrie constant of castor oil is 5,0 or five times as great as the dielectric constant of air. In order to determine the capacity of x parallel plate condenser, the following transposition is of value when the spacing between plates is known: where À =area of plates in square inches, K =diclectric constant of spacing material, € =capacity in miero-microfarads, t =thickness of dielectrie (plate spaeing) in inches. Where the area of the plates is definitely set, and when it is desired to know the spae- ing needed to secure 8. required enpacity, t -A X 0.2248 X K AL AA, o whexe all units are expressed just as in the preceding formula. This formula is not confined to condensers having only square or rectangular plates, but also applies when the plates are circular in shape. The only change will be the caleulation of the area of such circular plates; this ares can be computed by squaring the radixs of the plate, then multiplying by 3.1416, or “pi”, Expressed as an equation: 1416 X 13, where r = radius jn inches, The capacity of a multi-plate condenser can be calculated by taking the capacity of one section and multiplying this by the number of dielectric spaces. In such cases, PARALLEL CONDENSERS SERIES CONDENSERS Figure 8. however, the formula gives no considera- tion to the effects of edge enpacity so that the capacity as coleulated will not be en- tirely accurate. These additional enpacities will'be but a smal] part of the effective total capacity, particularly when the plates are ressonably large, and the fina) result yill, therefore, be within practical límits of ae- curacy. Equations for esleulating capacities of condensers in parallel connection are the same as those for resistors in series: C=C + Oy eto, Condensers in series connection are cal- culated in the same manner as arc resistors in parallel. The formulas are repested: (1) For two or more condensers of unegual capaciky in series: 1 c=— 0, 1 1 1 D4D4— O CO 6 1 1 1 1 e =D 4 4 GG (2) Two condensers of unegual capacity in series: G x c= C + Ca (3) Three condensers of equal capacity in series: 1 C == —, where Cy is the common capacity. 3 (4) Three or more condensers of equal ca- pacity in series: Yalue of common capacity Number of condensers in series (5) Six condensers in series parallel: 32 Fundamental Radio and Electrical Theory The RADIO E LOTE E Lo L lo; l Ca Te Te E «—— CONDENSERS IN SERIES-PARALLEL Figure 9. Voltago Rating of Condensers in Serles, Any good paper dielectrie filter condenser has such a high internal resistance (indicat- ing a good dielectrie) that the exact resistance will vary considerably from condenser to condenser even thongh they are made by the same manufacturer and ave of the same rat- ing. Thus, when 1000 volts d.e. is connected aeross two 1-nfd. 500-volt condensers, the chances are that the voltage will divide un- evenly and one condenser will receive more than 500 volts and the other less than 500 volts. Voltage Equalizing Resistors, By con- neeting a half-megohm 1-watt carbon resistor across each condenser, the voltage will be equalized becanse the resistors act as a volt- age divider and the internal resistances of the condensers are so much higher (many megohms) that they have but little effect in disturbing the voltage divider balance. Carbon resistors of the inexpensive type are not particularly accurate (not being de- signed for precision service); therefore it às advisable to check several on an aecurate ohmmeter to find two that are as close as pos- sible in resistance, The exact resistance is unimportant, just so it is the same for the two resistors used. Condensers in Series on À. 0, When two condensers are connected in series, alternat- ing eurrent pays no heed to the relatively high internal resistance of each condenser, but di- vides across the condensers in inverse propor- tion to the capacity. Because, in addition to the d.c. aeross a eapaeitor in a filter or audio amplifer eircnit, there is usually an a.e. or af. voltage component, it is inadvisable to series-connect condensers of unegual espaci- tance even if dividers are provided to keep the de. within the ratings of the individual capacitors. For instance, if 0, 500-volt 1-,fd. capacitor is used in series witk & 4-g$d. 500-volt con- denser across a 250-volt n.e. supply, the J-ufd. condenser will have 200 volts p.e. across it and the 4-ufd. condenser only 50 volts. Am equalizing divider to do any good in this case would have to be of very low resistance be- cause of the comparatively low impedanee of the condensers to ae. Such a divider would + POLARIZED CONDENSERS (ELECTROLYTIC) IN SERIES Figure 20. draw excessive current and be impracticable. The safest rule to follow is to use only con- densers of the same capacity and voltage rat- ing end to install matehed high resistance proportioning resistors across the varions eondensers to equalize the d.c. voltage drop across each condenser. This holds regard- less of how many capacitors are series connected. Electrolytic Condensers in Series. Simi- lar electrolytic eapneitors, of the same capae- ity and made by the same manufacturer, have more nearly nniform (and mueh lower) in- ternal resistance though it still will vary con- siderably. However, the variation is not nearly as great as encountered in paper con- densers, and the lowest d.e. voltage is across the weskest (leakiest) electrolytie condensers of a series group. As an electrolytie capacitor begins to show signs of breaking down from excessive volt- age, the lenkage current goes up, which tends to heat the condenser and aggravate the con- dition. However, when nsed in series with one or more others, the lower resistance (higher leakage enrrent) tends to put less d.e. voltage on the weakening condenser and more on the remaining ones. Thus, the capacitor with the lowest leakage current, nsually the dest capaeitor, has the kighest voltage aeross it. For this reason, dividing resistors are not essential across series-connected eleetrolytic capaeitors. Electrolytie condensers use a very thin film of oxide as the dieleetric, and are polarized; that is, they have a positive and a negative terminal which must be properly connected in a cireút; otherwise, the oxide will boil, and the condenser will no longer be of service. When eleetrolytic condensers are connected in series, the positive terminal is always con- neeted to the positive lead of the power sup- ply; the negative terminal of the condenser connects to the positive terminal of the neat condenser in the series combination. The method of connection is illustrated in figure 10. Alternating Current To this point in the text, consideration has been given primarily to & current consisting Handbook of a steady Ilow nf cleetrons in one direction, This type of current fow is Enown as undirec- tional or direct current, abbreviated de. Equally as important in radio work and more important in power practico is another and altogother different type oÉ current, known as altermuting current and abbreviated g.e. Power distribution from one point to another and into homes and factories is almost uni- versally ae. On the other hand, the plate supply to vacuura tubes is almost universally de. An alternating current begins to flow in one direction, meanwhile changing its amplitude from zero to a maximum value, then down again to zero, from which point it changes its diveclion, and again goes through the same procedure, ach one of Lhese zero-maximum- zero emplilude changes in a given direetion is called a half eyele. The complete change in two direetions is called a eyete, The num- ber of Limes per second that the current goes through a complete cyele is called the fre- quency. The frequency of common honse- lighting altermating current is generaly 80 eyeles, meaning that il goes through 60 complete eycles (120 reversals) per second. However, 25. and 50-eycle power is to be found quite Lrequently and 40-, 133-, and 240- eyele power is found in certain foreign com- tries. Radio Frequency A. O. Radio frequency currents, on the other hand, go throngh so many of these aliernations per section that the term cycle becomes unwicldy. As an example, it cam be said that a cerfain station in the Len-meter amateur band is operating on 28,810,000 eycles per second. However it is much more convenieni to say that the carrier frequency is 28,640 kiloeyeles, or 28.64 megacyeles, per second. A conversion + VOLTAGE TIME DIRECT CURRENT p— rerar — fes crie + “1 I TIME VOLTAGE o ALTERNATING CURRENT Figure 11. Graphical comparison of unidirectional or direct current and alternating current, Alternating Current 33 table for simplifying this terminology is given her 1,000 eyclos = 1 hilocyele. The abbreviation for kilocycle is o. 1/1,000 of a kilocyele, JO1 ke. or 1 megacycle = 1,000 kilocycies, or 3,000,000 excles, 103 ke. or 10º eyetes. kiloeycle — 1/1,000 megucycle, 001 mega- eyele, or 103 Me. The abbreviation for megacyeles is Me. Applying Ohm's Law to Alternating Cur- rent, Ohms law applies equally to direct or aliernating current, provided the cireuits under consideration are purely resistive, that is, cireuits which bave neither induetance (coils) nor capacitanco (condensers). Prob- lems which involve tnbe filamenis, drop re- sistors, electric lamps, heaters or similar resistive devices can be solved from Ohm's law, regardless of whelher the current is di- reet or alternating. When a condenser or a coil is made a part of the circuit, a property common (o either, called reactance, must be taken into consideration. Inductive Reactance. As was previously stated, when an allemating current flows through an inductance, a back- or counter- clectromotive force is developed; this force oppases any change in the initial em.£. The property o? an induetanee to offer opposition to change in current is mown ax its reuctanco or inductive reactance, This is expressed as Xp: Xp = 2yfL, where Ki = induclive reactance expressed in ohms. 31416 (2m — 6.283), frequeney in cyeles, induetance in henrys Indnctivo Reactance at BR, F. It is very often necessary to compute induetivo reaci- ance aí radio frequencies. The same formula may be used, except that the nnits in which the inductance and the frequency aro ex- pressed ill be changed. Indnciance can, therefore, be expressed in millikearys and frequency in kilocyeles. For higher fre- quencies and smaller values of induetance, frequency is expressed in megacyetes and in. ductance in microhenrys.” The basic equation red not be changed since the multiplying factors for inductanee and frequency appear in numerator and denominator, and henee are cancelled out. However, it is not possible 1 the same equation to express L in millihen. and f in cycles without conversion factors. Should it become desirable to know the value of inductance necessary to give q certain 36 Fundamental Radio and Electrical Theory AMPLITUDE F T TIME Figure 13. Graph showing the voltage output of a singte-tum conductor revolving in a magnetic feld, tinnes, the current becomes inereasingly grenter as the center of cach pole piece is approached by the loop. The ficld intensity between the two pole pieces is substantially constant from one side to the other. However, as the com- ductor is rotated it can easily bc seen that it will cut fewer magnetic lines of force when it is running essentially parallel to thc lines at either side of the pole pieces than it will cut when it is running cssentiully perpen- dicular to them as it is when in fhe center of thc pole picecs. After the conduetor has rotated through 180” it can be scen that its position with respect to the pole pieces ill be exzetly the opposite to that when it started. Henee, thc second 180º of rotation will produee «current starting from zero, rising to a maximum, and falling again to zero, but this emvrent will fow in the op- posite dircetion to that of thc first half- eycle of rotation. Actually the voltage does not increase direetly as the angle of rotation, but rather as thc sine of the angle; hence, such a eur- rent has the mathematical form of a sine wave. Although most electrical machinery does not produce a strictly pure sine curve, the departures are nsually so slight that the assumption can be regarded as fact for most practical purposes. Referring to figure 14, it will be sem that if a eurve is plotted for an alternating voltage, smeb a curve would assume the shape of a sine wave and by plotting ampli- tude against time, the voltage at any in- stant could be found. When dealing with alternating current of sine wave character, it becomes necessary to make constant usc of terms which involve the number of changes in polarity or, more properly, the frequency of the current. The instanta- ncous valuc of voltage at any given instant can be calenlated as follows: e = Emas sin Brft, where e =the instantaneous voltage, sin = the sine of the angle formed by The RADIO 1eveLe + rue T crer AMPLITUDE, revece= | docesd WMERE F = FREQUENCY IN CYCLES PER SECOND Figure 14. the revolving point P at the in- stant of time, t E = maximum erest value of voltage (figure 14). The term 2yÊ should he thoroughly under- stond because it is of basic importance, Returning again to the rotating point P (figure 13), it can be seen that when this point leaves its horizontal position and be- gins its rotation in a counter-clockywise direction, through a complete revolution back to its initial starting point, it will have traveled through 360 electrical de- grees. Instend of referring to this move- ment in terms of degrees, mathematical treatment dictates that the movement be expressed in radians or segments equal to the radins. Radians. 1£ radians must be considered in terms of degrees, there arc approximately 57.32 degrees in one radian. Tn simple lan- guage, the radian is nothing more than a unit for dividing a eirele into many parts. In à complete circle (360 degrces), there are 2r radiuns. Figure 15 shows Icsser divisions of a cirele in radians, When the expression 2y radians is used, it implies that the current or voltage has gone through a complete eirele of 360 elee- une: o (ruema)= emase nor -2mrr As E maDiaNs OR 000 B= RAdIANS On s60s €< AF ravians on 270» D= 27 RADANS O 3600 4 RADIAN = 87.224 OEGREES] Figure 15. Handbook Generation of Alternating Current 37 trical degrees; this rotation represents two complete chunges in direction during one cycle, as was previously shown. 2yf then represents onc cycle, multiplied by the num- ber of such eyeles per second or the frequency of the alternating voltage or current. The expression 2wft is a means of showing how far point P has traveled from its zero posi- fion toward a possible change of 2r radians or 360 cleetrical degrees. In the casc of an alternating current with a frequeney of 60 eyeles per second, the cur- rent must pass through twice 60 or 120 changes in polarity in the same length of time. This time can be expressed as! 1 2f However, the only consideration at this point is one half of one alternation, and be- cause the wave is symmetrical between O and 90 degrees rising, and from 90 degrees to zero when falling, the expression there- fore becomes: 1 4£ the actual time, t, in the formula is seen to he only a fractional portion of a second; 1 a 60-eyele frequeney would make — equal as 1 to — of a second at the maximum value, 240 and correspondingly less ut lower ampli- tudes. 2wft represents the angular velocity, and since the instantaneous voltage or cur- rent is proportional to the sine of this angle, a definite means is secured for caleulating the voltage at any instant of time, provided that the wave very closely approximates a sino enrve. Current and voltage are synonymous in the foregoing discussion sinee they both fol- low the same laws. The instantaneous eur- rent can be found from the same formula, except that the maximum current would De used as the reference, vi: i== mos sin 2rft, where i = instantancous current, Imex — Inaximum or peak enrrent. Effective Value of Alternating Voltage or Current. An alternating voltage or cur- zent in am ac. circuit is rapidly changing in direction, and since it requires a definite amount of time for the indicator needle on a de. measuring instrument to show a de- fieetion, such instruments cannot be used to measure altemating current or vollage. Even if the necdle had such negligible damping that it could be made to follow the ne. changes, it would inerely vibrate back and forth near the zero point on the meter scale. Alternating and direct current can be ex- pressed in similar terms from th standpoint of heating cffect. In other words, an alter- nating current will kave the same valne as a direct current in that it produces the same henting effect. Thus, am alternating cur- rent or voltage will have an equivalent value of one ampere when it produces the same heating effect in a resistanco as does one ampere of direct current. "Phis is known as the efectivo value; it is neither the maxi- mum nor the instantancous value, bué an en- tirely different value. This effcetive valuc is derived by taking the inslantaneous values of current over a cycle of alternating current, then squaring these values, thén taking an average of this value, and then taking the square root of average thus obtained. By this proce- duze, the efectixe value becomes known as the root mean square or rms. This is Lhe value that is read on alternating eument volt- meters and ammneters. The tons, value is 70.7 per cent of the peak or maximum in- stantancous value and is cxpr lows: Ec — 0.707 X Emas, Or Tea — 0.707 X Imax, whcrc Emos and Ima arc pesk values of voltage and current respectively, and Eeg and Top are effective or rm.s. values. The following relations are extremely use- ful in radio and power work: Erros = 0.707 X Emas, Emex = 1,414 X Erms. In order to find the peak value when the effective or rm.s. value is Imown, simply multiply the rms. value by 1414. Yrhen the peak value is known, multiply il by 0.707 to find the rm.s. value. Rectified Alternating Current or Pulsat- ing Direct Current. If an alternating cur- rent is passed through a full-wave reetifier, it emerges in the form of a current of vary- TIME AMPLITUDE Figure 16. Waveform output from a full-wave rectifier. 38 Fundamental Radio and Electrical Theory ing amplitude which flows in one direetion only. Such a enrrent is known as rectified ac. or pulsating de. A typical wave form of a pulsating direct current as would be obtained from the output of a fnll-wave ree- tifer is shown in figure 16. Measuring instruments designed for de. operation will not read the peak or instan- taneous maximum value of the pulsating d.e. output from the rectificr; it will read only the average vatue. This can be explained by assuming that it could be possible Lo cut off some of the peaks of the waves, using Lhe ent- off portions to fill in Lhe spaces that are open, thereby ohtaining an average de. vulue. À milliammeter and voltmeter connected to the adjoining eireuit, or across the output of lhe rectifier, will read this average value. Tt is related to peak value by the following ex- pression: Eavg = 0.036 X Emar Tt às thus scen that the average value is 63.6 per cent of the peak value. Relationship between peak, rm.s. or ef- fective, and average values. To sunmarize the Lhrec most significant values of an a.e. wave: Lhe peak valne ís equal to 1.41 time the ran.s. or effective, and the rms. value is equal to 0.707 times the peak value; the average value oí a full-wave rectified ae. wave is 0,636 times the peak value, and the average value of a rectified wave is equal to 0.9 times the rm.s. value. This latter fae- tor is of value in determining Lhe voltage out- put from a power supply which operates with a choke-input filtcr system. If the in- put choke is of the swinging type and is of ample inductance, the d.e. voltage output of the power supply will be 0.9 times the rms. ae. output of the used secondary of the transforner (one-half secondary voltage in the cusc of a full-wave reetifier and the full secondary voltage in lhe casc of bridge ree- tification) less the drop in the reclifier inhes (usually negligible) and the drop in the filter inductances. Phase. When an allemating current flows through a purely resistive cireuit, it will be found that the current will go through maximum and minimum in perfect step wilh the voltage. In this case the current is said to be in step or in phase with the voltage. For this reason, Ohm's lay will apply equally well for a.e. or for d.e. where pure resistances are concerned, provided that the effective values of a.c. are used in the ealeulations. If a circuit has capacity or induefunee or both, in addition to resistance, the current does not reach a maximum at the same in- stunt as the voltage; therefore Obm's law The RADIO CURRENT LAGCINE. VOLTAGE Bro TIME legoes) (erreurr contains PRE INDUCTANCE ONLY) CURRENT LEADING VOLTAGE By sor cormcurr commamino — hroged PURE CAPAGITY GMLX) Figure 17. The above two illustrations show the manner in which a pure inductance or a pure capacitance no resistance component in either) will cause the current in the circuit either to lead or to lag the voltage by 90º. will not apply. Tt has been stated that in- duciance tends to resist any change in eur- when an inductance is present in a eircuit through which an allernating current is flowing, it will be found that the enrrent will reach its maximum behind or later than the voltage. In electrica! terms, the current will Jag behind the voltage or, conversely, the voltage will lend the current. TÉ ihe circuit is pyrcly inductive, i.e., if it contains neither resistance nor capacitance, the current does not start until the voltage has first reached a maximum; the enrrent, therefore, lags the voltage by 90 degrees as im figure 17. The angle will be less than 90 degrees if resistance is in the cirenit, When pure capacity alone is present in an a.e. circuit (no inductance or resistance of any kind), the opposite effect will be encountcrcd; the current will reach a maxi- um ai the instant the voltage is startimg and, hence, will lead the voltage by 90 de- grces. The presence of resistance in the cir- cuit will tend to decrease this angle. Power Factor. It should now be apparent to the reader that in such eircuits that have reactance as well as resistance, it will not be possible to calculate the power as in à d.e. circuit or as in an a.e. eireuil in which cur- rent and voltage are in-phase. The reactive components cause the voltage and current to reach their maximuns af different times, as was explained under phase, and to eulen- late the power in such a circuit ye must uso a value called the power fartor im our com- putations. The power factor in a resislivo-resetive ae. circuit may be expressed as the actual watts (as measured by a wati-meter) divided by the product of voltage and entrent or: Handbook ao OD 2200 2300 Figure 20. Resonance curve showing the effect of resistance upon the selectivity of a tuned circuit. rent against the frequency either side ot resonance, the resultant curve becomes what is known as a resonance curve. Such a curve is shown ix figure 20, Several factors will have an effect on the shape of this resonanee curve, of which resist- ance and Lto-C ratio are the important con- siderations. The curves B and C in figure 20 show the cffeel of adding increasing values of resistance to the circuit. It will be seen thai the peaks become less and less promi- nent as the resistance is increased; thus, it can be said that lhe selectivity of Lhe eirenit is therchy decreased. Seleetivity in this case can be defined as the ability of a circuit to diseriminate against frequencies ad jacent to the resonant frequency. Reterring again to figure 20, it can be seen from curve À that a signal, for instance, will drop from 19 to 5, or more than 10 deci bels, at 50 ke. off resonance. Curve B, which represents considerable resistanec in the cir- euit, shows a signal drop of from 4 to 2.3, or approximately 4 decibels, when the signal is also 50 Kiloeyeles removed from the reso- nant point. Prom this it becomes evident that the siceper the resonant curve, the greater will be the change in current for a signal ve- moved from resonance by a given amount. The effect of adding more resistance to the cireuit is to flalton off the peaks without materially afecting the sides of the curve. Thus, signals far removed from the resonance frequeney give almost the same value of cur- rent, regardless of the amount of resistance present. Voltage Across Coil and Condenser in Series Circuit. Because the a.e. or r.£. volt- age across a coil and condenser is propor- tional to the reactance (for a given current), the actual voltages across the coil and across Resonant Circuits 41 the condenser may be many times greater than the terminal voltage of the eiremit. Furthermore, since the individual reactunces can be very high, the voltage across the con- denser, for example, may be high enough to cause flashover even Lhough the applied volt- age is of a value considerably below that at whieh the condenser is rated. Circuit Q—Sharpness of Resonance. An extremely important property of an induct- ance is its fnctor-of-merit, more generally «alled its Q. Etis this factor, Q which prima- rily determines the sharpness of resonance of a tuned circuit. “his factor can be ex- pressed as the ratio of the reaetance to the resistance, as follows: 2mtL + R where R = total d.e. and 2.£. resistanees. The actual resistance in a wire or induet- anee can be far greater than the de. value when the coil is used in a radio-frequeney circuit; this is because the curtent does not travel through the entire cross-section of the conduetor, but has a tendeney to travel closer and closer to the surface of Lhe wire as the frequency is increased. “This is known as the skin effect. The actual current-carrying portion of the wire is decreased, therefore, and the re- sistance is increased. This cieel becomes even more proncunced in square or rectangu- lar conduetors because the principal path of current flow tends to work outwardly toward the tour edges of the wire. Examination of the equation for Q may give rise to the thonght thai even though the resistance becomes greater with frequency, lhe inductivo reactanec does likewise, and that the Q might be a constant. In actual practice, however, the resistance usnally in- creases more rapidly with frequency than does the reaciance, with the result that Q normally deereases wilh increasing fre- quene; Parallel Resonance Tn radio circuits, parallel resonance is more frequenily encountered tham series resonance; in fact, it is the basic fonndation of veceiver and transmiter circuit operation. A circuit is shown in figure 21, The “Tank” Circuit. In this circuit, as contrasted with a circuit for series resonance, L (induetance) and C (capacitance) are con- nected in parallel, yet the combination can de considered to be in series with the remain- der of the circuit. This combination of L 42 Fundamental Radio and Electrical Theory My Ney” ua, & ' SI Figure 21, The parallel resonant tank circuit. L and € com- prise the reactive elements of the tank and R indicates the initial rf. resistance of the com- ponents, M; indicates what is called the “line current? or the current that keeps the tank in a state of oscillation. Mo indicates the “tank current” or the amount of current circulating through the elements of the tank, and C, in conjunction with R, the resistance which 45 prineipally included in L, is some- times called a tank cirenit because it cfec- tively functions as a storage tank when in- corporated in vacuum tube cirenits, rasted swith series resonance, there are of current which must be con- sidered in a parallel resonant circuit: (1) the line current, as read on the indicating meter My, (2) the circulating current which foys within Lhe parallel L-C-R portion of the cir- cuit. Sce figure 21 At the resonant frequency, the line current (as read on the meter M,) will drop to a very low value althongh the circulating current in the LC cirenit may be quite large. It is this line current that is read by the milliam- meter in lhe plate circuit of an amplifier or ntor stage of a radio transmitter, and it às because of this that the meter shows a sudden dip as the eirenit is tuned through its resonant frequency. The current is, there- fore, à Iminimum wheu a parallel resonant circuit is tuned to resonance, although the impedunce is a maximum af this same point, Tt is interesting Lo note that the parallel res- onant circuit acts in a distincily opposite manner to that of a series Tesonant circuit, in which the exrrent is at a maximum and the impedanee is minimum af resonance. If is for this reason that in a parallel resonant cir- cuit the principal consideration is one of impedance rather than current. Tt is also significant that the impedance curve for parallel circuits is very nearly identical to that of the exrrent curve for series resonance. The impedance at resonance is expressed as: SatL)* R impedanee in ohms, induetance in henrys, requeney in cyoles, distance in ohms. L= where Z The RADIO Or, impedance can be expressed as a fune- tion of Q as Z = QmtLeQ showing that the impedance of a cireuit is direetly proportional to its Q at resonance. The curves illustrated in figure 20) can be applicã to parallel resonance in addition to the purpose for which they are illustrated. Referenec to the impedanee curve will show that the effect of adding resistance to the cirenit will result in both a broadening ont and a lowering of the peak of lhe curve, Since the voltage of lhe cireuit is directly proportional to the impedanee, and since it às this voltage that is applied to the grid of the vacuum tube in a detector or amplifier circuit, the impedanee curve must have a sharp peak in order for the circuit to be selective.. If the curve is broadtopped in shape, both the desired signal and the inter- fering signals at close proximity to resonance will give nearly equal voltages on the grid of the tube, and the cireuit will then be non- selective; Le. it will tune broadly. Effect of L/C Ratio in Parallel Circuits. In order that the highest possible voltage can be developed across a parallel resonant circuit, the impedance of this eireuit must be very high. The impedanee will be greater when the ratio of induetance-to-capacitance is great, that is, when L is large as compared with €. When the resistance of the cireuit is very low, XL will equal XC at resonance and of course, there arc innumerable ratios of L and C that will have equal roactance, at à given resonant frequency, exactly as is the ease in a series resonant circuit. Contrasted with the necessity for a high L/C ratio for high impedance, the capacity for maximum selectivity must be high and lho inductanco tow. While such a ratio will result in lower gain, it will offer greater rejectivity to sig- nals adjacent to the resonant signal, In practice, where a certain value of in- ductance is tuned by a variable eapacitance over a fairly wide range in frequency, the 1/C ratio will be small at the lowest fre. queney and large at the high-frequeney end. The cirenit, therefore, will have unequal selee- tivity at the two ends of the band of fre- quencies wkich is being tuned. At the low- end of the tuning band, where the nee predominates, the seleetivity will be grester and the gain less than af the high- frequeney end, where the opposite condition holds true. Tncreasing the Q of the cireuit (lowering the series “resistanco) will ob- viously increase both the selectivity and gain. Circulating Tank Current at Resonance. The Q of a eireuit has a definite bearing on Handbook the cireulating tank current at resonance, This tank current is very nearly the value of the line current multipled by the cireuit Q. For example: an ré. line current of 0,050 amperes, with a circuit Q of 100, will give a eirenlating tank current of approximately 5 amperes. From this it can be seen that the inductanee and connceting wires in a circuit with a high Q must be of very low resistance, particularly in the case of high power transmitters, if heat losses arc to be held to a minimum. Effect of Coupling on Impedanee. If a parallel resonant eixemit is conpled to an- other circuit, such as an antenna output cir- cuit, the impedanee of the parallel circuit is decreased as the coupling becomes closer. The effect of closer (tighter) coupling is the same as though an actual resistanco were added to the parallel eirevit. The resistance thus coupled into the tank circuit can be con- sidered as being reftected from the output or load eirenit to the driver circuit. If the load across the parallel resonant tank eireuit is purely resistive, just as it might be if a resistor were shunted across part of the tank inductanee, the load will not disturb the resonant setting. If, on the other hand, the load is reactive, as it could be with too-long or too-short antenna for the resonant frequency, the setting of the tank tuning condenser will have to be changed in order to restore resonance. Tank Circuit Flywheel Effect, When the plate circuit of a class B or class O operated tube (defined in the following chapter) is connected to a parallel resonsnt circuit, the plate eurrent serves to maintain this L/C cir- cuit in a state of oscillation. TÊ an initial im- pulse is applied across the terminals of a parallel resonant circuit, the condenser will become charged when «ne set of plates as- sumes a positive polarity, the other set a negative polarity. The condenser will then diseharge through the inductance; the cur- rent thus flowing will cut across the turns of the induciance and cause a counter em.f. to be set up, charging the condenser in the oppasite direction. Tn this manner, an alternating current is set up within the L/C circuit and the oscilla- tion would continue indefinitely with the condenser charging, discharging and charg- ing again if it were not for the fact that the eireuit possesses some resistance, The effect of this resistance is to dissipate somc energy each time Lhe current flows Lram inductance to condenger and back, so thai Lhe amplitude of the oscillation grows weaker ahd wesker, eventually dying out completely. Parallel Resonant Circuits 43 The fregueney of the initial oseillation is dependent upon the 1/C constants of the cireuit. If energy is applied in short spurts or pushes at just the right moments, the L/C eircuit can be maintained in a constant os- cilatory state. The plate current pulses from class B and class € amplificrs supply just the desired kind of Eicks. Whereas the class B plate current pulses súpply a kick for a longer period, the short pulses from the class C amplifier give a pulse 0£ very high amplitude, thus being even more eftective in maintuining oseillation. So it is that the positive half eyele in the tank ci cuit will be reinforced by a plate current kick, Put sinee the plate current of the tube only flows during a half eycle or less, the missing half eyele in the tank circuit must be supplied by the discharge of the condens Since the amplitude of this half cycle will depend upon the charge on the plates of the condenser, and sinee this in tum will depend upon the” eapacitance, the value of capaei- tanec in usc is very important. Particularly is this true if a distorted wave shape is to be avoided, as would be the case when a trans- mitter is being modulated. The foregoing applies particularly to single-ended ampli- flers. £ push-pull were employed, the nega- tive half-cyele would secure an additional kick, thereby greatly lessening the necessity of the use of higher C in the L/C eireuit. Impedance Matching: Impedance, Voltage and Tums Ratio, A fundamental law of eleetricity is that the maximum transfer of energy results when thc impedance of the load às equal to the impedance of Lhe driver. Although this law holds true, it is not neces- sarily à desirable one for every condition or purpose. In many cases where n vacuum tube works into a parallel resonant circuit load, it is desirable to have the load impedance considerably higher than the tube plate im- pedance, so the maximum power will be dis- sipated by the load rather than'in the tube: On the other hand, one of the notable con- ditions for which the law holds true is in the matching of transmission lines to an antenna impedance, Often a vacuum tube cireuit requires that the plate impedaco of a driver circuit be matched to the grid impedanes of the tube being driven. When the driven tube operates in such a condition that it draws grid current, such as in all transmilter r.£. amplifier eir- cuits, the grid impedance may well be lower than the plate tank impedance of the driver stage. In this case it becomes necessary to tap down on the driver tank coil in order to seleet the proper number of tums that will 46 Fundamental Radio and Electrical Theory O ez + É empue é = = ee é 1trnmo O oz E 4 Figure 24. Impedance step-up and step-down may be ob= tained by utilizing the plate tank circuit of a vacuum tube as ar auto-transformer. equal to the square root of the ratio between the modulator load impodance and the ampli- fier load resistance; the transformer may be cither step-up or step-down as the case may e. The Anto Transformer. The type of transformer in figure 23 rwhen swound with heavy wire and over an iron cere is a common device in primary power cireuits for the pur- pose af inereasing or decreasing the line volt- age. In effcet, it is mercly a continuous winding with tops taken at various points along the winding, the input voltage being applieã to the bottom and also to one tap on the winding. TÊ the output is taken from this same tap, the voltage ratio will be 1-to-1; ie. the input voltage will be the same as the ontput voltage. On thc other hand, if the output tap is moved down toward the common terminal, there will be a step-down in the turns ratio with a consequent step-down in voltage, The opposite holds true if the ouput ter- minal is moved npward from the middle input terminal; there will be a voltage step-up im this case. The initial setting of the middle inpué tap às chosen so that the number of tnras will have sufficicnt reactance to Keep the no-load primary current at a reasonably low value. Tn the same manner as voltage is stepped up and down by changing the number of The RADIO ty ta fes ca + Figure 25. Two commonty used types of inductive coupling between radio-frequency circuits. turns in à winding, so can impedance be stepped up or down. Figure 24Á shows an application of this principle as applied to a vaguum tube circuit which couples one cir- euit to another. Assuming that the grid impedance may he of a lower value than the plate tank imped- anee of the preceding stage, 4 step-dowm ratio will be necessary in order to give maxi- mum transfer of energy. Tn B of figure 24, the grid impedanee is very high as compared wilh the tank impedance of the driver stage, and thus there is required a step-np ratio to the grid. The driver plate is tapped down on its plate tank coil in order to make this im- pedanice step-up possible. A driver tube with very low plate impedanee mnst be used if a good order of plate cfficiency is to be realized. Tn Q of figure 24, the grid impedanee very closely approximates the plate impedance and this connection is used when no transforma- tion is required. The grid and plate imped- anees ave not generally known in many prac- tical cases; henee, the adjustments are made on the basis of maximum grid drive consistent with maximum safe inpuí to thc driver stage. Inductive Coupling—The Radio-Frequency Transformer. Inductive conpling is often uscd when tivo cirenits ave to be coupled, This method of conpling is shown in figures 254 and 25B. The two inductances are placed in such in- duetive relation to each other that the Tmes of forve from the primary coil cut across the turns of the secondary coil, thereby inducing a voltage in the secondary. As in the case of eapacitive conpling, impedance transforma- tion here again becomes of importance. Jf two parallel tuncd cireuits are coupled very closely together, the circuits can in reality ho Handbook AMPLITUDE FREQUENCY Figure 26. Efect of coupling between circuits upon the resonance curve, Curve À indicates the curve wfien the circuits are under-coupled, B is the curve re- sulting from over-coupling, and C is the curve resulting from an intermediate value of coupling. Although the output amplitude would not be the same in all three cases, the curves have been drawn to the same maximum to illustrate more cleariy their relative shapes. overconpled. This is illustrated by the enrve in figure 26. The dotted line curve A is the original enrve or that of the primary coil alone. Curve B shows what takes place when two eireuils ave overcoupled; the resonance curve will have a definite dip on the peak, or a double hump. This prineiple of overcou- pling is advantageonsly utilized in bandpass cirenits where, as sbown in O, the conpling is adjusted to such a value as to reduee the peak of the curve to à virtual fat top, with no dip in the center as in B. Some undesirable capacitive coupling will result when cireuits are closely or tightly coupled; if this capacitive conpling is appre- ciahle, the taning of the cireuits will be af- feeted. The amount of capacitive coupling can be reduced by so arranging the physical shape of the induetances as to enable only à minimum surface of one to be presented to the other. Another method of accomplishing the same purpose is by elcetrical means. À curtain of closely-spaced parallel wires or bars, con- neeted together only at one cnd, and with this end connected to ground, will allow electro- magnetic coupling but not cloctrástatic cou- pling. Such a device is called a Faraday sereen or shield. Link Coupling. Still another method of deercasing capaeitivo coupling is by means of a coupling link circuit between two parallel zesonant cireuits, The capacity of the cou- pling link, with its onc or two turns, is so small as to be negligible, Also, one side of the link is oíten grounded to reduco further any eapacitive coupling that may cxist. Link conpling is widely uscd in transmittor eireuits because it adapís itself so univorsally Circuit Coupling 47 LINK COUPLING antem 1H — Hr TAP ON AND LINK COUPLING prenono 4 [EM a ul UNITY COUPLING Figure 27, Two types of link (inductive) coupling and (C) unity coupling. and eliminates the need of a radio-frequency choke, thereby reducing a sourec of loss. Link coupling is very simple; it is dia- grammed in À and D of figure 27. Im À of figure 27, there is an impedance step-down from the primary coil to the link eireuit. This means that the line which con- neets the two links or loops vil have a low impedanee and therefore can he carried over a considerable distance without introduction o£ appreciable loss. A similar link or loop is af the ontput end of the line; this loop is conpled to the grid tank of the driven stage. Still another link conpling method is shown in B of figure 27. Tt is similar to that of A, with the exception that the primary line is tapped om the coil, rather than heing ter- minated in a link or loop. Unity Coupling, Another commonly used type of coupling is that known as mnity com pling, by reason of the fact that the turns ratio between primary and secondary is one-to-one, This method of «oupling is illustrated in G of figure 27. Only one of the windings is tuned although the interwinding of the two coils gives an effect in the untuncd winding as though it were actually tuned with a con- denser. Unity conpling is used in some types of ultra-high-frequency cirenits although the mechanical considerations are somewlat diff cult. The secondary, when it serves as the 48 Fundamental Radio and Electrical Theory grid coil, is placed inside of a copper tubing coil; the latter serves as the primary or plate coil, Conduction of an Electric Current So fax this chapter has dealt only with the conduetion of current by a stream of electrons through a conductor or by cleetrostatic cou- pling through a capacitor. While this is the most common method of transmission, there are other types of conduelion which are equally important in Lheir respective branches of the field. An electric enrrent may also be transmitted by Lhe motion of minute particles of matter, by lhe motion of charged atoms called ioxs, and by a stream of eleetrons in a vacuum. The estrring OL current br, ehargod par. ticles, such as bits of dust, is only of academic interest in radio. However, there is a com- mercial process (called the Cottrell process) which mães this type of conduetion in indus- trial dust precipiation. A highly charged wire inside a grounded metal chamber is placed so that the dust-laden fue gases from certain industrial processes (usually metal- lurgie zefining) must pass through the cham- ber. The dust partieles are first altracted to the wire; there they attain a high elecric charge which causes them to be ailraeted to the sides of the chamber where thoy arc pre- cipitated and subseguently collected. A small electric current between the center elecirode and the chamber is the result of the carrying of the charges by the dust partieles. Conduction by Ions. When a high enough voltage is placed between two terminals in air or any other gas, that gas will break down suddenly, the resistanco betwcen the two points will drop from an extremely high valne to à few hundredths or thonsandihs of ohms, and a comparatively large electric current ill dow to the accompaniment of an amount of visible light either as a flash, an are, à spark, or a colored discharge such as is found in the “neon” sign. This type of conduetion is due to gas ions which arc generated when the electric stress betweeu Lhe to points be- comes so great that eleelrons are tora from the molecules of the gas with thc production of quantily of posilively charged gas jons an negative electrons. The breskdown volt- age for a particular gas is dependent upon the pressure, the spacing of the clectrodes, and the type of electrodes. Lightning, tank condenser fashovers, and ignition sparks iv an automobile are such discharges that occur at atmospherie pressure or above. However, the pressure of the gas is usually reduced to facilitate tho ease of breale- The RADIO down of the gas as in the “neon” sign, merenry-vapor lamp, or voltage regulator tubes such as the VR-150-30. If a heated fila- ment is used as one electrode in Lhe discharge chamber, the breakdoyn voltage is further reduced to a valuc called the ionization po- tential of lhe gas, This principle is used in the 866, the 83, and other merenry-vapor reetifiers. Through the use of the heated cathode the break-down potential is redueed from about 10,000 volts to approximately 15 volts and the conduetion of clegtrie current is made unidireetional, enabling the discharge chamber Lo be nsed as a reetifier. 'Phe appli- calions of the principle of ionic conduelion in vacuam tubes (along with discussion of eleetronie conduction) will be covered in more detail in the chapter devoted to Facuum Tube Theory. The cmission of colored light which ae companies an electric discharge lhrongh a gas is due to the re-combination of the iunized gas molecules and the free electrons to form neu- tral gas molecules. There is a definite color speelrum which is chatacteristie of every gas and for that maiter for every element when it is in the gaseous state. For neon this color is orange-red, for mereury il is blne-violet, for sodium, almost pure yellow throngh the list of the clements. “This prin- ciple is used in the spectroscopic idenlifica- tion of elements by their characteristio lines in the spectrum (enlled Wraunhõfer lines). Electrolytic Conduction. Nearly all in- orgenie chemical compounds (and a few or- ganic ones of certain molecular sirmeture) when dissolved in water undergo a chemical- electrical change known as electrolytio dis- association which results im the production of ions similar in certain properties to those formed as a result of the electric breakdown of a gas. For example, when sodium chlor- ide or table salt is dissolved in water x cer- tain percentage of if ionizes or breaks down into positively churged sodium ions, or sodium atoms wilh a deficiency of one electron, and negatively charged chloride ions, or chlorine atoms »wilh one excess electron. Similarly, sodium hydroxide disassociates into positive sodium ions and negative hydroxy] ions—sul- furie acid into positive hydrogen ions and negative sulfate jon: his solution of an ionized compound and waler renders the aqueous solution a con- duetor of electricity. (Water in the pure form is a good insulator.) The condnetivity of the solution is proportional to the mobility of the ions and to the quantity of them ayail. able in the solution. Maximum conduetivity is had not when there is a maximum of the Types of Emitters 51 the pure metal. Subsequent improvements have resulted in the highly eficient earbu- vized thoriated-tungsten filament as used in virtually all medium-power transmitting tubes in use today. Thoriated-tungsten emitters consist 0£ a tungsten wire containing about one per cent thoria. The new lilament is first carburized by heating it to a high temperature in am at- mosphere containing a hydrocurbon at reduced pressure. Then the onvelope is highly evacu- ated and the filament is ashed for a minute or two at about 2600º K before being burned at 2200º K for a longer period of time. The Hashing causes some of the thoria to he Te- duccd by the carbon to metullie thorium. The attivating at a lower temperature allows the thorium to diffuse to the surface of the gire to form a layer of the metal a moleente thick. Te is this single-molecule layer of thorium which reduces the work function of the tung- sten filiment to such a valne that the elee- trons will be emitted from a thoriated filament thousands of times more rapidly than from a pure tungsten flament operated at the same temperature. The carburization of the tungsten surface seems to form a layer of tungsten carbide which holds the thorium layer much more firaly lhan the plain tungsten surface. This allows the filament to he operated af higher tcmperatrre, with consequent greater cm ion, for the same amount of thorium evapo- ration. Phoriun evaporation from the sur face is à natural consequence of the operation of the thoriated-tungsten flament. The car- bnrized layer on the tungsten wire plays an- other role in acting às à reducing agent to produce new thorium from the thoris to re- Place that lost by evaporation, This new thorium continually diffuses to the surface duzing the normal operation of the flament. One thing to remember abont any type of filament, particulariy the thoriated type, is thaf the emitter deteriorates praetically as fast whén “standing by” (no plate current) as it does with any normal amont of emis- sion load, However, a thoriated Alament may be cither temporarily or permanently dam- aged by a heavy overload which may strip the surface emilting layer 0£ thorium from the filament., Beactivating Thoriated-Tungsten Fila. ments. Thoriated-tungsten flaments (and onty thoriated-tungslen filaments) which have gone “Hat” as a result of insuílicient filament voltage, à severe temporary over- load, a less severe extended overload, or even normal operation may quite frequently he reactivated to their original characteristics by a process similar to that of the original activation. However, only flaments which have been made by a reputable manufacturer and which have not approached too close fo the end of their useful life may be suecess- fully reactivated. The filament found in certain makes of tubes may often be re- activated threc or four times before the fila- ment will «case to operate as a thorinted emitter. The actual process of reactivation is simple enongh and only requires a filament trans- former with taps allowing voltage up to about 25 volts o The tube which has gone flat is placed in a socket to which only the two filament wires have been connected. The fila- ment is then “Aushed” for about 20 to 40 seconds at from 1% to 2 times normal rated voltage. The filament will become extremely brigbt during this time and, if there is still some thoria left in Lhe tungsten and i£ the tube didn't originaliy fail as a result of an air leak, some of this thoria will be reduced to metalico thorium. The filament is then burned aí 15 to 25 per cent overvoliage for from 30 minutes to three to four hours to bring this new thórium to Lhe surface. The tube should then be tested to sec if it shows signs of rencwed life. TE it does, but is still weak, fhc burning process should be continned at abont 10 to 15 per cent overvolt- age for a few more hours. This should bring it back almost to normal, TE the tube checked still very low after the first attempt at reac- tivation the complete process can be repezted as à last effort. Thoriated-tungsten filameuts are operated at about 1900º K or at a bright yellow A burnout at normal filament voltage most am unheard of occurrence, The ratings placed upon tubes by the mannfncturers are figured for a life expectaney of 1000 hours. Certain types of tubes may give much longer Bife than this but the average transmitting tube will give from 1000 to 5000 honrs of nse- ful life. The Oxide-Coated Filament The most eficient of all modem filaments is the oxide-coated type which consists of a mixture of barium and strontium oxides coated npou à wire or strip usually consist- ing of a nickel alloy. This type of filament operates at a dull-red to otange-red tempera- ture (1050º to 1170º K) at which tempera turo it will emit large quantities of electrons, The oxide-coated filament is somewhat more efficient than the thoriated-fungsten type in small sizes and it is considerably less ex- pensive to mannfacture. For this reason all  52 Theory and Operation of Vacuum Tubes reeciving tubes and quite a number of the low-powered transmitting tubes use the oxide-conted filament. Another advantage of the oxide-conted emitter is its extremely Jong life—the average tube can be expected to run from 3000 to 5000 hours, and when loaded very lightly tubes of this type have been known to give 50,000 hours of life before their characteristies changed to any great ex- tent. The oxide-conted filament does have the disadvantage, however, that it is unsuitable for use in tubes which must withstand more than about 600 volts of plate potential. Some years back transmitting tubes for operation up to 2000 volts were made with oxide-coated filaments but they have beem discontinued. Much more satisfactory operation is obtain- able at medimn plate potentials with thoriated filamenta. Oxide filaments are unsatisfactory for use at high plate voltages because (1) their ne- tivity is sexionsly impaized by the high tem- perature necessary to bombard the high- voltage tubes and, (2) the positive ion hom- hardment which takes place even in the best evacuated high-voltage tule causes destrue- tion of the oxide layer on the surface of the Bilament, Oxide-coated filaments operate by virtue of à, mono-moleeular layer of alkaline-carth metal (barium and strontium) which forms on the surface of the oxide coating. Such filaments do not require reactivation since there is always sufficient reduction of the oxides and diffusion of the mctals to the sur- of the filament to morc than meet the emission needs of the cathode, Indirectly Heated Filaments— The Heater Cathode The hester type enthode was developed as a result of the requirement for a type of emitter which could be operated ftom alter. naling current and yet would not introduce a.e. ripple modulation even vhen used in low- level stages, Tt consists essential!y of a small mickel-alloy eylinder with a conting of slrontium and barium oxides on its surface similar to that used on tho oxide-conted fila- ment. Inside the eylinder is an insulated heater element eonsisting usually of a double spiral of tungsten wire. The heater may op- erate on any voltage from 2 to 117 volts although 6.3 is by far the most common valve. The heater is operated at quite a high tem- perature so that the cathode itself may be brought to operating temperature in a matter of 15 to 30 seconds. Heat coupling betyecn the heater and the cathode is mainly by radia- The RADIO tion, although there is some thermal condue- tion through the insulating coating on the henter wire, as this coating is also in contact with the esthode thimble. Indirectly heated cathodes are employed im all a.c. operated tubes which are designed to operate at a low level either for r.Ê. or af. use However, some receiver poyer tubes use honter cathodes (616, 6V6, GW6, and 6846) as do some of the low-power trane- mitter tubes (802, 807, T21, and RK39). Heater cathodes are employed exelusively when a number of tubes are to be operated in series as in an a.e-de. receiver. A heater often called a uni-potential cathode because there is no voltage drop along its length as there is in the flamenttype cathode. Types of Vacuum Tubes If a enthode capable of being heated either indirectly or directly is placed in an cvacuated envelope along with a plate, such a two-element vacuum tube is called a diode, The diode is the simplest of all vacuum tubes and is the fundamental type from which all the others are derived; hence, the diode and its characteristies will be discussed first. Characterístics of the Diode. When the cathode within a diode is heated, it will be found that a few of the elecitons leaving the cathode will leave with sufficient velocity to reach the plate HE the plate is electrically connected back to the cnthode, the electrons which have bad sufficient velocity to arrive at the plate will fow back to the cathode througi (he external circuit. This small amount of initial plate current is an effect found in all two-element vacuum tubes. Ef a battery or other source of d.e. voltage is placed in the external circuit between the plate and cathode so that the battery voltage places a positive potential on the plate, the flow of current from the enthode to plate will be increased. This is de to the strong at traetion offered by the positively churged plate for any negatively charged particles. Ff the positive polential on the plate is in- ercased, thc flow of electrons between the cathode and plate will also increase up to the point of seturation. Saturation current flows when all of the clectrons leaving the cathodo are attracted to the plate, and no inereuse in plate voltage con increase the number of eleetrons being attracted. The Space Charge Effect. As a cathode is heated so that it begins to emit, those clee- trons which have been discharged into the surrounding space form in lhe imnediate Handbook Types of Vacuum Tubes 53 [saTuRaT ion NUMBER OF FLECTRONS REACHING PLATE — PLATE VOLTAGE T0* Figure 1, CURVE SHOWING NUMBER OF ELEC- TRONS REACHING THE PLATE OF À DIODE PLOTTED AS A FUNCTION OF THE PLATE VOLTAGE. h will be noticed that there is a small fow of plate current even with zero voltage. This initial flow can be stopped by a small negative plate potential. As the plate voltage is increased in a positive direction, the plate current increases approximately as the 3/2 power of the plate volt- age until the saturation point is reached. At this point all the elecirons. being emitted from the cathode are being attracted to the anode. vicinity of the cathode a negative charge which acts to repel those elcetrons which normally would be emitted wcre the charge not present. This cloud of clectrons around the cathode is called the space charge. The cleeteons comprising the charge are contin- uously changing, since those elcetrons mak- ing up the original charge fall hack into the guthode and are replaced by others emitted py it. The effeel of the space charge is to make the enrrent through the tube variable with respect to the plate-to-cathode drop acr it. As the plate voltage is increased, the p itive charge of the plate tends to nentralize the negative space charge in the vicinity of the cathode. This neutralizing action upon the space charge by tho increased plate volt- age allows a greater number of eleetrons to be emitted from the cathode wbich, obvionsiy, cunses à greater plate current to flow. Then the point is reached at which the spaco charge around the eathode is neutralized completely, all the electrons that the cathode is capable of emitting avc being attraeted to the plate and the tube is said to have reached saturation plate current as mentinned abovo. Insertion of a Grid—The Triode. If an element consisting of a mesh or spiral of wire is inserted coneentrie with the plate and be- twecn the plate and thc cathode, such an element will have an effect on the cathode-to- platc current of the tube. The new clement is eommonly called a grid, and a vacuum O Aer Figure 2. ILLUSTRATING THE SPACE CHARGE EFFECT IN A DIÓDE, (A) shows the space charge existing in the vicinity of the cathode with zero or a small amount of plate voltage. A few high-velocity electrons will reach the plate to give a small plate current even with no plate voltage, (B) shows how the space charge is neutralized and aíl the electrons emitted by the cathode are attracted to the plate with a battery suficient to cause saturation plate current. tube containing a cathode, grid, and plate is commonly called a three-element tube or, more simply, a triode, IÉ this new element through which the elec- trnns must pass in their course from cathode to plate, is made negative wilh respect to the filament, the charge on this grid will in effect aid the space charge surrounding the cathode and hence will reduec the plate current of the tube. As a matter of fact, if the charge om this grid is made sufficiently negative the spaee charge will be increased to such an ex- tent that all the eleetrons leaving the cathode ill be repelled haek to it and the plate cur- rent will be reduced to zero. Any de, volt age placed upon à grid (especially so when speaking of à control grid) is called & bias. Hence, the smallest negative voltage hich causes cutoft of plate current is called the value of eutof? bias. Figure 3 illustrates the menner in which the plate eurent of a typical triode will vary with different values of grid bias, This show; graphically the cutoft point, the approri- mately linear relation between grid hias and plate current over the operating range of lhe tube, and the point of plate current satura- tion. However, the point of plate current saturatinn comes at a different position with a triode as compared to a diode. Plate current non-lincarity or saturation may begin either at the point where the full emission capa- bilities of the flament have been reached or at the point where the positive grid voltage begins tn approach the positive plate voltage. This latter point is commonly referred to as the diode vena and is caused by the posi- tive voltage of lhe grid allowing it to rob from the current stream cleetrons that would normaliy go to the plate. When the plate 56 Theory and Operation of Vacuum Tubes much above 20 Mc. because of difficulties en- countered in the oscillator section. Special Purpose Mixer Tubes, Notable among the special purpose multiple grid tubes is the 6L7 heptode, used principally as a mixer in superheterodyne cireuits. This tube has fixe grids: control grid, screens, sup- pressor and special injection grid for oseil. lator input. Oscillator coupling to control grid and sereen grid circuits of ordinary pen- todes is effective as far as mixing is con- cerned, but has the disudvantage of consid- erable interaction between oscillator and mixer, The 617 has a special injection grid so placed that it has reasonahic cffect on the electron stream without the disadvantage of intoraetion between lhe sereen and control grid. Tho principal disadvantage is that ib requires fuiriy high ascillator input in order to realize its high conversion conductance, Tt may also he nsed as an r.£. pentode amplifier. The 6J8G and 6K$ are two tuhes spe- cifically designed for converter service. They consist of a heptode mixer unit and a triode unit in the same envelope, internally con- nected to provide the proper injection for conversion work. While both tubes function as a triode oscillator fcedinys a beplode mixer, the meihod of injcetion is different in the two tubes. In the 6486, the control grid of the oscillator is connected intermally to a special shielded injector grid in the heptode section. Tn the 6KS, the number one grid of the hep- tode is connected internally to the control grid of the oseillator triode. Single-Ended Tubes. Wrom the introduc- tion of the sereen-grrid tube to the present time it has heen standard practico to bring the control grid (or the no. 1 grid as it is called) of all pentodes and tetrodes designed for radio frequency amplificr use in receiver through the tup of the envelope, This prac- tice was started because it was much casier to shield the input from the output circuit when one was at the top and the other at the bottom of the envelope. This was trnc both of the clements and of their associated cir- cuits, With the introduction of the oetal-bascd metal tube it became fensible to design and manufacture highgain v£. amplificr and mixer tubes with all the terminais brought out the base. The metal envelope gives ex- cellent shiciding of the elements from ex ternsd fields, and through the usc of a small additional shicld inside the locating pin of the oetal socket, the diametrically opposite grid and plote pins of the tuhes are well shielded from each other, The 6847 and The RADIO 6SK7 are conventional rf. amplifer pen- todes exemplifying this type of design, the 1852 (6AC7) and 1853 (6AB7) are televi- sion amplilier pentodes, the 68A7 is a new, greatly improved pentagrid converter tube, thc 6807 is a diode-high-p triodo and the 68C7 is a dual triode, Dual Tubes. Some of the commonly known yacuum tubes are in reality tvo tubes in one, Le, in à single glass or metal envelope. Twin triodes, such as the types 53, 646, 68CT, and 6NT are examples. À disadvantago of these twin-triode tubes for certain applica- tions is the fact that the cathodes of both tubes are brought out to the same base pin. Of a different nature arc the GHS twin diode and the 6P8G and 6C8G twin triodes, The cathodes of cach of these tubes are brought to à separate base pin on the socket, thns making Lhem true twin triodes. Other types combine the funetions of double diode and cither Jow or high yu triode in the same en- velope, as well as a similar combination with a pentode instead of a triode. Slill other types combino a pentode and a triode, à pen- tode and a power supply rectifier, and clectron-ray indicating tubes (magic eyes) with their self-contained triode d.e, voltage amplifier, Manufacturer's Tube Manual. The Iarger tuhe manufacturers offer at a nominal cost tube manuais which are very complete and give much valuable data which, because of space limitations cannot be included in this handbook. Those especially interested in vacumm tubes are urged to purchase one of these books as a supplementary reference, APPLICATION OF THE VACUUM TUBE The preceding section of this chapter has heen devoted to the theory of vacuum tubes and to the various forms in which they com- monly appear. The sucecedivg section wil) be devoted to the application of the char- aeteristies and abilities of the vacuum tuhe to the problems of amplification, oscillation, rectification, detection, frequeney conver- sion, and electrical measurements. The Vacuum Tube as an Amplifier The ability of a grid of a vacuum tube to control large amoumts of plate power with a small amount of input energy allows the vacuum tuhe to be used as an amplifer. Tt is the ability of the vacuum tube to amplify an extremely small amount of energy up to Handbook Application of the Vacuum Tube 57 almost any amount without change in any- thing except amplitude which makes the vacuum tube such an extremely uscful ad- jumet to modern industry and communica- tion. The most important considerations of a vacunm tube, asido from its power handling ability (which will be treated later on), are amplification fuctor, plate resistance, and imutual conduetance, Amplification Factor or Mu. The empli- fiction factor or mu (p) of a vacunm tube is the ratio of a change in plate voltage to a change in grid voltage, either of which will esuse the same change in plate current. Expresscd as a differential equation: dEp dEs The p can be determined cxperimentally by making a slight change in the plate voli- age, thus slightly changing the plate current. The plate current is then returned to its orig- inal value by a change in grid voltage. The ratio of the increment in plate voltage to the increment in grid voltage is the u of the tube, The foregoing assumes that the experiment is conducted on the basis of rated voltages as shown in the mannfacturer's tube tables. The plate resistance can also be determined by the previous experiment. By. noting the change in plate current as it occurs yhen the plate voltage is changed, and by dividing the latter by the former, the plate resistance can then be determined. Expressed as an equa- tion: dp Ro = dlp The mutual conduetanee, also referred to as transconductance, is the ratio of the am- plifieation factor (p) to the plate resistance: Ep wu dEs dp m=— = Ro dE, dE, dIp The amplification factor is the ability of the tube to amplify or increase the volfages applicd to the grid. The amount of voltage amplificntion Lhat can be obtained from tube is expressed as follows: Br, Ro + By where Rx is equal to the plate eireuit load resistance, As a practical example, supposc wc take the case of a 65 tuhe with a plate resistance ot 66,000 obms and an amplifeation factor of 100 operating into a load resistance of 50,- 000 ohms. The voltage amplification of the stage as caleulated from the above equation would be: 100 X 50,000 >>> =8 50,000 + 66,000 From the foregoing it is sem that an input of 1 volt to the grid ot the tube will give am output of 43 volts (a.c.). Audio-Frequency Amplifiers Amplifiers designed to operate at a low lcvel at radio, intermediate, and audio fre- quencies arc almost invariably of the class À type. Higher level audio amplifiers can be of the class A, class AB, or class B type; these elassifications and their considerations will be considered first. The class B and class G amplifers as used for medium and high- level radio-trequency work will be considered under Rudio-Frequency Amglifiers. The Class A Amplifer. À class 4 ampli- fier is, hy definition, an amplifier im tolich the grid bias and clternating grid voltages are such that plate current in a specific tube fous at al timos. The ontput waveform from a class A amplifier às a faithful repro- duetion of thc exciting a.e. voltage upon the grid. For the above conditions to be the case it is necessary that the grid bias, or the ope ating point, of the amplifier be chosen with care to allow maximum output with minimum distortion. Figure 5 shows the operating character- istic of a typical vacuum tube Té will be noticed that thc curve cê plate current with varying grid voltage is quite lincar within certain limits—outside these limits it is no longer a straight line, For an amplifer to be able to put out a voltage wavcform which is a faithfu] reproduction of the input wave- form, if is necessary that the range over which the grid voltage will be varicd shall give a linear variation im plate current. Also, a class A amplifier must not draw grid cor zent; so the operating point must be midway between the point of zero grid bias and the point on the operating characteristic where the curvature becomes noticeable, Such a point has been chosen graphically in figure 5. When the grid bias is varicd around this operating point the Aluctuation in grid po- 58 Theory and Operation of Vacuum Tubes The RADIO tential zesults in a corresponding fuctuation im plate current. When this current flows throngh a suitable load device, it produces a varying voltage drop which is a replica of the original input voltage, although considerably grenter in amplitude. Should the signal voltage on the grid be permitted to go too far negative, the negative half eycle in the plate output will not be the same as the positive half eyele, In other words, the output wave shape will not be a duplicate of the input, and distortion in the output will ibercfore result. The fundp- mental property of class A amplifieation is that the bias voltage and input signal level must not advance heyond the point of zero grid potential; otherwise, the grid itsclf will become positive. Electrons will then flow into the grid and through its external eirenit in much fhe same mauner as if the grid were actually the plate. The result of such a flow of grid current is a lowering of the input impedance of the tube so that power is re- quired to drive it. Since class À amplifiers arc never designed to draw grid current they do not realize the optimum capabilities of any individual tube. Iuspection of the operating characteristic of fignrc à reveals that there is a long streich of linear characteristio far into the positive grid region. As only the small portion of the operating characteristic below the zero grid bias line can be used, the plate circuit clficicney of a class À amplifer is low. How- ever, they arc used because they have very little or negligible distortion and, sinec only Figure 5. te OPERATING POINT 1 Pu oureur core da SIGNAL, Pd -——- No POINT AT WHICH GRIO CURRENT MATE PLS ! t 1 q inpuT SGNAL, CLASS A OPERATION an infinitesimal amount of power is required on the grid, a large amount of power ampli: fication may be obtained. Low-level audio and radio frequency amplifying stages in receivers and audio amplifiers are invariably operated class À. The correct values of bias for the operation of tubes as class A ampli- fiers are given in the Tube Tables. The Class AB Amplifer, A class AB amplifer is one in which the grid bias and alternating grid voltages are such that plate current im à specifo tube flows for appreci- ably more than half but less than the entire electrical eyele ahen delivering maztmum output, Tn a class AB amplifier, the fixed grid bias is made higher than would be the case for a push-pull class A amplifier, The resting plate eurrent is thereby reduced and higher values of plate voltage can be used withont exceeding the rated plate dissipation of the tube. The result is an increase in power output. Class AB amplifiers can be subdivided into class AB, and elass AB, There is no fow of grid current in a class AB, amplifier; that is, the peak signal voltage applied to each grid does not execed the negative grid hias voltage. in a class ABa amplifier the grid signal is greater than the bias voltage on the pesks, and grid current flows. The class AB amplificr should be operated in push-pull if distortion is to be held to « min- imum. Class AB, will furnish more power output for a given pair of tubes than will class AB;. The grids of a class AB, ampli- fier draw current, which calls for a power driver stage. The Class B Amplifier, A class B ampli fier is one in which the grid bias is approx- imately equal to the cutof value so thal the plate current is very low (almost zero) awhen no exciting grid vollage is applica and so that plate current in a specific tube flows for approximately one half of each cycle when am alternating grid voltage is applied. A elass B audio amplificr always operates with tiwo tubes in push-pull. The bias volt- age is increased to the point where but very hittle plate eurrent flows. This point is called the eutof' point, When the grids are fed with voltage 180 degrees ont of phase, that is, one grid swinging in a positive direetion and the other in a negative direction, the two tubes will alternately supply current to the load. Wen the grid of tube no. 1 swings in a positive direction, plate current fows in this tube. During this process, grid no. 2 swings negatively beyond the point of cutoff; hence, no current flows in tube no. 2. On the other Handbook Radio-Frequency Amplifiers 61 shown in E; this arrangement is generally chosen for high permenbility audio trans- formers of small size and where if is neces- sary to prevent the plate current from flowing through the transformer primary. The plate circuit in the latter is shunt-fod. A resistor of appropriate value is often sub- stitnted for the impedance in the eireuit shown in E, RADIO-FREQUENCY AMPLIFIERS Radio-frequeney amplifiers as used in transmitters invariably fall into the “power” classification. Also, since they operate into sharply tuned tank eireuits which tend to take out irregularities in lhe plate current waveform and give a comparatively pure sinesrave output, more efficient conditions of operation may be used than for an audio amplifier in which the output wavelorm must he the same as the input over a wide hand of frequencics. [he class IB and class € r.£, amplificrs fall into this grouping. The Class B R.F. Amplifier. The defini- tion of a class B r.£. amphfier is the same as that of a class B amplifer for audio use. However, the r.f. amplifer operates into à tuned circuit and covers only a very small range of frequency while the audio type works into an untuned load and may cover 8 range of 500 or 1000 to 1 in frequeney. Class B radio-frequeney amplifiers are used primarily as linear amplifiers whose function is to increase the output [rom à modulated class C stage. The bias is adjusted to the culoff value. In a single-ended stage, the rf. plate current flows on alternate half eyeles. The power output in class E r.L, amplifi proportional to the square of the grid cxci tion voltage. The grid voltage excitation is doubled in a linear amplifier at 100% modu- lation, the grid exeitation voltage being sup- plied by the modulated stage; hence, Lhe power output on modulation peaks in a linear stage is increased four times im value. In spite of the fact that power is supplied to the tank cireuit only on alternate half eyeles, the fiywheel efiver of the tuncd tank eireuit sup- phies the missing half eyele of radio fre- queney, and the complete waveform is repro- duced in the output to the antenna, The Class O R.P. Amplifer. A class € amphificr is defined as an amplifier in which the grid bias is appreciably greater tham the cutof? value so that the plate current in each tuhe is zero when no altemating grid voltage is applied, and so that plate current in a specific tube flows for appreciably less than one half of ench eyele when an alternating grid voltage is applied. oureur SEMAL É mcuseme á ã PoITAT wmGH caio, CURRENT STARS TO FLOW CLASS € OPERATION Figure 8. Angle of Plate Current Flow, The class C amplificr differs from others in that the bias voltage is inercased to a point well beyond eutoff. When a tuhe is hiascd to cutolf, as in a class B amplifier, it draws plate current for a half eyele or 180º. As this point of opera- tion is carried beyond eutoff, that is, when the grid bias becomes more negative, the angle of plate current flow deereases. Under normal conditions, the optimum value for class € amplifier operation is approximately 120º. The plate current is at zero value during the first 30º because the grid voltage is still ap- proaching cutof. From 30º to 90º, the grid voltage has advanced beyond cutofl and swings to a maximum in a region which al- lows plate emrrent to flow. From 90º to 150º, the grid voltage retums to cutoft, and the plate current decreases to zero, From 150º to 180º, no plate current flows since the grid voltage is then beyond eutofr. The plate curtent in a class C amplifer flows in pulses of high amplitude, but of short duration. Effciencies up to 75% are realized under these conditions. Tt is possible to convert nearly all of the plate inpul power into rf, output power (approximately 90% efficicney) by increasing the excitation, plale voltage and hias to extreme values, Linearity of Class C Amplifiers. The rf. plate current is proportional to the plate volt- age; hence, the power output is proportional to the square of the plate voltage. Class € amplifiers are invariably used for plate modu- lation hecause of their high cfficicney and be- cause they reflect a pure resistance load into the modulator, The plate voltage of the class € stage is douhled on the peaks at 100% modulation; the power output af this point is consequentIy increased four times, Figure É illustrates graphically the op- eralion of a class C amplifier with twice cut- off bias and with the peak grid swing of such ag 62 Theory and Operation of Vacuum Tubes The RADIO a value as just to approach the diode bend im the plate charaeteristic. When the excitation voltage is inercased beyond this point the plate cmrent waveform will have a dip at the crest due to the taking of electrons from the plate current stream by the grid on its highly positive penks. The Vacuum Tube as an Oscillator The ability of an amplifier tube to control power cnables it to funetion as an oscillator or à generator of alternating current in a suitable circuit. When part of the amplified output is coupled back into the input circuit, sustained oscillations will he generated pro- vided the input voltage to the grid is of the proper magnitude and phase with respect to the plate. The voltage that is fed back and applied to the grid must be 180º out of phase with the voltage across the load impedance in the plate circuit. The voltage swings arc of a frequency depending upon cirenit constants, TÉ a parallel resonant eireuit consisting oÉ an inductanco and capacitance is inserted in series with the plate circuit of an amplifer tube and a connection is made so that part of the potential drop is impressed 180º out of phase on the grid of the same tube, amplifica- tion of the potential across the L/C circuit =vill result. “?he potential would increase to an unrestricted value were it not for the limited plate voltage and the limited range of lincarity of the tube characteristio, which causes a reversal of the process after a certain point is reached. The rate of reversal is de- termincd by the time constant or resonant frequency of the tank circuit. The frequency range of an oscillator can be made very great; thus, by varying the cir- cuit constants, oscillations from a few evelos per second up to many millions can be gener- ated. A number of different types of oscilla- tors are treated in detail under the section de- voted to Transmitter Theory. The Vacuum Tube as a Rectifier It was stated at tho first of this chapter that shen the potential of the plate of a two- clement vacunm tube or diode is made posi- tive with respect to the cathode, cleetrons emitted by the cathode will be attracted to the plate and a current will fow in the ex- ternal circuit tha returns the electrons to the euthode. I£, on the other band, the plate is made negative with respect to the cathode the electron fow in the external circuit will ecase due to the repulsion of the electronic stream AMPLITUDE E HALF=WAVE RECTIFICATION TUBEA O TUBEA TusEs | rue [tum | E = FULLOWAVE RECTIFICATION AMPLITUDE Figure 9. within thc tube back to the exthode, From thisis derived a valuable property, namcly, the ability of a vacuum tube to pass current in one direction only and hence to funetion as a rectifier or a device to convert alternatinss eur- rent into pulsating d.e. The Half-Wave Rectifer. Figure 9A shows a half-wave rectifier circuit. For con- venicnee of explanation, a conventional power rectificr has been chosen although the same diagram and explanation wonld apply to diode rectificution as employed in the detector eir- enits of many receivers. When a sine-wave voltage is induced in the secondary of the transformer, the reetifer plate is made alternately positive and nega- tive as the polarity of thc alternating emrent changes. Electrons are attraeted to the plate from'the cathode when the plate is positive, and current then flows in the external eireuit. On the suececding half eyele, the plate be- comes negative with respect to the euthode, and no current fows, Thus, there will be an interval before the suceceding half eyele occurs xhen the plate again becomes positive. Under these conditions, plate current once more be- gins to flow and there is another pulsation in the output circuit. For the rcason that one half of the com- plete waye is absent in the output, the result às hat is known as half-wave rectification. The output power is the average value of these pulsations; it will, therefore, be of a low value hecansc of the interval between pulsa- tions. Full-Wave Rectification. In a fullazavo circuit (figure 9B), the plate of one tube is positive when the other plate is negative; al- though the current changes its polarity, one of the plates is always positive. One tube, therefore, operates effcetively om cuch half Handbook Rectification-Detection 63 eyrore Bs am ae | la Ec €B) PLATE perecTION Figure 10. ILLUSTRATING DETECTOR OPERATION IN UPPER AND LOWER-BEND PORTION OF THE CHARACTERISTIC CURVE. cyele, but the ontput current is in the same direction. In this type of cireuit the rectifi tion is complete and therc is no gap between plate current pulsations. This output is known as rectificd q.c.0r pulsating d.e. Mercury Vapor Rectifiers, If a two- clement elcetron tube is evacuated and then filled with a gas such as mereury vapor, its charaeteristies and performance will differ radically from thosc of an ordinary high- vacumm diode tuhe. The principle upon which the operation of a gas-filled reetifier depends is known as the phenomenon of gascous ionization, which was discussed under Fundumental Theory. In- vestigation has shown that the electrons emitted by a hot cathode in a mercury-vapor tube ave aecelerated toward the anode (plate) with great velocity, These electrons move im the clectrical field betwcen the hot enthode and the ande. In this space they collide with the mercury-vapor moleeules which are present. TÉ the moving electrons attain a velocity so great as to cnable them to break through a potential difference of more than 10.4 volts (for mercury), they will literally knock the clectrons out of the atoms with which they colide. As morc and more atoms arc broken up hy eollision with electrons, the mercury vapor =vithim the tube becomes ionized and transmits a considerahlo amount of current. The ions are repelled from the snode when it is posi- tive; they are then attracted to the cathode, thus tending to nentralize the negative space charge as long as saturation current is not drawn. This effect neutralizes the negative spaee charge to such a degree that the voltage drop across the tube is reduced to a very low and constant valnc. Furthermore, a con- siderable reduetion in heating of the diode plate, as well as an improvement in the volt- age regulation of the load current, is achicved. The cfficicncy of rectification is thereby in- ereastd because the voltage drop across any rectificr tube represents a waste of power. Detection or Demodulation Detection is the process by which the audio component is separated from the modulated radio-frequency signal carrier at the receiver. Detection always involves either rectification or nonlinear amplifieation of an alternating current. Two general types of amplifying detectors arc nsed in radio eireuits: The Plate Detector. The plate detector or bias detector (sometimes called a power detector) amplifies the radio-frequeney wave and then reetifies it and passes the resultant audio signal component to the suceceding audio amplificr. “he deteetor operates on the lower hend in the plate current characteristic, because it is hiased closc to the eutolf point and therefore could be called a single-ended elass B amplifier. The plate current is quite low in the absence of a signal and the audio component is cvidenced by an incrense in the average unmodulated plate current. See figure 10. The Grid Detector. The grid detector dif- fers from the plate detector in that it rectifies in the grid cireuit and then amplifies the restiltant audio signal. The only source of grid bias is the grid leak so that the plate emexent is maximum when no signal is present. This form of detector operates on the upper CHAPTER FOUR Radio Receiver Theory A radio receiver may be defined as a de- vice for reproducing in the form of useful output the intelligence conveyed by radio waves applied to it. Usually an antenna is a necessary adjunct to the receiver. The an- tenna will not be discussed in this chapter, however, as the function and design of an- temas is thoroughly covered in Chapter 20. Receiver Tubes. The tube manufacturers have boon lavish in their produetion af tuhes for use in radio receivers. Many similar tubes are made in different forms, such as metal tubes, glass tubes with standard bases, glass tubes with octal bases similar to those used on metal tubes, glass tubes with tubular envelopes, glass tubes encased in metal shelis and fitted with octal bases and tubes with similar characteristics but diffcring in their heater or filament voltage and current ratings. Some tubes are designed for dry- battery filament supply, others for automo- bile scrvice and another group for operation from am 4.0, source. Tn general, there are certain distinct classes of tubes for particular purposes. Sereen- grid tubes were primarily designed for radio- froqueney amplífers, yet they are oftem employed for regenerative detectors, mixers and highgain voltage audio amplificrs. General purpose triode tubes are used as oscillators, detectors and audio amplifers. Power triodes, tetrodes and pentodes are em- ployed for obtaining as much power output as possible in the output audio amplifer stage of a radio receiver. Diodos are de- signed for use as power supply rectifiers, radio detectors, automatie volume control eir- euits and noise suppression eireuits. In addition to these general types of tubes, thore are a great many others designed for some particular service, such as oscillator- mixer operation in a superheterodyne re- eciver. Vacuum tubes require a source of power for the filament and other electrodes. Cer- tain components in a radio receivor are for 66 the purpose of supplying direct-current cnergy to the clectrodes of tie tubes, such as the plate and screcn cireuits. In nearly all eirenits, the control grid of the vacuum tube is biased negatively with respect to the cathode, for proper amplifer action. This bias is obtained in sevoral ways, such as from a solf-biasing resistor in series with the cathode, fixod bias from the power sup- ply or grid leak bias for somo oscillators and detectors. By-pass and coupling condensers are found in different portions of the circuits throngh- out a radio receivor. By-pass condensers provide a low impedance for r.£. or audio frequencies around such components as re- sistors and choke coils. Coupling condensers provide a means of connection between plate and grid circuits in which the de, voltage components are of widely different values. The coupling condenser offers an infinito im- pedance to the d.e, voltages, and a relatively low impedance to the rf. or af. voltages, Sercen-prid tubes have a higher plate im- pedanee than triodes and, therefore, require a much higher value of plate load impedance in order to obtain the grentest possible amount of amplification in the audio or radio circuits. Screen-grid tubes are normally used in all x.f. and if. amplifiers because the control grid is electrostatically serecned from the plate eireuit, Lack of this soreening would cause self-oscillation in the amplifer; xhen triodes are used in radio-frequency amplifers, the grid-to-plate espacities must be neutralized, The rf. amplification from a triode amplifer in a radio receiver is so much less than can be obtained from a screen- grid tube amplifier that triodes are no longer used for this purpose. Detection. AIl receivers use some sort of detector to make audible the intelligence im- pressed on the radinted carrier wave at the transmite. Tho process of impressing the intelligence on the carrier wave is Inown as modulation, and as the detector soparates this Detection 67 modulation from the carrier, itis often known as a demodulator. One of the simplest practical receívers consists of à tuned circuit for selecting the desired radio signal and a detector £or separating the modulation from the carrier. The detector may be either a mineral such as galena or carborundum, or else a vacunm tube. Figure 1 shows such a receiver using a diode vacuum tube as a detector. The sensitivity of this receiver, or in other words its ability to make audible weak signals, would be very low, but it is useful to illustrate the basic action of all receivers. Resonant Circuits such as are formed by eoil To and condenser C, are almost always used to couple the antenna to the first tube in & radio receiver, When the current in- duced in the antenna is cansed to pass through a coil, such as Ey in figure 1, a volt- age is induced aeross the coil Tt will bo recallek from chapter 1 that this voltage aeross the coil is equal to the produet of the current and the impedanee of the coil. The impedanec of a non-resonant coil such as E is made up principally of its reactance. This ronetance is a funetion of the coil dimensions and the frequency of the impressed enrrent. Coils Ly and Lo in figure 1 are said to be inductively coupled, as radio-frequeney energy is transferred from one to the other by virtue of the fact that the alternating in- ductive field around Ly links and unlinks grite the turns of Lo, thus inducing a voltage in Lo. Disregarding the tube, V, for the moment, the current flowing through Lo of figure 1 is limited by the reactances of the coil and condenser C. The reactance of the coil in- ercases With frequency while the reactance of the variable condenser decreases with fre- quency. For any setting of C there is a frequency at which the capacitive reactance and the inductive reaciance are equal. Theso two. reactances are opposite in effect and neutralize each other at this frequeney, re- sulting in a circuit having zero reactance, and a condition known as resonancs. At rosonance the current fowing hack and forth between E, and C is limited only by their resistances, and since the resistance of modern condensers is very small, the current. is actually limited by the resistance of the coil. The bigh radio frequency (rf.) cur- rent flowing through the coil and condenser causes an r.f, voltage to be developed aeross them equal to the prodnck of the eurrent and the impedance of the circuit. As the im- pedance of the parallel tuned eircuit at res- onance is high, the voltage across it is also mn dam Ev o o ANT. tl La emo Figure 1 DIODE DETECTOR RECEIVER. While it would make a poor receiver, this type of circuit is useful in illustrating how the detector separates the modulation from the carrier wave. high. Thus, it may be seen that at its resonant frequency the voltage aeross a tuned cireuit may be very much higher than what might bo expeeted from looking at the dia- gram and assuming that a simple trans- former action took place hetween the pri- mary and secondary. The voltage step-up in the tuned circuit is illustrated by the drawings representing the modulated carrier wave above the dif- feront portions of the receiver circuit in figure 1. “A” represents the radio signal as it is picked up at the antenna, while “B” represents the same wave considerably in- ereased in amplitude after it has passed through the tuned circuit. Rectification of the radio-frequency carrier takes place in the diode vacuum tube, V, and a pulsating d.e. voltage as illustrated at “C” is passed through the earphones. The pulsa- tions in this voltage correspond to the modu- lation voltage originally placed on the carricr wave at the transmitter. As the diaphragms in the esrphones vibrate back and forth following this pulsating d.e. volt- age they audibly reproduce the modulation on the carrier. Eegenerative Receivers The Triode Detector. The simple re- ceiver shown in figure 1 would be en ex- tremely poor one, being suitable for use only in the immediate vicinity of a transmitting station. The sensitivity of the receiver may de increased considerably by replacing the diode detector by a triode in a regenerative detector eircnit as shown in figure 2. The regenerative receiver has been quite popular in high-freguency work for many years. Tt combines high sensitivity, simplie- ity, Jow cost, good signal-to-noise ratio and 68 Radio Receiver Theory Fao Ca Figure 2. TRIODE REGENERATIVE DETECTOR. The regenerative detector makes the simplest practical high-frequency receiver. reliability. Tts principal disadvantage, how- ever, and the one which has caused it to as- sume a secondary role in the high-frequency receiver picture, is its Iack of selectivity when subjected to large signal inputs. Operation. The regenerative detector, diagrammed in figure 2, operates as follows: In the absence of a signal in the input cir- cuit and with the proper voltages applied to the filament and plate, the plate current as- sumes a value near the upper bend of the tubes plate characteristic. When a signal voltage is applied across the input circuit the plates on the coil side of the grid condenser, €, become positive (lose some of their electrons) each hulf-cyele of the signal volt- age. When this side of the grid condenser goes positive, electrons from the filament flow to the grid and into the plates on the grid side of C, the resulting excess of elec- trons trapped on the grid causing it to as- sume a negative potential and reducing the plate enrrent. To prevent the grid from becoming more and more negative as eleetrons accumulate on the condenser, a high-resistance grid leak, R, is connected across the condenser. This resistor allows the negative charge on the grid to become cumulative only during the number of r.f. eycles that constitute one-half an audio cycle, thus allowing the plate cur- rent to follow the modulation on the im- pressed signal. This type of grid-leak de- iector gives high audio output, since rectifi- cation takes place in the grid eireuit and the amplifying properties of the tube are uti- lized. Unfortunately, however, this type of detector is prone to give rather high distor- tion when signals having a large percentage of modulation are impressed on it. The grid- leak detector is not limited to triodes; either tetrodes or pentodes may be used, these gen- erully having greater sensitivity than the triodes. The RADIO Aupio oureyr CATHODE TAP REGENERATION, Ser SCREEN =gRID. REGENERATION CONTROL E 7 Lp) remos rume Amo oureur né zm camsope-cou necentaanos ur SEREENCERIO BEE ENERATIGN CONTROL = -B 48 CONS wouND In SME DIRECTO PLATE-TICHÇER QEGenEnATIO JiTt TEROTELE-CONDENSER REGENERATIOH CONTROL +8 AUDIO qureur Auto qureur SesEEN-GSi-TICALER Figure 3. REGENERATIVE DETECTOR CIRCUITS. These circuits illustrate some of the more popular regenerative detectors. Values of one to three megohms for grid ieaks are common. The grid condenser usually has a capacity of .0001 sfd., white the screen by-pass is 0,1 mid. Pentode detectors operate best when the feedback is ad- Justed so that they start to oscillate with from 30 to 50 volts on the screen grid. For the reception of em. (constant-ware telegraphy) signals, it is necessary to provide some means of securing à heferodyne, or “beat note” with the incoming signal. n the autodyne detector this is done by eoupling some of the radio-frequency energy in the plate cireuit back into the grid circuit and allowing the tube to oscillate weakly. The feedback or tickler, coil, Ly, is closely coupled to the grid coil and thus provides the feed- baek necessary to make the stage oscillnte. Handbook Superregenerative Receivers 71 of “sclf-quenched” detector the grid leak is usually retrened to the positive side of the power supply (throngh the coil) rather than fo the cathode. representative self. quenched superregenerative detector circuit is shown in figure 7. Both types of superregenerative detectors act as small transmitters and radiate broad, rough signals unless they are well shielded and preceded by an r£ stage. For this rea- son they arc not too highly recommended for use on frequencies below 60 Me. However, there are oteasionally cases where their use is justificd on the 56-0-60 Me. band. The superregenerative receiver tunes very broad- Iy, receiving a band at least 100 ke, wide. For this renson it is widely popular for the reception of unstable, modulated oscillators at ultra-high frequencies, Frequency modulation reception is possible with superregenerative receiver, although with the amount of “swing” ordinarily uscd in frequency-modulated transmitters the audio outpué of the receiver is comparable to that obtained when the signal is amplitude modu- Inted at a rather low percentage. Tf a rela- tively wide swing is used in the transmitter, however, the audio output of the receiver will compare favorably with that obtained from a fully amplitude modulated carrier of equiv- alent strength, Practical superregenerative receiver cireuits along with a further discussion of their opera- tion will be found in Chapter 18. Superheterodyne Receivers Because of its superiority and nearly uni- versal use in all fields of radio reception except at the cxtremely high “micro wave” frequen- cies, the theory of operation of the super- heterodyne should he familiar to every radio experimenter, whether or not he contemplates building a receiver of this type. The follow- ing discussion concerns superheterodynes for amplitude-modulation reception. Tt is, how- ever, applicable in park to receivers for fre- queney modulation. The points of difference between the two types of receivers together with circuits required for F.M. reception will he found in Chapter 9. Principle of Operation. In the super- heterodyne, a radio-frequency cirenit is tuned to the frequency of the incoming signal and the signal across this cirenit applied to a vacuum-tuhe mixer stage. In the mixer stage the signal is mixed with a steady signal gen- erated in the receiver, with the result that a, signal bearing all the modulation applied to the original but of a freguency equal to the to avo AeLiER. Figute 7. SUPERREGENERATIVE DETECTOR. This extremely sensitive self-quenched detector atrangement is often used at ultra-high frequen- cies, The plate blocking condenser must have low resctance at the quench frequency; à value of «006 xd. is common. difference between the local oscillator and in- coming signal frequencies appears in the mixer output circuit. The output from the mixer stage is fed into a fixed-tune inter- mediate-freguency amplifor, where it is amplificd and detected in the usual manner and passed on to the audio amplifier. Fig- ure 8 shows a block diagram of the funda- mental superheterodyne arrangement, Superheterodyne Advantages, The ad- vantages of superhoterodyne reception are directly attributable to the use of the fixed- tune intermediate-froqueney (1.£.) amplifier. Since all signals arc converted to the interme- dinte frequency, this section of the receiver may he designed for optimum selectivity and amplification without going into the ex- tremely complicated tunablo band pass arrangements or the number of stages whieh would. be necessary if the signal-frequeney tuning eircuiís were designed to have a com- parable dogree of selectivity and gain. High amplification is easily obtained in the intermediate-freguency amplifler, since it operates at a relatively low frequency, where conventional pentode-iype tubes give à great deal of voltage gain. À typical if. amplifier stage is shown in figure 9. From the diagram it may be seen that both the grid and plate circuits are taned. Tuning MixeR PEsequena Av ney E oem AUPURIER ecror AMPOIFIER OSCULATOA| BLOCK DIAGRAM OF SUPERHETERODYNE Figure 8. THE ESSENTIAL PARTS OF A SUPER- HETERODYNE RECEIVER, There are several possible variations of this arrangement. Rf. amplifier stages often are used ahead of the mixer, Occasionally the 17. amplifier stages are omitted in simple superheterodynes. memos qem qe mm 72 Radio Receiver Theory The RADIO Figure 9. INTERMEDIAÇE, RERRENCY AMPLI- 6K7 and 65KT varios tados are usually used as i.f, amplifer tubes. These types require cathode and sereen resistors of approximately 300 and 100,000 ohms, respectively. The higher transconductance types such as the 1851-52-53 will require lower values of cathode and screen resistors for hest operation. By-pass condensers are usually .05 or 0.1 afd, bath cireuits in this way is advantageous in two ways: it increases the selectivity, and it allows the tubes to work into a high-impedance resonant plate load, a very desirable condition where high gain is desired. The tuned cir- cuits used for coupling between i.f. stages are known as if. transformers. These will be more fully discussed later in this chapter. Choice of Intermediate Frequency. The choise of a frequency for the if. amplifier involves several considerations. One of these considerations is in the matter of seleetivity; as a general rule, the lower the intermediate frequency the better the selestivity. On the other hand, a rather high intermediate fre- queney is desirable from the standpoint of image climination and also for the reception of sipmals from television and F.M. transmit- ters and modulated self-controlled oscillator, all of which oceupy & rather wide band of frequencies, making a broad seleetivity char- aeteristic desirable. Images arc a peculiarity common to all superheterodyne receivers, and for this reason they are given a detailed dis- cussion later in this chapter. While intermediate frequencies as low as 30 ke. were common at one time, and fre- quencies as high as 20,000 ke. are used in some specialized forms of receivers, most present- day communications superheterodynes nearly always use intermediate frequencies around either 455 ke, or 1600 ke. "Two other freguen- cies which are sometimes enconntered im broadeast-band receivers aro 175 ke. and 262 e. Generally spesking, it may be said that for maximum selectivity consistent with a reason- able amount of image rejeetion for signal fre- quencies np to 80 Me, intermediate frequen- cios in the 450-470 ke. range are used, while for a good compromise between image rejee- tion and selectivity the i.f, amplifier will often operate at 1600 ke. For the reception of both amplitude and frequency modulated sigmals above 30 Me., intermediate fraquencies near 2100, 3000 and 5000 Ke, are most often used. The intermediate amplifiers in television re- ceivers will usually be found to operate in the region between 8000 and 15,000 Ke. Arithmetical Selectivity. Aside from al- loving the use of fixed-tune band pass am- plifier stages, the superheterodyne has an overyhelming advantage over the tr.f. type of receiver bocanse of what is com- monly known as arithmetical selectivity. This can best be illustrated by considering two reccivers, one of the tr.£. type and one of the superheterodync type, both aitempting to receive a desired signal at 10,000 ke. and eliminate a strong interfering signal at 10,010 ke. In the tr£. receiver, separating those tro signals in the tuning cireuits is practically impossible, since they differ in fregueney by only 0.1 per cent. However, in a super- heterodyne yith an intermediate frequency of, for example, 1000 ke., the desired signal will be converted to a frequency of 1000 ke. and the interfering signal will be converted to a frequency of 1010 ke., both siguals appearing at the input of the if. amplifler. Tn this case the two signals may be separatcd much more readily, since they differ by 1 per cent, or ten times as much as in the first case. Mixer Gircuits. The most important single scetion of the superheterodyne is the mixer. No matter how mueh signal is applied to the mixer, if the signal is not converted to the intermediate frequency and passed on to the if. amplifer it is lost. The tube manufactur- ers have released a large variety of special tubes for mixer applications and these, as well as improved cireuits with older type tubes, have resulted in highly efficient mixer arrange- ments in present-day receivers. Figure 10 shows several representative mixer-oseillator eireuits. At “A” is illustrated controlgrid injection from an cleetron- coupled oscillator to the mixer. The mixer tube for this type of circuit is usually a remote- ent-of? pentode of the 576]? type. The coupling condenser, C, between the oseillator and mixer is quite small, usually 1 or 2 uyfd. This same cireuit may be used with the os- cillator output being taken from the oscillator giid or cathode, The only disadvantage to this method is that interlocking, or “pulling,” be- tween the mixer and oscillator turing controls is liable to take place, A rather high value of esthode resistor (10,000 to 50,000 ohms) is usually used with this cirenit,  “164 ã Mixer-Oscillator Circuits 73 Figure 10. MIXER-OSCILLATOR COMBINATIONS. The various oscillators do not have to be used with the mixers with which they happen to be shown. The triode oscillator shown at E couíd replace the pentode circuit shown at 8, for instance. Injection of oscillator voltage into mixer clements other than the control grid is illus- trated by figures 10B, C, D and E. The circuit of 10B shows injection into the sup- pressor grid of the mixer tube. The sup- pressor is biased negatively by connecting it directly to the grid of the oscillator. An alternative method of obtaining bias for the suppressor, and one which is less prone to cause interlocking betwecn the oscillator and mixer is shown in figure 10. Tn this cireuit the suppressor bias is obtsined by al- lowing the rectifed suppressor-grid current to flow through a 50,000- or 100,000-ohm re- sistor to ground. The conpling condenser be- tween oseillator and mixer may be 50 or 100 gutd. with this cireuit, depending upon the frequeney. Output from the oseillator may be 76 Radio Receiver Theory E ususeo éra fito +B Figure 12. USING A SEPARATE OSCILLATOR WITH A DUAL-PURPOSE CONVERTER TUBE. A separate oscillator may also be connected into the mixer circuits shown in figure 11 at the points marked “X” receive a signal at 14,100 ke. Assuming an i£-amplifer frequency of 450 kc., the mixer input eirenit will be tuned to 14,100 ke. and the oscillator to 14,100 plus 450, or 14,550 c. Now, a strong signal at the oscillator plus the intermediate frequency (14,550 plus 450, or 15,000 ke.) will also give a difference fre- queney of 450 ke. in the mixer ontput and will be received just as thongh it were actually on 14,100 ke., the frequency of the desired signal. The image is always twice the intermediate frequeney away from the desired signal. The only way that the image could be elim- inated in fhis particular case would be to make the selectivity of the mixer input cireuit and any eircuits preceding it great enough so that the 15,000-ke, signal would be eliminated with theso eircuits tuned to 14,100 ke. For any particular intermediate frequency, image interference troubles become increas- ingly greater as the frequency to which the signal-Prequeney portion of the receiver is trmned is increased. This is due to the fact thut the percentage difference between the de- sired frequency and the image frequency de- ereases as the receiver is tuned to a, higher frequency. The ratio of strength between a sigual at the image frequeney and a signal at the froqneney to which the receiver is tuned required to give equal output is known as the image ratio. The higher this ratio, the better the receiver in regard to imege-interference troubles. With but a single tuned circuit between the mixer grid and the antena, and with 400-500 ke, if. amplifiers image ratios of one hundred and over are easily obtainable up to frequen- cies around 5000 ke. Above this frequency grenter selectivity in the mixer grid eireuit The RADIO (throngh the use of regeneration) or addi- tional tuned circuits between the mixer and the antenna are necessary if a good image ra- tio is to be maintained. R.F. Stages, Since the necessary tuned cirguits between the mixer and the antenna can be combined with tubes to form rf. am- plifier stages, the reduction of the effects of mixer noise and the increasing of the image ratio can be aecomplished in a single section of the receiver. When incorporated in the re- ceiver this section is known simply as an 4.7. amplifier; vshen it is a separate unit with a separate tuning control if is known as a pre- selector. Either one or two stages are com- monly used in the preselector or r.f. amplifier. Some single-stage preseleetors and a few two- stage units use regeneration to obtain still greater amplification and selectivity. Double Conversion, As previonsly men- tioned, the use of a higher intermediate fre- quency will also improve the image ratio, at the expense of i.f. selectivity, by placing the desired signal and the image farther apart. To give both good image ratio at the higher frequeneies and good selectivity in the Lf. amplifier, a system known as double conver- sion is sometimes employed. Tn this system the incoming signal is first converted to a rather high intermediate frequency, such as 1600 ke. and then amplified and again con- verted, this time to a much lower frequeney, sugh as 175 Fe. The first if. frequency sup- plies the necessary wide separation between the image and the desired signal while the second one supplies the bulk of the if. se- leetivity. Regenerative Preselectors, R.f. amplifiers for wave-lengths down to 30 meters can be made to operate effeiently in a nonregenor- ative condition. The amplification and se- leetivity are ample over this range. For higher frequencies, on the other hand (wave- lengths below 30 meters), controlled regener- ation in the r.£. amplifier is often desirable for the purpose of increasing the gain and selee- tivity. The input impedanee of the grid circuit of a radio-freguency amplifier tube consists of a very high capacitive reactance which becomes part of the tuning capacity for longer wave- lengths. However, in very short wave receiv- ers the input impedance of a tube may drop to very low valucs, such as a few thousand ohms. This low impedance across the input tuned circuit reduces the amount of amplifica- tion that can be obtained from the complete rf. stage to a very low valne. A small amount of r.£. fecdhack can be in- troduced to compensate for this tube loss. Handbook Mixer-Oscillator Circuits 77 Regeneration can be carried to the point of actually creating the effect of negative resist- anee in the grid circuit, and thereby balaneing the resistance introduced across the tuned cir- cuit by the relatively low parallel tube re- sistance, Jixcessive regeneration will result in too much negative resistance, which will cause the rf. amplifler to oscillnte. Operation shonld always be below the point of self. oscillation. As previously discussed, a disadvantage of the regenerative r.f. amplifer is the need for an additional regeneration control, and the difficulty of maintaining alignment between this circuit and the following tuned cireuits. Resonant effects of antenna systems usually must be taken into account; a variable an- tenna eoupling device can sometimes be used to compensate for this effect, however. An- other disadvantage is the increase in hiss, or internal noise. The reason for using regeneration et the higher fregnencies and not at the medium and low frequencies can be explained as follows: The signal-to-noise ratio (output signal) of the average rf. amplifier is reduced slightly by the incorporation of regeneration, but the signalto-noise ratio of the receiver as a whole is improved at the very high frequencies be- cause of the extra gain provided ahead of the mixer, this extra gain tending to make the signal ontpnt a larger portion of the total signal.-plus-noise output of the receiver. Signal-Frequency Tuned Circuits The signal-frequeney tuned cireuits in su- perheterodynes and tuned radio frequeney types of receivers consist of coils of either the solenoid or universal-wound types shunted by variable condensers. It is in these tuned eir- cuits that the causes of success or failure of a receiver often lie, The universal-yound type coils usually are used at frequencies below 2000 ke.; above this frequency the single- layer solenoid type of coil is more satisfae- torp. Impedance and Q. The two factors most affecting the tuned cireuits are impedance and Q, which, as explained in Chapter 2, is the ratio of reactance to resistance in the circuit. Since the resistance of modern condensers is low at ordinary freguencies, the resistance usually can be considered to be concentrated in the coil. The resistance to be considered in making Q determinations às the r.f, resistance, not the d.c. resistance of the wire in the coil, The latter ordinarily is low enough that it may be neglected. This r.£, resistance is influenced by such factors as wire size and type and the proximity of metallic objects or poor in- sulators, such as coil forms with high losses. Jt may be seen from the enrves shown in Chapter 2 that higher values of Q lead to better selectivity and increased r.f. voltage across the tuned circuit. The increase in volt- age is due to an inerease in the circuit im- pedance with the higher valnes of Q. Frequently it is possible to secure en in- creage in impedance in a resonant eireuit, and conseguently an increase in gain from an amplifer stage, by increasing the reactance throngh the use of larger coils and smaller tuning condensers (higher L/G ratio). The Q of the coil probably will be lowered by this process, but the impedance, which is a function of both reactance and Q, will be greater be- cause for small increases in reaetances the re- actance wil! increase faster than the Q de- ereases. The selectivity will be poorer, but in superheterodyne receivers sclectivity in the signal-frequeney eireuits is of minor im- portanee where sigmals on adjacent channels are concemed. On the other hand, the t.r.f. type of receiver requires good seleetivity in the tuned cireuits, and a compromise between impedance and Q must be made. Input Resistance, Another factor which influences the operation of tuned eireuits is the deerease with increasing frequency of in- put resistance of the tubes placed across these cireuits. At broadeast frequencies the input resistance of most tubes is high enough so that it is not bothersome. As the frequeney is in- creased, however, the input resistance be- comes lower because the transit time required by an electron traveling between the cathode and grid becomes sn appreciable portion of the time required for an r.f. eyele of the signal voltage. The result of this effect is similar to that which would be caused by placing a re- sistance between the grid and cathode. Because of the lower input resistance of tubes at the higher frequencies, there is a limit to the maximum impedance necessary to obtain maximum voltage across the tuned cir- cuits when these circuits are shunted by the tube's input resistance. These considerations often make it advisable to design the con- centrie tuned cirenits often used at thc higher requencies for maximum Q rather than for maximum impedance, The tube input re- sistanee remains constant, and increasing the tuned circuit impedanee beyond two or three times the input resistance will have but little effect on the net grid-to-gronnd impedance of the amplifer stage. The limiting factor in r.f. stage gain is the ratio of input conduetance to the tube trans- conduetanee. When the input condnctanee  78 Radio Receiver Theory The RADIO - orago fa ro eva " MIXER 3 OScILLATOR Figure 13. CIRCUIT FOR REDUCING GRID LOAD- ING EFFECTS. Tapping the grid down on the coil will increase the gain and selectivity obtained with high- transconductance tubes at high frequencies. becomes so great that it equals the transcon- ductanee, the tube no longer ean act as an amplifer, There are two ways of increasing the ratio of transcondnetance to input con- duetance. One of these methods is exempli- fied by the “acorn” type tube, in whieh the input conductance is reduced through the use of a smaller element structure while the trans- conduetance remains nearly the same as that of tubes ordinarily used at lower frequeneies, Another method of aceomplishing an inerease in transconduetanee-input conductance ratio is by greatly increasing the trarisconductance at the expense of a proportionately small in- crease in input conduetance. The latter method is exemplified by the so-called “tele- vision pentodes” which have extremely high transconductance and an input conductance several times that of the acorn tubes. The diffenltes presented by input- resistance effects may be partially obviated by tapping the grid down on the coil, as shown in figure 13. This circuit is commonly employed with high-transconductance tubes when oper- ating on the 28-30 Me, amateur band, and nearly always with such tubes on the 56-60 Me. band. Acorn tubes, due to their smaller dimensions and lower capacities, are consid- erably better than the conventional types at ultra-high frequeneies and it usually will not be found necessary to tap their grids dowm on the tuned eireuit until frequencies around 200 Me. are reached. Superheterodyne Tracking. Because the detector (and r.£. stages, if any) and the oscil- lator oporate on different frequêncies in super- heterodynes, in some cases it is necessary to make special provisions to allow the oseillator to track with the other tuned eircuits when similar tuning condensers are used. The usual method of obfaining good tracking is to oper- ate the oscillator on the high-frequeney side of the mixer and use a series “tracking con- denser” to slow down the tuning rate of the oscillator. The oseillator tuning rate must be sloywer because it covers a smaller range than semES TRACRINÉ CONDENSER Figure 14. OSCILLATOR SERIES TRACKING CON- DENSER ARRANGEMENT. The series condenser allows the oscillator to tune at a slower rate of capacity change than the mixer. does the mixer when both rangres are expressed as a percentage of frequency. At frequencies above 7000 Kc. and with ordinary if. fre- aueneies, the difference in percentage between the two tuning ranges is so small that it may be disregarded in receivers designed to cover only a small range, such as an amateur hand. À mixer and oscillator tuning arrangement in which a series tracking condenser is pro- vided is shown in figure 14. The value of the tracking eondenser varies considerably with different intermediate frequencies and tuning ranges, enpacities as low as 0001 ufd. being used at the lower tuning-range frequencies, and values up to .01 ufd. being nsed aí the higher frequencies, Bandspread Tuning. The frequency to which a receiver responds may be varied by changing the size of either the coils or the condensers in the tuning cirenits, or both. In short-wave receivers a combination of both methods is ususlly employed, the coils being changed from one band to another and vari- able condensers being used to tune the receiver across each band. In practical receiver, coils may be changed by one of two methods: A switch, controllable from the front panel, may be used to switeh coils of different sizes into the tuning circuits or, alternatively, coils of different sizes may he plugged manvally into the receiver, the connection into the tuning eireuits being made by suitable plugs or the coils. Where there are several “plug-in” coils for each band they are sometimes arranged on a single mounting strip, allowing them all to be plugged in simultaneously. Tn receivers nsing large tuning condensers to cover the short-wave spectrum with a mini- mum of coils, tuning is liable to be quite dif- ficult owing to the large frequency range covered by a small rotation of the variable condensers. To alleviate this condition, some Handbook IF. Tuned Circuits 81 latier representing all-wave sets with band- switching, large tuning condensers, and con- ventional tubes. LF, Tuned Circuits AM if. amplifers employ bandpass cir- cuíts of some sort. A bandpass eireuit is exactly what the name implies—a circuit for passing 2 band of frequencies. Bandpass ar- rangements can be designed for almost any degree of selectivity, the type used in any par- ticular application depending upon the use to which the if, amplifer is to be put. Bandpass Circuits. Bandpass -eireuits consist essentially of two or more tuned cir- euits and some method of conpling the tuned cireuits together. Some representativo ar- rangements are shown in figure 16. The cir- euit shown at À is the conventional i.f. trans- former with the coupling, M, hetween the tuned cireuits being provided by indnctive coupling from one coil to the other. As the coupling is incréased, the seleetivity curve becomes less penked, and when a condition of over-coupling is reached the top of the curve flattens out. When the conpling is increased still more, a dip oceurs in the top of the curve. The windings for this type of if. transformer, as well as most others, nearly always consist of small, fiat universal-wound pies mounted either on a piece of dowel to provide an air core or on powdered-iron impregnated bake- lite for “iron core” transformers. The iron-core transformers generally have some- what more gain and better selectivity than equivalent air-core units betwcen 175 and 2000 ke. The cireuits shown at B and C are quite similar. Their only difference is the type of mutual coupling used, an induetanec being used at B and a capacitance at C. The opera- tion of both eireuts is similar. Three reso- nant eircuits are formed by the components. In B, for example, one resonant eireuit is formed by Ly, Cy Ca, and Ly all in series. The frequeney of this resonant eireuit is just the same as that of a single one of the coils and condensers, since the coils and condensers are similar in both sides of the circuit and the resonant frequency of the two condensers and the two coils all in series is the same as that of a single coil and condenser. The second resonant frequency of the complete cir- cuit is determined by the characteristics of each half of the cireujt containing the mutual coupling device. Tn B, this second frequency will be lower than the first since the resonant frequeney o£ Ly, C; and the inductance, M, or Lo, Go and Mis lower than that of a single a SE sig + Figure 16, LF. AMPLIFIER BAND-PASS CIRCUITS. The ordinary 1. transformer circuit is shown at A. The oiher circuits are intended to give a straight-sided, flat-tapped selectivity characteris- tic to the it. amplifier. coil and condenser, due to the induetance of M being added to the cirenit. The opposite effect takes place at O, where the common conpling impedance is a condenser. Thus at O the second resonant frequency is higher than the first. In either case, however, the circuit has two resonânt frequencies, result- ing in a flattopped selectivity curve. The width of the top of the curve is controlled by the reactance 0£ the mutual eoapling com- ponent. As this reactance is increascd (in- ductance made grester, capacity made smaller) the two resonant freguencies become farther apart and the curve is broadened. The circuit of figure 16D is often used where a fairly high degree of bandpass action is required and the number of i.f. transform- ers used ranst be kept at a rainimum. En this circuit there is induetive coppling between the center coil and each of the outer coils. The 82 Radio Receiver Theory it x É Tê Z E our [o Figure 17. CRYSTAL FILTER EQUIVALENT CIRCUIT. With a constant input voltage, the r.f, voltage developed across Zy depends upon the impedances of Z, X and à result of this arrangement is that the center coil acts as a sharply tuned coupler between the other two. À signal somewhat off the resonant fregnency of the transformer will not induce as much voltage in the center coil as will a signal of the correct frequeney. When a smaller voltage is induced in the center coil, it in turn transfers a still smalier voltage to the output coil. In other words the coupling of the three coils increases as the resonant frequency is approached and remains nearly constant over a smail range and then decreases again as the resonant band is passed. Another very satisfactory bandpass ar- rangement which gives a very straight-sided, flat-topped curve is the negative-mutual ar- rangement shown at E. Energy is transferred between the input and output eiruits in this arrangement by both the negative-mutnal coils, M, and the common capacitive reset ance, C. The negative-mutual coils are inter. wound on the same form and eonneeted “baekward)” as shown. Crystal Filters. The selectivity of the intermediate-frequeney amplifer may be in- creascd grestly throngh the use of an ex- tremely high Q piezo-eleetrie series resonant circuit. The piezo-eleetric quartz crystal, together with its coupling arrangement, is gencrally known as a erystal filter. The electrical equivalent of the basic crystal filter eireuit is shown in figure 17, while the elee- trical equivalent of the erystal itself is showm im figure 18. At its resonant frequency, the crystal, E, may be replaced by a very small resistance, and thus at this frequeney the eurrent flow- ing through the eirenit, Z, X, Z, reaches a maximum and the ontput voltage Eous is also at its mazinum valne. At frequencies slightly off resonance the crystal impedance becomes quite high and the eurrent flowing through the circuit, and consequently the volt- age Eout developed across Z,, drops to a low valne. Jt is the ratio of Eou at resonanee The RADIO to this voltage at freqnencies away from resonanee that determines the selectivity charaeteristie of the crystal filter. This rabo may be shown to depend upon the values of the impedanees Z and Z4. These impedances remain nearly constant for frequencies near resonance, and the seleetivity of the filter cir- cuit as a whole may be altered by changing the resonant frequency values. The variable seleetivity crystal filter eireuits quite often used in communications superheterodynes operate on this principle. Practical Filters. In practical erystal fil. ters is is necessary to balanee out the capaeity across the erystal holder (Cy in figure 18) to prevent hy-passing around the erystal of un- desired signals off the crystal resonant fre- queney. The balancing is done by a phasing circuit which takes out-of-phase voltage from Figure 18. CRYSTAL EQUIVALENT. The crystal is equivalent to a very targe inductance in series with a very small resistor and condenser. Figure 19. VARIABLE-SELECTIVITY CRYSTAL CIRCUIT. In this circuit the setectivity is at a minimum when the input circuit is tuned to resonance. emvsraL E acesa ” PE Mm p— + a balanced input circuit and passes it to the output side of the crystal in proper phase to neutralize that passed through the holder capacity. A representative practical filter arrangement is shown in figure 19. The phasing condenser is indicated in the diagram by PC. The balanced input circuit may be obtained either through the use of a split- stator condenser as shown or by the use of a center-tapped input coil. Variabie-Selectivity Filte: In the eir- cuit of figure 19 the selectivity is minimum with the crystal input cireuit tuned to reso- nanee, since at resonanee the input eireuit is a pure resistance effeetively in series with the voltage applied to the erystal. As the input Handbook Crystal Filter Circuits 83 cireuit is detuneã from resonanee, however, the resistive component of the input imped- ance decreases and the selectivity becomes grenter, Tn this circuit the ontput from the crystal filter is tapped down on the if. stage grid winding to provide a better match and lover the impedance in series with the crystal. The cirenit shown in figure 20 also achieves variable seleetivity by adding an impedance in series with the crystal cireuit. In this case the variable impedance is in series with the crystal output circuit. The impedanee of the output tuned circuit is varied by varying the Q. As the Q is reduced (by adding re- E € WIDE-RANGE VARIABLE-SELECTIVITY CRYSTAL FILTER, SELECTIWTY CONTROL The selectivity is varied by changing the impedance of the output circuit by changing its Q. Figure 21. DEGENERATIVE 1.F. STAGE, Degeneration in the 1£. stage following the crystal filter às desirable to avoid input capacity changes when the gain is varied, To envstaL Pita E TOAMC. TOMANUAL VOLME CONTROL sistance in series with the coil) the impedance decreases and the selectivity becomes greater. A variation of the circuit shown at figure 20 consists of placing the variable resistance across the coil and condenser, rather than in series with them. The result of adding the resistor is a reduetion of the ontpué im- pedance and an increase in selectivity. The circuit behaves oppositely to that of figure 20, however; as the resistance is lowered the se- lectivity becomes greater. Interference Rejection. The erystal filter phasing condenser can be adjusted so that parallel resonance between it and the crystal / causes a sharp dip in the response curve at some desired point, such as 2 ke. from the desired signal peak. This effect can be uti- lized to eliminate completely the unwanted sideband 1 Ec. away from zero beat for cw, reception. The b.fo. then provides a true single signal effeet, that is, a single beat fre- queney note. This effeetively increases the number of e.w. channels that can be used in any short-wave band. 1600-Kc. Crystal Filters. Since the se- leetivity of a series crystal resonator varies approximately directly with frequency, erys- tal filters for use with if. amplifers in the 1500- to 1600-ke. range are approximately three times as broad as their maximum se- lectivity setting as 465-ke. crystal cirenits. This is no great disadvantage, as a well. designed 1600-Ke. filter may be made to have 300-eyele seleetivity at its maximum setting. For radiotelephone reception the 1600-Ke. filter actually is advantageous, becanse its minimum selectivity permits a much wider band than a 465-ke. unit. The wider avail- able pass band allows the crystal to be left in the cirenit at all times and the seleetivity merely varied to suit the kind of reception desired. Variable-selectivity circuits of the type shown in figure 19 require special con- sideration when used sith 1800-Kc. exystals, however. This is due to the fact that tho capaeity across the crystal holder, and conse- quently the capacity of the phasing con- denser, is much higher, due to the thinner exystal required at 1800 ke. As the phasing condenser and the crystal are actually in series across the input cireuit and selectivity control, any change in setting of the phasing condenser will alter the se- leetivity. This diMiculty may be eliminated by using a special form of phasing condenser which sets as a capacity potentiometer and maintains equal capacity across the input cir- cuit and at the same time varies the capacity in the phasing branch. Reducing Input Capacity Variations, As the previous discussion on crystal filters has indicated, the selectivity of the crystal filter can be altered by changing the impedance of the crystal ontput eireuit. Since the im- pedance at crystal frequency of the output circuit can be varied by detuning it as well as by varying its Q, it is important that the input capacity of the tube following the filter remain constant when the gain of this stage is varied. The input capacity may be stabi- lized with respect to changes in the tube's am- plification by employing a small amount of degeneration, as illnstrated in figure 21. The amount of degeneration which can be nsed 86 Radio Receiver Theory The RADIO used to set the meter for minimum indication xvhen no signal is being received. Beat-Freguency Oscillators. The beat- frequeney oseillator, usnelly called the b.f.0., is a necessary adjunct for reception of cw. telegraph signals on superheterodynes which do not use regenerative deteetors. The oseil- Jator is conpled into the second detector cir- cuit and supplies a weak signal of nearly the same frequency as that of the desired signal from the if. amplifier. If the if. amplifier is tuned to 465 ke., for example, the b£.o. is tuned to approximately 464 or 466 ke. in order to produce a 1000-cyele beat note in the output of the second detector of the receiver. The carrier signal would otherwise be inaudible. The b.£.o. is not used for voice reception, except as an aid in searching for wcak stations. The b.£.0. input to the second detector nced only be sufficient to give a good beat note on an average signal. Too much coupling into the second deteetor will give an exeessively high hiss level, masking weak signals by the high noise background. A method of manually adjusting the b.£.o. output to correspond with the strength of re- ccived signals is shown in figure 27. A var- iable b. output control of this sort is a useful adjunct to any superheterodyne, since it allows sufficient b.f.o. output to bc obtained to give a “beat” with strong signals and at the same time permits the b.f.o. output, and consequently the hiss, to bo reduced when at- tempting to receive weak sigmals. The cir- cuit shown is somewhat better than those in which one of the electrode voltages on the b£.o. tube is changed, as the latter usually change the frequency of the b.£.o, at the same time they change the strength, making it necessary to reset the trimmer each time the output is adjusted. Tn nearly all receivers in which both a.v.c. and a b.£.o. are uscd it is necessary to discon- nect the ave. circuit and manually control the gain when the b.f.o. is tumed on. This is hecanso the b£o. nois exactly like 2. strong signal and puts a.v.6. bias on the stages on the a.v.. line, thereby lowering the gain of the receiver. Noise Suppression The problem of noise suppression con- fronts the listener who is located in such places «here interference from power lines, elec- trical applianeés and automobile igmition sys- tems is tronblesome. This noise 1 often of such intensity as to swamp out signal from desired stations. There are three principal methods for re- dueing this noise: (1) Ae. line filters at the souree of inter- ferenee if the noise is erented by an elec- trical appliance. (2) Noise-balaneing cireuits for the redue- tion of power-leak interference. (3) Noise-limiting eirenits for the reduction, in the receiver itsclf, of interfcrcnee of the type caused by antomobile ignition systems. Power Line Filters. Numcrous household appliances, such as electric mixers, heating pads, vaeuum sweepers, refrigerators, oil burners, sewing machines, doorbells, ete., erente an interference of an intermittent na- ture. The insertion of a line filter near the source of interfcrence often will cffeet a com- plete cure. Filters for small appliances can consist of a 01-nfd. condenser connected aeross the 110-volt a.e. line. Tso condensers im series across the in seres soros the Tino, with the midpoint with the midpoint SIGNAL ANTENHA RED cur E To Figure 28. JONES NOISE-BALANCING CIRCUIT. Ti dlrcui, when properiy adfuste, reduces the intensity of pomer-leak and similar interference. Norge antena connected to ground, can be used in conjune- tion with ultra-violet ray machines, refriger- ators, ail burner fumaces and other more stubborn offenders. In severe cases of inter- ferenee, additional filters in the form of heavy-duty r.f. choke coils must be connected in series with the 110-volt a.c. line on both sides of the line. Noise Balancing. Power line noise inter- ferenee can be greatly reduced by the instal- lation of a noise-balancing cireuit ahcad of the receiver, as shown in figure 28. The noise-balancing circuit adds the noise com- ponents from a separate noise antenna in such a manner that this noise antena will buck the noise picked up by the regular receiving antenna. The noise antenna can consist of a connection to one side of the .e, line, in some cases, while at other times an additional wire, 20 to 50 feet in length, can be run paral- Jel to the a.c. house supply line. The noise antenna should pick up as much noise as pos- síble in comparison with the amount of signal Handbook Noise Suppression 87 pickup. The regular receiving antenna should be a good-sized out-door antenna, so that the signal to noise ratio will be as high as possible. When the noise components are balanced out in the circuit ahead of the re- ceiver, the signals will not be appreciably attenuated. This type of noise balancing is not a simple process; it requires a bit of experimentation in order to obtain good results. However when proper adjustments have been made, it is possible to reduce thc power leak noise from 3 to 5 R points without reducing the signal strength more than one R point, and in some cases there will be no reduction in signal strength whatsoever. This means that fairly weak signals can be received through terrifie power leak interference, Hash type interference from electrical appliances can be reduced to a very low value by means of the same cireuits. The coil should be center-tapped and con- nected to the receiver ground connection in most cases. The pickup coil consists of four turns 0£ hookup wire 2” in diameter which can be slipped over the first rf. tuned coil in most radio receivers, A two-turn coil is more appropriate for 10- and 20-meter operation, though the four-turn coil is suitable if care is taken in adjusting the condensers to avoid 10-meter resonance (unless very loose in- ductive conpling is used). Adjustment of CG, will generally allow a noise balance to be obtained when varying Cs and Cs in nearly any location. One an- tenna, then the other, can be removed to check for noise in the receiver. When properly bal- anced, the usual power line buzz can be bal- anced down nearly to zero without attenuating the desired signal more than 50%. This may result in the reception of an intelligible dis- tant signal through extremely bad power line noise. Sometimes an incorrect adjustment will result in balancing out the signal as well as the noise, A good high antenna for signal reception will ordinarily overcome this effect. With this circuit some readjustment is necessary from band to band in the short- wave spectrum; noise-balancing systems re- quire a good deal of pationee and experi- menting at cach particular receiving location. Noise-Limiting Cireníte, Several differ- ent noise-limiting circuits have become pop- ular. These circuits are beneficial in over- coming automobile igmition interforenee. They opcrate on the principle that each in- dividual noise pulse is of very short duration, yet of extremely high amplitude. The pop- Ping or clicking type of noise from electrical ignition systems may prodnce a signal ten to twenty times as great as the incoming radio signal, As the duration of this type of noise peak is short, the receiver can be made inoperative during the noise peak without the human ear detecting the total Joss of signal. Some noise limiters, or eliminators, actually punch a hole in the signal, while others merely limit the maximum peak signal which reaçhes the head- phones or londspcaker. The noise peak is of such short duration that it would not be objectionable except for the fact that it prodnces an overloading effcet on the receiver, which incroases ifs time con- stant. A sharp voltage peak will give a kick to the diaphragm of the headphones or speaker, and the momentum or incrtia koeps the diaphragm in motion until the dampen- ing of the diaphragm stops it. This move- ment produees à popping sound which may completely obliterate the desired signal. Tf the noise peak can be limitod to an amplitude equal to that of the desired signal, the re- sulting interferenee is practically ncgligible. AF, Peak Limiters, Remarkably good noise suppression can be obtained in the audio amplifier of a radio receiver by using a de- layed push-pull diode suppressor. Any twin diode tube can be used, though the type 84 high vacuum full-wave roetifer tube seems to be the most effective. The circuit in figure 29 can be used to do- seribe the operation of this general type of noise suppressor or limiter. Each diode works on opposite noise voltages; that is, both sides of the noise voltage (-) and — portions of the 9.º. components) arc applied to diodos which short-cireuit the load when- ever the applied voltage is greater than the delay voltage. The delay bias voltage pre- vents diode current from flowing for low-level audio voltagos, and so the noise cirenit has no effect on the desired signals except during the short interval of noise peaks. This in- terval is usually so short that the human ear will not notice a drop in signal during tho small time that the load (headphones) is short-circuited by the diodes, ISTF TUBE jezeuro. Figure 29, A.F. NOISE LEMITER. A fimiter such as this is effective in reducing short-duration noise pulses, suck as automobile ignition Interference,  88 Radio Receiver Theory Delay bias voltage of 14 volts from a small finshlight cell will allow any signal voltage to operate the headphones which has a peak of less than about 14 volts. Noise pesks often have values of from 5 to 20 times as great as the desired signal; so these penks operate the diodes, esusing current to flow and a sudden drop in impedance across the headphones. Diodes have nearly infinite impedance sehen no diode current is flowing; however, as soon as current starts, the impedance will drop to a very few kundred ohms, whish tends to damp out or shoré cireuit the audio output. The final result is that the noise level from automobile ignition is limited to values no greater than the desired signal, This is low enough to cause no trouble in un- derstanding the voice or cw. signals. A push-pull diode circuit is necessary be- cause the noise peaks are of an 2.º, nature and are not symmetrical with respect to the zero a.e, voltage reference level. The negative peaks may be greater than the positive peaks, depending on the bias and overload charae- teristics of the audio amplifier tube. If a single diade is uscd, only the positive (or negative) peaks could be suppressed. In dg- ure 29 the two bias dry-cells are arranged to place a negative bias on each diode plate of 14 volis. À positive noise voltage peak at the plate of the audio amplifler tube will overcome this negative bias on the top diode plate and cause diode current to flow and lower the impedance, A negative noise volt- age peak will overcome the positive bias on the other diode cathode and cause this diode to net as a noise suppressor. A positive bias on the cathode is the same as a negative bias on the diode plate. The 6H6 has two sepa- rate calhodes and plates, hence lends itself readily to the simple circuit illusirated in figure 29. Cireuits of this type are very effective for short-pulse noise elimination because they tend to punch a hole in the signal for the duration of a strong noise voltage peak. À peak that will causc a loudspeaker or head- phones to rattle with a loud pop will be re- duced to « faint pop by the noise-suppression system. The delay bias prevents any atten- uation of the desired signal as long as the signal voltage is less lhan the bias, With this type of noise limiter it is possible to adjust the audio or sensitivity guin controls so that the anto ignition QRM seems to drop out, leaving only the desired signal with a small amount of distortion. Lower gain set- tings will allow some noise to get through but will eliminste audio distortion on voice or music reception. At high levels the speech The RADIO AFA ggozsuro, +ozsou. Figure 30. ADJUSTABLE NOISE LIMITER. With this circuit the bias on the fimiter diodes is adjustabie for different noise levels. The center- tapped choke may be the primary of a small pentode output transformer. af oureur dE RECEIVER sa SMALL CL poa a OUTPUT TRANSTORMER E CPRIMARY AND SEcONDAR + REVERSED) « Figure 32. NOISE LIMITER FOR USE WITH LOUDSPEAKER, The high bias on this dual-diode noise limiter allows it to be used on high-level audio stages. or music peaks will be aticnuated whenever they exceed the d.e. delay bias voltage. Faint ignition ratéle will always be sudible in the background with any noise-suppressor cireuit sineg some noise peaks are too small to oper- ate the systeras, yet are still audible as à weak rattle or series oÉ pops in the headphones. Figures 30 and 31 show two noise-limiter cireuits which can be nsed as separate units for connection to any receiver. The unit shown in figure 30 can be connected ucross any headphone output as long as there is no direct current fowing through the phones. A blocking condenser can be connected in series with it 1£ necessary, though beiter noise sup- pression results when the blocking condenser is in series with the plate lead to the head- phones. Delay bias is obtained from the plus B supply through a 15,000-ohm 10-watt re- sistor and a 200-chm ire-wound variable resistor. The cathode or cathodes are made a volt or so positive with respect to ground and minus B conncetion. The diode plates are connected through a centertapped low resistance choke to ground as far as bias voltage is concerned. Any push- pull to voice coil outpnt transformer can be used for the center-tapped choke in figure 30. The secondary can be left open. The delay bias is adjustable from O up to about 3 volts and once sef for some noise level, can be left in that position. Handbook Receiver Adjustment 91 devote to the operation. There are no short cuts; every circuit must be adjusted individ- ually and aceurately if the receiver is to give peak performance. The precision of each adjustment is dependent npon the aceuracy with which the preceding one was made. Superhet alignment requires (1) a good signal generator (modulated oscillator) cov- ering the radio and intermediate frequencies and equipped with an attenustor and B-plus switch; (2) the necessary socket wrenches, serewdrivers, or “nentralizing tools” to ad- just the various if, and rf, trimmer con- densers, and (3) some convenient type of tuning indicator, such as a copper-oxide or electronic voltmeter. Throughont the alignment process, unless specifically stated otherwise, the a.f. and r.f. gain controls must be set for maximum out- put, the beat oscillator switched off, the R- meter cut ont, the crystal filter set for min- imum selectivity and the a.v.e. turned off. If no provision is made for a.v.e. switehing, the signal generator output must be reduced to the proper level by means of the attenuator. When the signal output of the receiver is ex- cessíve, cither the attenuator or the a.f. gain contro! may be turned down, but never the r.f. gain control. LF. Alignment. After thc receiver has been given a rigid electrical and mechanical in- spection and any faults which may have been found in wiring or the selection and assembly of parts corrected, the if, amplifer may be aligned as the first step in the checking opera- tions. The coils for the r.f. (if any), first detector and high-frequeney oscillator stages must be in place. It is immaterial which coils are in- serted, since they will serve during the if. alignment only to prevent open-grid oscilla- tion. With the signal generator set to give a mod- ulated signal on the frequency at which the if. amplifer is to operate, clip the output leads from the generator to the last i.£. stage; “hot” end through a small fixed condenser to the control grid, “cold” end to the receiver ground. Adjust both trimmer condensers in the last i.f. transformer to resonance as in- dicated by signal peak in the headphones or speaker and maximum defleetion of the out- puí meter. Each i.f. stage is adjusted in the same man- ner, moving the hot lead, stage by stage, back toward the front end of the receiver and back- ing off the attenuator as the signal strength increases in ench new position, The last ad- justment wil! be made to the first if. trans- former with the hot lead connected to the control grid of the first detector, Occasion- ally, it is necessary to disconnect the 1st de- tector grid lesd from the coil, grounding it through a 1,000- or 5,000-ohm grid leak and coupling the signal generator through a small cnpacitance to the grid. When the last if, ad justment has been com- pleted, it is good practice to go hack through the if. channel, re-peaking all of the trans- formers. Tt is imperative that this recheck be made in sets which do not include a crystal filter and where necessarily the simple align- ment of the if. amplifer to the generator is final, LF. with Crystal Filter, There are several ways of aligning an L.f. channel which contains a crystalfilter circuit. However, the folloying method is one which has been found to give satisfactory results in every case: JUSTA ds cAvETAL f náo Pre — astosav + Figure 35, CRYSTAL, TEST OSCILLATOR CIRCUIT. The receiver's crystal may be placed in this oscillator for a rough alignment of the i.f. am] fier to the crystal frequency. Thé tank circuit is made up of à winding from à b.fo. transformer and a 350-matd, broadrast condenser, T£ the if. chonnel is known to be far out of alignment or if the initial alignment of a new receiver is being attempted, the erystal itself should first be used to control the fre- queney of a test oscillstor.. The eircuit shown in figure 35 cam be used. A b£o. coil, as shown in the diagram, ean be used for the plate inductance. Tf none is handy one wind- ing of an if. transformer may be used. In either case, it is necessary to discomnect the trimmer across the winding unless it has suf- ficient meximum capacity to be used in place of the 350-unfd. tuning condenser indicated in the diagram. A milliammeter inserted in the plate cireuit will indicate oseillation, the plate eurrent dip- ping as the condenser tnnes the inductance to the resonant frequency of the erystal, Some erystals will require additions) grid-plate capacity or oscilistion; if so, a 30-upfd. mica 92 Radio Receiver Theory trimmer may be conneeted from plate to grid of the oscillator tube. The oseillator is then used as a line-up oscillator as deseribed in the preceding section by using a.e, for plate supply instead of batteries. The a.e. plate supply gives a modulated signal suitable for the preliminary lining-up process. For the final if, aligoment the crystal should be replaced in the receiver and the phasing condenser set at the “phased” setting, àf this is known. TÊ the proper setting of the phasing condenser is unknown it can be set at half capacity to start with. Next, a sig- nal generator should be connected across the mixer grid and ground and, with the receiver's a.v.e. circuit operating and the beat oscilla- tor turned “off,” the signal generator slowly timed across the if. amplifier frequency. As the generator is tuned through the erys- tal frequency, the receiver's signal strength meter will give a sudden kick. Should the re- ceiver not be provided with a signal-strength meter, a vacuum-tnbe voltmeter, such as shown in Chapter 22, can be connecteã across the a.v.e. line; if the receiver has neither a.v.e, nor a tuning meter, the vacuum-tube voltmeter may be connected between the second detector grid and ground. Tn any esse a kick of either the tuning meter or the vacuum-tube volt- meter will indicate crystal resonance. Tt is quite probable that more than one resonance point will be found if the receiver is far ont of alignment. The additional points of res- onanee are spurious crystal peaks; the strongest peak should be chosen and the sig- nal generator left tuned to this frequency. The phasing condenser should next be ad- justed for minimam hiss or noise in the re- ceiver output and the seleetivity control, if any is provided, set for maximum selectivity. From this point on, the alignment of the i.f, amplifier follows conventional practice, ex- cept that the a.v.e. eircuit is used as an align- ment indicator, each circuit being adjusted for maximum output. If the reeciver is of the type having no a.v.c. or tuning indicator, and the vacuum-tube voltmeter must be con- neeted across the second-deteetor grid circuit, it will be necessary to remove the vacuum-tube voltmeter and make the final adjustment on the last i£. transformer by ear after the other transformers have been aligned. B.F. O. Adjustment. Adjusting the best oscillator is relatively simple, Tt is only necessary to tune the receiver to resonanee with any signal, as indicated by the tuning in- dicator, and then turn on the b.f.o. and set ifs trimmer (or trimmers) to produce the desired beat note. Setting the beat oscillator in this way will result im the beat note being stronger The RADIO on one “side” of the signal than on the other, which is what is desired for maximum selee- tivity. The b£.o. should not be set to “zero beat” with the receiver tuned to resonance with the signal as this will cause an equaily strong beat to be obtained on both sides or resonance. Front-End Alignment. The alignment of the “front end” of a manufacturcã receiver is a somewhat involved process and varies con- siderably from one receiver to another and for that reason will not be discussed here. Those interested in the alignment of such re- ceivers usually will find full instructions in the operating manual or instruction book sup- plied with the receiver. Likewise full align- ment data are always given when em “all wave” tuning assembly for incorporation in home-built receivers is purchased. In aligning the front end of a home- construeted superheterodyne which covers only the amatenr bands the principal prob- lems are those of seenring proper bandspread im the oscillator, and then tracking the signal- frequeney cireuits with the oscillator, The simplest method of adjusting the oscillator for proper bandspread is to tune in the os- cillator on an “all wave” receiver and adjust its bandspread so that it covers a frequency range equal to that of the tuning range de- sired in the receiver but over a range of fre- quencies equal to the desired signal range plus the intermediate frequency. For example: If the receiver is to tune from 13,950 to 14,450 ke. ta cover the 14-Me. amateur band with a 50-kc. lecway at cach end, and the inter. mediate frequeney is 465 ke, the oscillator should tune from 13,950---465 ke. to 14,450-+ 465 ke, or from 14,415 to 14,915 ke. (Note: The foregoing assumes that tho os- cillator will be operated on the high-frequeney side of the signal, wkich is the usual condi- tion. Tt is quite possible, however, to have the oscillator on the low-frequency side of the signal, and if this is desired the inter- miediate frequency is simply subtracted from the signal frequency, rather than added, to give the required oscillator frequency). T£ no calibrated auxiliary receiver is avail- able the following procedure should be used to adjust the -oscillator to its proper tuning range: A modulated signal from the signal generator is fed into the mixer grid, with mixer grid coil for the baná being nsed in place, and with the signal generator set for the highest frequency in the desired tuning range and the bandspread condenser in the receiver set at minimum capacity, the oseilla- tor bandsetting condenser is slowly decreased from maximura capacity until a strong signal Handbook Receiver Adjustment 93 from the signal generator is picked up. The first strong signal picked up will be when the oseillator is on the low-frequency side of the sigual. T£ it is desired to use this beat, the oseillator bandsetting condenser need not be adjusted further. However, if it is intended to operate the oscillator on the high-frequeney side of the signal im accordance with usual practice, the bandsetting condenser should be decreased in capacity until the second strong signal is heard. When the signal is properly located the mixer grid should be next timed to resonance by adjusting its padder con- denser for maximum signal strength. After the high-frequeney end of the band has thus been located the receiver bandspread condenser should be set at maximum capacity and the signal generator slowly tuncd toward the low-frequency end of its range until its signal is again picked up. Jf the bandsprend adjustment happens to be correctly made, which is not probable, the signal generator ealibration will show that it is at the low- frequency end of the desired tuning range. Tf calibration shows that the low-frequency end of the tuning range falls either higher or lower than what is desired, it will be ncces- say to make the required changes in the band- spread circuit deseribed under the section on Bandspreaã and repeat the checking'process until the tuning range is correct. Tracking. After the oscillator has been set so that it covers the correet range, the tracking of the mixer tuning may be tackled. “With the signal generator set to the high- frequeney end of the tuning range and loosely eoupled to the mixer grid the signal from the generator should bc tuned in on the receiver and the mixer padding condenser adjusted for maximum output. Next, both the receiver and the signal generator should be tuned to the low-frugneney end of the receiver's range and a check made to see if it is necessary to reset the mixer padder to secure maximum output. TE the tracking is correct it will be found that, no change in the padder capacity will be neces- sary. If, however, it is found that the out- put may bc increased by retuning the padder it will be necessary to readjnst the mixer bandspread. An increase in signal strength with an in- ercase in padding capacity indicates that the bandspread is too great and it will be neces- sary to increase the tuning range of the mixer. An increase in sigual strength with a decrease in padding capacity shows that the mixer tuning range is too great and the bandspread will have to be increased. . When the mixer bandspread has been ad- justed so that the tracking is correet at both ends of a range as narrow as an amateur band, it may be assumed that the tracking is nearly correct over the whole band. The signal generator should then be transferred to the grid of the rf. stage, if the receiver has one, and the procedure described for tracking the mixer carried ont in the r.. stage. Series Tracking Condensers, The above discussion applies solely to receivers im which a small tuning range is covered with each set of coils and where the ranges covered by the oscillator and mixer circuits represent nearly equal percentages of their operating fre- quencies, i.e., the intermediate frequeney is low. When these conditions are not satisfied, such as in continuous-coverage receivers and in receivers in which the intermediate fre- queney is a large proportion of the signal frequency, it becomes neçessary to make special provisions for oscillator tracking. These provisions usually consist of ganged tuning condensers in whieh the oscillator sec- tion plates are shaped difterently and have a different capacity range than those used across the other tumed eircuits, or the addi- tion of a “tracking condenser” in sorios with the oscillator tuning condenser in conjune- tion with a smaller coil. While series tracking condensers are scl- dom used in home-constructed receivers, it may somctimes be necessary to employ one, as in, for example, a receiver using a 1600-kc. if. channel and covering the 3500-4000 ke. amateur band. The purpose of the series tracking condenser is to slow down the oscil- lator's tuning rate when it operates on the high-frequency side of the signal This method allows perfect tracking at three points throughout the tuning range. The three points usually chosen for the perfect tracking are at the two ends and center of the tuning range; between these points the track- ing will be elose enough for all practical pur- poses. In home-constructed sets the adjustment of the tracking condenser and oscillator coil in- ductanee is largely a matter of cut-and-try, requiring a large amount of patience and an understanding of the results to be expected when the series capacity and the oscillator inductanee are changed. Receivers with AV.. When lining up à receiver which has automatic volume con- trol (a.v.e.), it is considered good practice to keep the test oscillator signal near the thresh- old sensitivity at all times to give the effcct of a very weak signal relative to the audio amplificr output with the audio gain control on maximum setting. Testing. In checking over a receiver, cer- tain troubles are often difficult to locate. 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