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May 3, 1938. P. o. FARNHAM 2,115,844 AUTOMATIC TRANSMISSION CONTROL Filed April '7, 1932 D_l >l-_Hlj A @§43 2 sneèts-sheet 1 May 3, 1938. 2,115,844 P. O. FARNHAM AUTOMATIC TRANSMISSION CONTROL Filed April '7, 1932 2 Sheets-Sheet 2 %\ 0225; gym/wm: @d w. Patented May 3, 1938 2,115,844 UNITED STATES PATENT OFFICE 2,115,844 AUTOMATIC TRANSMISSION CONTROL Paul 0. Farnham, Boonton, N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application April 7, 1932, Serial No. 603,849 19 Claims. (Cl. 250-20) This invention relates to automatic control cir which includes an automatic gain control for cuits of the type employed in wave transmission maintaining a substantially constant output for systems, particularly in radio receivers, and more all values of signal input above a critical value especially to methods of and circuit arrangements and an automatic transmission control for sup for suppressing all output from the transmission pressing all output for radio input voltages below system when the magnitude of the received Wave that level at which the noise output becomes sig or signal falls below a predetermined level. nificant in comparison to the signal output. The invention will be described in connection These and other objects and advantages of the with the reception of radio broadcast signals but invention will be apparent from the following it will be apparent that the invention may be ap specification when taken with the accompanying l0 plied to other types of signal transmission sys drawings in which: tems. Due to the high gain of receivers of the Fig. 1 is aschematic diagram illustrative of the automatic gain control type, the noise output is novel method of controlling the transmission undesirably high when the receiver is tuned off through a radio receiver; a carrier wave or when, through fading, the re ceived signal energy falls to a low value. A large part of this noise is “fluctuation” noise arising directly in or from the tubes and circuits at the voltages at the detector of the order of four or five volts may result from this cause. In addition to receiver noise, there is an often greater com stages of a complete radio receiver are repre power lines. For satisfactory reception, it Would be desirable to effect operation of an automatic gain control receiver in such manner that the audio output will be completely suppressed When ever the received signal energy falls below that level at which the noise output is significant in comparison to the signal output. A negative bias on the anode of a diode de tector will suppress all audio output until the peak input voltage on the diode exceeds the bias $3 Ol voltage but, unfortunately, the continued pres 'ence of the bias voltage required for best noise suppression may be so high as to produce some , ship between audio output and signal input for several adjustments of the circuit of Fig. 2. receiver, such as from electrical disturbances on n ‘ Fig. 3 is a curve sheet illustrating the relation beginning of the amplifying system, and peak ponent due to interference arising outside of the lil Ul Fig. 2 is a fragmentary circuit diagram of one embodiment of the invention, and In the schematic diagram, Fig. 1, the several N) 5 30 3 Oi particularly, its relationship to the automatic gain control unit J. distortion in the audio output. The gain control unit J comprises an automatic Objects of the present invention are to provide bias-developing agency taking its input from the ¿o improved methods of and circuit arrangements point l in the transmission system and furnish for controlling the transmission through an elec ing a control upon the radio frequency ampli trical wave transmission system. Further objects- ñcation of the system preceding its input point I are to provide methods of and circuit arrange through the channels a, b and c. Since the input ments for automatically controlling the transmis to control unit J is in the form of a modulated 5v sion in such manner that a substantially constant radio frequency voltage, and the output is usually and satisfactory output is obtained for all input in the form of a direct current bias suitable for voltages above a critical value, and the output is application to the vacuum tubes of the preceding completely suppressed for all inputs below that stages, it is apparent that the operation of the critical value which will produce a satisfactory unit J involves the process of rectification though output. Further specific objects are to provide it may, in addition, include amplification pre a transmission system in two sections of which ceding or following the rectification. the gain is automatically varied in opposite direc In accordance with the invention, a second> tions by the incoming signal. More specifically, bias-developing agency K is connected to the an object is to provide a transmission system, for .example a radio receiver of the high gain type, 20 sented by the units A to H which are arranged between the antenna or input system and the reproducer l. These stages may be combinations of radio frequency amplifiers, a detector and audio amplifiers, or certain of the stages may be fre quency changers and intermediate frequency am pliñers such as are employed in superheterodyne receivers. The design of the several stages and the general method of adding an automatic gain control system, indicated as an auxiliary unit J, to receivers of various types are well known. The present invention relates particularly to the provision, in such a system of the automatic gain or transmission control unit K, and more v 40 45 ` 50 transmission system at some point 3 which may,v in one special case, be at the voltage level point l _55 2 ’2,115,844 but which, in the general case, may precede, coin cide with, or follow the point I. The output from the unit K is employed through one or more channels e, f, to control the transmission of the . Cn diode rectiñer of control unit K will not develop a direct current output until the peak input Volt age E1 exceeds the bias Voltage Eg. The output circuit of the demodulator includes receiver system subsequent to point I, and in a radio frequency choke L1 and an output resist such manner that, for radio frequency voltage levels E1 at point I less than a critical Value, the unit K does not permit transmission through the remainder of the system to the audio output at reproducer I. In general, a suppression of trans-` mission at some point subsequent to the auto matic gain control connection I is to be preferred .to a suppression at a preceding point and, for insuring a sharper cut-oí, it is desirable to have the control unit K permit free` transmission of ance R3, the low potential terminal of the output resistance being grounded through an> audio fre quency by-pass condenser I8. To suppress the audio output for weak input voltages, a delay bias voltage is provided between the cathode C 10 and the grid G1 of the demodulator and this bias voltage is controlled automatically by the control unit K. The direct current potential developed across resistance R2 by the diode rectifier C-G2 is impressed upon the control grid of a direct 15 signals, with increasing radio frequency input, current amplifier I9 which has output circuit before the normal gain control of the unit J is initiated. In the particular circuit arrangement shown resistors R4, R5 in the plate and screen grid cir cuits, respectively. Positive potentials are im pressed upon the plate and screen grid by a lead 20 which connects the junction of the resistances 20 in Fig. 2, the two control units J and K are con~ nected to the transmission system at the same point I, which point coincides with the output voltage across the tuned circuit of unit E of the receiver system as shown diagrammatically in Fig. 1. Dio-de rectification is employed and, for economy and simplicity, the rectifier elements of .the units F, J and K are enclosed in a common envelope, i. e., the multiple element tube I0 func tions as three diode rectiiiers in parallel. R4, R5 to the point Y on the direct current source I4. The low potential terminal of the audio out~ put resistance Ra is connected to the plate of tube I9 through a filter resistance 2|. A‘tap 22 on the audio output resistor R3 per 25 mits any desired portion of the available audio output .to be impressed upon the amplifier tube 23 >of the succeeding unit G of the system. The cathode of the amplifier tube is connected by 30 lead 24 to a point on the direct current source I4 30A nal through a condenser I2. Tube I9 may be a special tube having three anodes but a commer cial screen grid tube, with a control grid G1 and screen grid G2 may be employed. The elements 40 will be designated as grids in the following de of the point Y and the grid of the tube is con nected, through a filter comprising resistances 25 and condenser 26, to the screen grid of the direct current ampliñer I 9. A blocking condenser 21 35 is included between tap 22 and the grid to isolate the grid from the direct current potential of the resistance R3. Audio frequency by-pass condensers 28 are pro vided between the anodes of the two control unit 40 rectifiers and the anode G1 of the audio frequency The radio frequency voltage E1 developed across the input impedance il by the amplifier action of the preceding stages is impressed on tube Ill by connecting the high signal potential terminal of impedance H to the cathode C of 35 the tube, and grounding the low potential termi scription but it will be apparent that the elements function independently as anodes for rectifica tion. Grid G1 and cathode C constitute the audio 45 frequency rectifier or »demodulator of the system, plate P and cathode C constitute the rectiñer of the automatic gain control unit J, and grid G2 and. cathode C constitute the rectifier of the transmission control unit K. The direct current circuit of the gain control 50 rectifier is completed by the output resistance R1 which is connected between plate P and ground, and a connection I3 from the low potential ter minal of the input impedance Il to a point of 55 positive potential with respect to-grouncl, such as point Y on the Voltage divider resistance I4 of a rectifier-filter system (not shown) which is en ergized, as is common practice, from a source of alternating current. With this arrangement, the 60 cathode C is more positive than plate P by a steady delay-bias voltage Ep, andno rectification takes place until the peak input voltage E1 eX ceeds the bias voltage Ep. When the input volt age E1 exceeds the bias voltage Ep, the rectified 65 voltage developed across resistance R1 is returned, through a filter comprising resistance i5 and condenser I6, to a preceding stage or stages as a gaincontrol voltage. The direct current circuit of the control unit K 70 includes the resistance R2 which is connected between the grid G2. and a point S, preferably adjustable by tap I‘I, on the positive potential source I4. The direct current potential of point S is less than that of the cathode connection point Y by lan amount Eg, andY therefore the which is at a greater positive potential than that rectifier. In the absence of a rado frequency input at theA antenna, the plate current flow in the direct cur rent amplifier I9 is high and the delay bias volt 45 age on grid G1 is substantially the same as the delay bias voltage Eg. So long as the peak input voltage E1 to tube I0 is less than the delay bias Eg on the grid G2, this condition obtains and the high bias on the demodulator renders the transmission 50 system inoperative beyond the point I of the schematic system of Fig. 1, i. e., the tube I0 of Fig. 2. For peak values of E1 less than the delay bias Ep on the gain control unit J, there will be no reduction in the gain of the radio frequency 55. amplifier system preceding the point I. For val ues of E1 greater than Eg but less than Ep, a direct current potential is developed across the resist ance R2 of the control unit K, thus reducing the plate current flow in the resistor R4 of tube I9 60 which reduces the delay bias between the demod ulator cathode C and grid G1. This permits the transmission system to regain its normal opera tion beyond the point I or tube IU and up to the blocking condenser 21. 05 The audio transmission is also blocked, for Weak signals, by the high negative bias established on thegrid of the amplifier tube 23. As the value of the input Voltage E1 increases in the range be tween Eg andrEp, the reduction of current flow 70 in the screen grid system of the direct current amplifier I9 decreases the bias on tube 23 to per mit its operation as an audio amplifier. The cir cuit elements comprising the control unit K there fore suppress Vall audio transmision through the 2,115,844:> system for weak signals but restore the normal transmission characteristics for rectiñer- input voltages E1 above a predetermined value Eg. Ad justment of the point S along the resistor I4 makes possible a selection of the carrier level at the antenna below which no receiver output is obtained. The effect of the two automatic control units on the transmission through'the receiver system 10 will be apparent from the several curves of Fig. 3, which curves shown the relationship between an tenna input and audio output, at reproducer I, for several values of delay bias voltage Eg. In obtain ing the data for these curves, the carrier modula tion was set at 10% and the tap I'I was adjusted along the direct current source I4 to vary the delay bias Eg on the rectifier `element of unit K between zero and a maximum of 40 volts, the bias for each curve being indicated by the legends 20 such as “Eg=10” appearing adjacent the curves. The delay bias on the automatic gain control unit J was such that no gain control voltage was devel oped for antenna' input voltages less than about 15 microvolts. 25 The typical output-input characteristics of pre vious types of sensitive automatic gain control receivers are Well indicated by the curve for zero transmission suppression bias. It will be noted that the audio output does not fall 01T uniformly 30 as the antenna input decreases below the critical value at which the automatic gain control comes into operation. In the range of very Weak signals, the total audio output decreases at a relatively low rate, the greater part of the output being due 35 to fluctuation noises arising within the receiver. Reception over the range of antenna voltages up to about 10 microvolts is not satisfactory due to the noise component and, over the greater part of that range, it is not possible to distinguish the sig 40 nal output as it is completely masked by the noise output. The total audio output in this range of signals is reduced when the delay bias Eg on the control unit K is adjusted to suppress transmission for very weak signals. The curve for Eg=l0 volts shows that the audio output decreased rapidly, With decreasing signal strength, for carrier inputs of less than 4 microvolts. Increasing the delay bias Eg to l5 volts provided a much sharper cutoff 50 for inputs below '7 microvolts, the remaining curves showing sharp cuto?fs at higher carrier in puts. The curve for the value of Eg=30 volts in dicates that all transmission beyond the control point was suppressed until the automatic gain control through the unit J was fully established, i. e., substantially constant audio output is ob tained for all ranges of antenna> input voltages above that critical value corresponding to a recti fier input voltage E1 suñicient to initiate the re 60 moval of the blocking action of the control unit. An examination of the curves for delay bias voltages of 30, 35 and 40 volts will show that, in terms of increasing antenna input, the effect of an increase of the delay bias from 35 to 40 volts has more effect than an increase from 30 to 35 volts. This results from the fact that the automatic gain control unit J is functioning, in the upper half of this range, to prevent the rectifier input E1 from increasing at as rapid a rate as the increase in the antenna input. The several curves illustrate the varying trans mission characteristics which may be obtained by adjustment of the critical input voltage below which the audio output is reduced or eliminated. 75~ The appropriate adjustment for any given re 3. ceiver will depend upon the absolute magnitude of the iiuctuation noises arising in the receiver and the noises arising from outside disturbances. With the receiver for which the curves of Fig. 3 were plotted, a delay bias Eg of l5 volts was suin cient to suppress receiver noises and the suppres sion of additional noises due to local disturbances may be effected by increasing Eg to 30 volts or 35 volts. i It will be apparent that embodiments of an automatic transmission control system such as shown in Fig. l may take other forms than that shown in the circuit diagram oi Fig. 2. As stated above, either or both control units may employ ampliñcation before and/ or after rectification. 15 As shown in Fig. 2, the unit K includes the recti I'ler C-Gz and the direct current ampliñer but it will be apparent to those familiar with the design of wave transmission circuits that the tube I9 may be made a rectifier by appropriate changes in 20 the circuit connections. ' In its broader aspects, the invention comprises the addition to a transmission system of two gain control units which automatically determine the> transmission through the system. With such 25 control units, two'separate portions- of the trans mission system are normally biased, i. e., in the absence of a radio frequency input, in opposite senses, one portion being biased for maximum gain while a second portion is biased to suppress transmission. The control units are preferably connected across the system at a point intermedi ate these sections, and with increasing input volt ages, the control units operate in opposite sense to reduce the normal gain of one portion of the 35 system and to remove the blocking action normal ly impressed on the other section. The net eiTect of the two controls is to maintain a substantially constant output for all signals of an order sum cient to produce a satisfactory output, and to eliminate all output for signals of lesser magni tude. Itis to be understood that there is a consid erable latitude in _the design of the control units and, obviously, in the types of transmission sys 45 tems in which such units are incorporated. I claim: „ i. In a carrier Wave receiver, the combination with a diode demodulator, and means impressing a bias potential on said demodulator to prevent the development of an audio output voltage until the impressed carrier voltage exceeds a predeter mined value, of means for removing said bias potential as said impressed carrier voltage in creases above said predetermined value and ad 55 ditional means for adjusting said value. 2. In an electrical wave transmission system, a double diode rectiñer having a cathode and two anodes, a single input impedance for said rec tifier, circuit elements connected to the respec 60 tive> anodes to constitute substantially independ ent audio frequency vand direct currentv output circuits, means for impressingy a negative poten. tial on one of said anodes to prevent the develop- ' ment of current flow in the output circuit 'there 65 of until the impressed radio voltage exceeds a predetermined magnitude, and means auomati cally varying said impressed negative potential with variations in the radio voltage impressed on said rectifier and means for selecting the radio 70 voltage value at which said last means is op erative. ' 3. In a transmission system, the combination with a demodulator having an anode, of means for impressing on said anode >a negativedirect 75 l 2,1 15,8442 current. potential, of means automatically reduc ingsaid impressed potential. to a negligible value when the received .signal energy exceeds a pre determined level and additional means for select. . ingthe signal energy level .at -which said auto matic means becomes operative.. 4. In. a radio. receiver, the combination with. a radioy frequency ampli?er, andan audio fre quency amplifier, of a demodulator connected 10: between said amplifiers, and means including a pairof control units for automatically control lingthetransmission through the respective am plifiers in opposite sense and. asa function of the received radio energy and one of said control 15; units having means to control the demodulator operation. 5. The invention asset forth in claim 4, where in said controlunits eachzinclude a rectifier, and one of saidrectifiers ris in parallel with said de .; modulator. 6. A radio receiver of thetype including a radio. frequency amplifier, a detector, control current. .voltage .developed by? the: second dioder across its. associated output resistance and means for .preselecting the radio voltage-level'at. which said seconddiode isabletorectify. - . 11..A= radio :receiver as claimed in'claim 10, Ul wherein .said last means comprises a second tube having a cathode cooperating with a control grid and.ianode,~-and means .impressing between the said control grid and cathode the direct current. voltage developed across the output resistance of 10 the second diode, the said biasing resistance being connected between'the cathode and anode of the second tube. 12. Aradioreceiver as claimed in claim 10,1.' wherein saidlast means includes a direct current amplifier, and mea-nsimpressing upon the same the direct current .voltage developed by said sec ond diode, thesaid biasing resistance being» in‘ eluded inthe output circuit of said direct `current amplifier. . ;quency voltage rises above a critical value, means normally. biasing. said detector to suppress all ode‘for automatically controllingthe gain of said amplifier in accordance with variations in the strength. of received signals, and biasing means including a third diode Yfed with signals by said automatically to remove the suppression bias on tude. of received signals in that sense which ren said detector when the received radio frequency voltagefexceeds a critical value, characterized by meansv for adjusting .the critical voltage value at. ders said‘demodulator diode inoperative for Weak signals and operative for signals in excess of a threshold value. means operative vautomatically to adjust the gain ofthe amplifier when the received. radio fre 20. 13. In. a-Lradio receiver, a. radio. amplifier, a; demodulator diode, .means includinga Ysecond di audiooutput, and;control means independent-of amplifier for impressing .upon-said-demodulator the action of the iirstcontrormeans operative.: diode a bias voltage that varieswith the magni which one of said` control means becomes op erative independently >of the critical voltage value 1 of the second controlmeans. 14. A radioireceiver `as» claimed in claim 13, in. combination vwith means adjustable to control'the threshold value below which said biasing means 7. The invention as set forth in claim 6, where in. said adjusting. means. regulates the critical value ofthe radio frequency voltage at -which said suppression control means becomes ,opera renders said demodulator diode inoperative. tive to remove the suppression bias from said de. means normally 'impressing a bias voltage on said 40 tector. 8. In a radio receiver, the combination with a radio. frequency amplifier, a detector, an audio frequency amplifier, means automatically con trolling the gain of said radio frequency ampli fier, control means rendering said .gain control means inoperative when the received radio fre quency voltage lfalls below a critical value, means normally biasing said detector and audio ,fre ,1. quency amplifier to render the same inoperative, and means operative to remove said normalbias when .the received radio frequency voltage rises above a critical value, of meansfor adjusting the critical voltage value at4 which one control means .ibecomes operative independently of the critical voltage value of the. othercontrol means. 9. The invention as set forth'in claim 8,. where in said. adjusting means regulates. the4 critical. voltage value at which said second control means . 15...*In ,a radio receiver, the combination with a radiofzfrequencyamplifier, a demodulator stage, and a .stage'of audio frequency amplification, amplifier to produce maximum gain, and means normally impressing upon:.one of said stages-a bias voltage of a magnitude eiiective to prevent transmission therethrough, of control means op erating automatically-when the receiver input 45 exceeds a predetermined value to maintainthe amplified Voltage level at a point in said ampli’ fier at a substantially ñxed'value over a wide range of `¿received‘signal voltages, andcontrol means operable automatically when the received radio frequency voltage rises to a critical value to re move .said transmission-preventing bias voltage, wherein each of said control means includes a rectifier and means impressing thereon a bias voltage which rendersl the rectifier :inoperative until the peak radio input to therectifier exceeds the bias voltage thereon,` the biasing means of the rectifier of usaidiirst control -means impressing thereon a biasvoltage less than that impressed on 60 the .other rectifier by itsbiasing means. 16. Ina radio receiver, the combination with a said amplifier. . 10. In a radio receiver, the` combination With. radio frequency amplifier, a diode detector anda diode rectifier vonlwhich radio frequency volt a radio amplifier, of transmission control means comprising .a tube-having a cathode cooperating. ages are> impressed .by said amplifier, and means Götwith a pair of anodes to function as two diodes, including a resistance in the circuit of said de 65 input circuit means impressing uponeach diode tector for applying thereto a delay bias voltage to a radio voltage developed by said radio amplifier,` render the detector inoperative for. radio input an output resistance for each diode, and means voltages. below acriticalvalue, of means includ impressing upon the first of said diodes a bias ing said rectifier and a direct current amplifier for `voltagetending to prevent rectiñcationthereby; establishing a current flow in said resistance to 704 702 reduce said applied .bias voltage when the radio said biasingmeans comprising a biasing resist ance between the cathode and the anode element input voltage on said »rectifier exceeds a predeter-. of the said-ñrst diode, andmeans forxpassing mined valuefin combination with a third rectifier through saidbiasing resistancea direct current for automatically controlling the gain-of saidam'e. ,which varies -as a .function .of the. rectified` direct pliñer in. accordance « withÀ variations in the ûOárbecomes operative toremove >the normal biasv on 5 2,115,844 strength of the radio frequency voltage impressed output terminals of said radio frequency amplifier, on said receiver. one rectifier developing an audio frequency volt age for application to said audio amplifier, means 1'?. In a radio receiver, the combination with a radio frequency amplifier, a diode detector and a Ul diode rectifier on which radio frequency voltages are impressed by said amplifier, and means in cluding a resistance in the circuit of said detector for applying thereto a delay bias voltage to ren der the detector inoperative for radio input volt 10 ages below a critical value, of means including said rectifier and a direct current amplifier for estab lishing a current flow in'said resistance to reduce said applied bias voltage when the radio input voltage on said rectifier exceeds a predetermined 15 value, in combination With an audio frequency amplifier Working out of said detector and means impressing thereon a bias voltage to render the same inoperative, and wherein said direct current amplifier automatically removes the bias voltage 20 on said audio frequency amplifier when the radio input voltage on said receiver exceeds a prede termined value. 18. In a radio receiver, the combination with a radio frequency amplifier and an audio frequency 25 amplifier of three rectiíiers having their input terminals connected in parallel and across the energized by the direct current voltages developed by other rectiiiers for automatically controlling 5 respectively the radio frequency and the audio frequency transmission through said receiver, said first rectifier being normally biased to prevent operation thereof, circuit elements cooperating with said means to render said first rectifier op 10 erative when the amplified radio frequency volt age impressed upon one of said other rectifiers reaches a predetermined value. 19. In a radio receiver, the combination with a diode demodulator, of means impressing a delay 15 bias voltage on said demodulator, and means for automatically varying the said bias voltage as a function of the radio voltage input to the de modulator, said automatic means including a rec tifier and direct current ampliiier for developing 20 a direct current voltage of greater magnitude than that obtainable by rectification of the radio volt age on said demodulator, and means for adjust ing the radio input level at which said rectifier is operative. 25 PAUL O. FARNHAM.