US2144304A - Delayed automatic gain control circuit - Google Patents

Description

R. A. BRADEN 2,144,304

DELAYED AUTOMATIC GAIN CONTROL CIRCUIT Filed Nam-19, 193e @wwiwlv ff@ lm A l m Jan. 17,1939.

Patented Jan. 17, 1939 PATENT AOFFICE DELAY-ED" AUTOMATIC? GAIN (L'ONTROL` CIRCUIT' Rene A Braden, Collingswood; N J assigner to ItadioOorporatibnof America, a corporation of Delaware.

Application November 19, 1936-, Serial-No. 111,569

7 Claims. (Cl: Z50-20)' My present invention relates to automatic gain` control; arrangements for radio' signalling systems, and: more particularly to a novel' andi improvedi type of delayed automatic volume control circuit for a radio receiver.

One of the main objects of my present invention is to provide an automatic volume control arrangement for a radio receiver, wherein Vthe volume controll action is delayed' until: signals of a predetermined amplitude are received, the volume control?` circuit essentially comprising adevice of uni-directional conductivity whose conductivity is directly controlled by a signal. rectiler of the diode type.

Another important object of the.` invention may be stated to` reside in the provision of a delayed automatic volume' controlv circuit for a radio. receiver, wherein the control circuit comprises a diode rectiiier` circuit adapted to. render a second diode circuit operable and conductive upon receptionl of signals above a predetermined amplitude, the current flow` in the second diode circuit being utilized to provide thevolume controly bias'.

Still other objects of my invention are to improve generally the efliciency of operation ofdelayed automatic gaincontrol? circuits for radio receivers, and more especially to provide such control circuits in an economical and reliable construction capable of readyassemblyin radio receivers of present day design.

The novel features which I believe to be characteristic of my invention arer set forth inr particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which'Ihave indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to theaccompanying drawing, there is shown in the latterv a superheterodyne receiver of a well known type. The receiver embodies the usual signal collector A, such as a grounded antenna circuit; the latter impressing the collected signals upon oneyor more, stagesr of tunable radio' frequency amplification. The radio frequency amplifier is denoted by the refer-'i ence numeral I, and' the tunableinput circuit, thereof includes the tuningcondenser 2. 'i The grounded cathode circuit of amplifier l may include the customary biasing network 3 which` provides initial bias for the signal` grid of the am.. plirler, and the amplified `output of amplifierll is impressed uponthe tunable input circuit 4 oi'.

the mixer; or convertenstage. The variable; tuning, condenser 5 of" the input circuit 4 has* its rotors' arranged for uni-control adjustment with the rotors. of' condenser 2. The dottedline Bwillf be understood as designatingthe uni-control adjusting elementof the receiver tuning mechanism. The converter stage has been conventionally representedybut the reference numeral 1 is to4 ber understood as designating the usual rst detector and local oscillator circuits which are provided in present day superheterodyne receivers. i

For example, the numeral 1" may designate a tube of theZA'? type whose circuits are arranged to enable it` to function as a pentagrid converter.- In such case it Will= be understood that the condenser rotors of the oscillator` tank circuit and detectorinput circuit willbe arranged Vforunicontrol by the element 6. Of` course, and as is well known, the 'network' 1- mayemploy separate first detector and-local oscillator tubes. Thesev circuitsl are` so=well known to-those skilledin the artV thatl further descriptionis not believed neces-y sary. It is merel-y essential to point out that in the output circuit 8 of the converter network there appears signal energy of intermediate frequency; and the circuit 8t is tuned to the operating IF. Thev latter may be chosen from a range of fre uency values of 75to-500 k. c. Of course,

higher requency ranges may be used. The IF amplifier may comprise as many stages as `desired, and the numeral 9 denotes conventionally one, or more, such stages of IF 'amplication TheIF-tuned input circuit l0 is'reactively coupled to the output circuit 8- of the converter network, and the resonant output circuit Il., tuned tothe IF, is reactively 'coupled to the resonant input 'circuit I2 of the last stage of IF amplifica:- tion I3.

'Ihecircuit IZ is tuned to the IF; and the reso# nant output circuit I4 of amplifier I3 is similarly tuned to the operating IF; The numeral I5 denotes a signal grid biasing network disposed in the grounded cathode circuit of amplifier I3, and it will be understood that similar biasing net- 45' Works are employed to provide the normal signal grid bias in each of networks 1 andA 3. The tuned input circuit I6 of the second detector stage l1` is reactively coupled to the output circuit, and' 50 is also resonated to the operating IF. The detected signals of the network Il which includes a signal detector of suitable type such as a diode, may be impressed upon one, or more, audio frequency ampliers, and the amplified audio curu,

55 tube I 9. The signal grid I8 is connected to the.y

after referred to as AVC) is employed. The function of this volume control circuit is to vary the gain of each of the amplifying tubes in suchl a manner that the signal carrier amplitude at the input circuit I 6 of the second detector is maintained substantially uniform over a Wide range of signal carrier amplitude variation at the signal collector A. Early AVC systems were aected by the modulation of the carrier which resulted in compression of the Vvolume range of the received signal.

diode instead of a triode, or pentode/rectifier to generate the AVC bias. Systems Whichget audio voltage and AVC voltage from the same diode have several faults. voltage is not large enough, or does not increase rapidlyV enough with increase in signal at the diode. This can be partially corrected by driving the diode harder and introducing delay. Delay in this circuit results in amplitude distortion, and it is Ydifficult to get enough delayY to make the AVC characteristic really ilat. A better scheme is to use separate diodes for signal and for AVC, working at a fairly low signal level,'and amplifying'thefAVC diode output with a direct current amplifier.V` Any amount of delay can then be introduced. The principalbad featureof this circuit is that it almost invariably motorboats. A minor drawback is that the SPU-'hasto be connected to give both negative and positive voltages with respect to ground.V A less troublen some procedure is to'tap off signal at the second detector, and pass it through an i111'v amplier followed by a diode detector. Most circuits using this arrangement attempt to apply the Anecessary delay to this signal-rectifying diode;V This results in dependence on the modulation,` if the delay is large enough to be Vvery effective 'in nattening the output variationV curve. Mypresent arrangement applies the delay to a second, non-'rectifying diode, and thus largely Yavoids effects of the modulationV on the AVC voltage. Considerable help in this direction is given by the reduction in modulation whichy occurs in the output crcuit of the rectifying diode, because of the long time Vconstant of this circuit. 1

Signal energy for the automatic volume con-- trol circuit is taken off from the'plate circuit of amplifier I3, and such IF energy is impressed upon the signalY grid I8 of the multiple function tap 2I on resistor 20, resistor ZII'being connected to ground through condenser IUI andA at i the other-end being connected to the plate side oi' output circuit I4 through condenser 22. Asubstantially fixed grid bias is maintained by connecting grid I8 through the high resistance |02 to the negative end of the cathode resistor 25. Thetube I9 is of the well known duo-diodepentode type, but the two anodes of the tube are strapped together and are conventionally represented by a single anode designated by the numeral- 23. As `is well known,such tube comprises a pentode section and two diodes, all three having their cathodes connected together,. but being otherwise independent.

. The plate of the pentode section of tube I S is connected to a desired point ofY positive potential through the primary Winding `T1 of a coupling transformer T. The secondary winding T2Y of the coupling transformer is connected in ors 25 and 21.

This is avoided .by using af K resistor Pare connected to points of predeter- Generally, the AVCy series between the cathode bias resistor 25 of tube I9 and the diode anode 23. In series between the cathode 24 and diode anode 23 there are arranged the resistor 25, shunted by condenser 26; the resistor 21, shunted by condenser 28; and the secondary winding T2. The junction of resistors 25 and 21 is adjustably connected to a desired point on potentiometer P, and it will be understood that the reference numeral 3U denotes a tap connected to the junction of resist- This tap can be adjusted to a desired point on potentiometer P which has a ,predetermined positive potential with respect to ground.

. 'I'he terminals. of potentiometer, or bleeder,

mined positiveand negative direct current po- 'tenti'aL the negative terminal being at ground potential. Those skilled in the art will readily understand that the bleeder resistor P may be included in the usual direct current power supply circuit of the receiver. A second diode device 40 has its cathode connected by a conductor 4I to the anode sideV of resistor 21. The anode of diode 40 is connected to ground through resistor 42,` a condenser 43 shunting the resistor 42 to ground.

The anode side of resistor 42 is conductively connected to the signal grid circuits of the variousfsignal transmission tubes whose gains are to be automatically controlled, by means of the conductor designated by the reference letters AVC.* The AVC or control circuit includes a filter network comprising a series resistor 50 and shunt condenser 5 I. An additional bias circuit vfor controlling the last IF tube is connected from a tap on grid circuit of tube I3. A circuit for controlling the converter tube is connected from the same tap, or a separate tap, as shown, on resistor 42, through lter IUE- I 01, to the signal grid circuit of the converter tube included in the converter system 1. Appropriate direct current blocking condensers are disposed in the input vcircuits of the controlled tubes as exemplified by condenser |04.

In considering the operation of the delayed AVC circuit disclosed herein, it is rst pointed out that the IF signal voltage is impressed on the signal grid` I8 of the pentode section of tube I9 by means of the tapped resistor 20. Tap 2| regulates the input signal voltage impressed on the gridof tube I9, which will generally be in the range from to 50% of the signal voltage developed across tuned circuit I4. The upper limit of permissible input is that at which tube I9 is on thev verge of overloading when signal collector A is picking up the maximum signal which the receiver is required to receive and control. The IF energyis amplified by the pentode section of tube I9, and is impressed on the rectifier circuit including diode 24-23, through transformer T. It isdesired thatthe selectivity at the diode anode 23 be nogreater than that applied to the signal detector I1. This is conveniently arranged by connecting I8 to primary circuit I4 rather than tofsecondary circuit I6, and by making T much less selective thanfcircuit I6, preferably by making T1 and T2 s'o large that they are approximately tuned to the IF by the distributed capacities plus the electrode capacitiesof the tube elements connected to them. The resistor 21, shunted by the condenser 28, functions as the diode load, and the cathode and anode electrode of the diode are both maintained at awcommon positive potential with respect to'ground by connection 30. The diode Since `diode 40and resistor i4f2- are cori'zreete'd across resistor 2l and to fgroundjthe vlt'fa'geEz,

tvpped-oii fromlacross bleederfresis'tor P, is in series withv the diode '40 in such a sensefa'sfto bias 'theanode of diode `40negativ`ely `ivi/"ith respect toits cathode. Thisresults in the'diode`i`40 being nonthe -fact that the direct current voltage developed across resistor L21 by l'the 4iio'w bf'irectid current therethrough, is opposite Lin `'polarity lto the voltage E2. f

due to the increase in IF voltage impressed 'beltween the diode electrodes 24-423 which `occurs if the signal applied to Ais gradually increased Ifrom a smallvaluath'e anode of diode "40 becomes AS the voltage drop across `resistor`21 increases,

Iless negative. Eventually it `"becomes -positive with respectto the cathode, Vwiithmthe "result that current begins to flow through "the diode v4!! and the load resistorf42. From 'thatlpointonftl'e direct current voltage across resistor"42increases in `step with that across resistor 2'! proportionally to the nCreaSein-input voltageimpressed on I9,

butl is smaller in magnitude than` the voltage `thevoltage drop across resistor 21. "Onthe other lhand, if the drop across `2" is smaller than Ez, "there is no voltage across 42.

It will, therefore, be seen thatthe signal grids *of the controlled signal transmission "tubes" do fnot have bias applied to them "from the AVC cir- `cuit until the diode becomes conductive, current begins to now "through resistor 42.*In other Words, the AVC actionis delayed 'uiitilyfth'e `signal. amplitude applied to the pentode section of tube I9 attains that value which `willcause the cathode of diodell to become negative with respect to the anode thereof.` The AVC bias applied to the controlled signal transmission tubes increases in magnitude as the` reeeivedsignal carrier amplitude increases. The 4VC bias is im.- pressed on the controlled stages Yafter suitable Vfiltration. The lterll-'l denotes a `lter network which is used in each ,of the grid 'connections, and the function of the filter .network is to suppress audio frequency fluctuations of ithe AVC bias. In practical use it might be desirable to insert a separate lter, similar to `--L-5'l, as to prevent feedback among Hthe various tubes connected to one point on 42. a

The condenser 43 should ordinarily Vbe very small, or zero, as its effect is to cause 4 rec'zi'iiica tion of the audio component of the voltage impressed on diode 40, andthe audio component may be quite large in comparison with the steady component. If the condenser 43 is too large the AVC bias will increase when the received signal is strongly modulated, and decrease a'sthe modulation decreases thereby reducing the fluctuation of the audio signal level. This last effect is not customarily desired, because it'aino'unts to a re` duction of the volume range of the output. Preferably, a fraction of the total AVC voltage devel-` oped across 42 is applied to theV last IF tube 3, since the gain of this tube must notbe reduced to such a degree that the amplied signaliin:l pressed on the 2nd detector, inaly'be'f the 'oranger 1oV tomeokvolts, ististbrted. Tube I3 is theref' controlled by the voltage "at a tap onresisto .f Thefarnountcf control (if any) "applied totlie signal circuit ofthe converter 1 :must also b'e `limitedl to a relatively vsmall amount, by similar connection, to avoid amplitude dist'ortion "of the 11F, output.

The diode `24%#23should be capable of generating steady voltage -of '50 to 100 volts across 'load resistor `2l. `'Ihe delay voltage E2 can then be 15 to 30 volts which leaves 35 to 70 volts for AVC faction. If only two or three tubes are controlled in gain, the` higher limit is satisfactory, but if live `or six tubes are `controlled or if `some of the amplifiers Vare of the 6L? or similar type with control applied to two successive grids, the lower value of control voltage would be'adequate for broadcastreceivers. The delay in the AVC action is adjustable by virtue of the tap 3i). Tap 36 should be so adjusted, in cooperation with tap 2l, that diode 4U becomes conductive at an inputsignal level slightly below that which establishesthe desired signal voltage at the second detector. Delay voltage can, alternatively, be increased to limit the noise produced between stations when tuning.

The condenser 2t maybe quite large so as to take out a great deal of the audiocomponent in the voltage impressedon diode 4D, the limit to the size of condenser 28 being determined by the magnitude of resistor 2l and b-y the permissible leg in the' operation of `the system. The larger the elements` 28 and 21, the greater will be the time lag between a change in signal level applied tothe pentode section of` tube `I9 and the resulting change in AVC bias.` The lag should be great enough so that the lowest frequencies which it is `desired to reproduce `in the signal output channel, including the second detector, will not cause fluctuation of the AVC bias, for this would causefthev output at these frequencies to be reduced, VThe permissible'and necessary delay must be divided between network 28-21 and 5th-5I in suitable proportions such that suitable lag is produced whilen audio frequencies` are prevented freniA reaching the signal grids of the controlled ons:` y y Y While I have indicated and described a system for carrying my invention into eifect, it will be apparent to one skilled in the art that my invention is by no means limitedrto the particular organization shown and described, but thatmany modifications may be made without `departing from the scope of my invention, as set forth in theappended claims.

What I claim is: x

" 1. In combination with a signal transmission tub`e,uja source of signal waves connected to 'the input electrodes of the tube, a signalutilization network coupled `to the output electrodes of the tube, a signal rectication network, responsive to waves impressed upon said Atransinission tube, for producing a direct currentvoltage varying in magnitude with the signal wave amplitude, a device of uni-directional conductivity, a source of j direct current voltage connected in circuit with Asaid device and the rst direct current voltage,the ltwo direct currentuvoltages being of opposite polarity andrendering said device nonconductive for signals of less than a predetermined amplitude, V'and an impedance in ,circuit withsaid device `which is adapted to havea direct `(rzurrent voltage ...developed thereacross when rst direct current Yvoltage exceeds the iiightud Of th rSfnlld voltage ad'said device rent voltage from the rectified signal current, a

lbecomes conductive, and means for impressing `the third direct current voltage upon said transmission tube as a gain control voltage.

2. In a receiver of the type provided with a signal amplifier and a detector, an automatic volume control circuit of the delayed type, said control circuit including a diode rectifier network coupled to a point between said amplifier and detector for the impressionof amplified lsignals on the rectifier network, a load resistor in the rectifier network adapted to develop a direct current voltage from the rectified signal current, a second diode, a source of fixed direct current voltage connected in series with said load resistor between the electrodes of the second diode, the two direct current voltages being in polarity opposition and rendering said second diode nonconductive for signals of less than a predetermined amplitude, a second load resistor disposed in the space current path of the second diode and adapted to develop a direct current voltage thereacross when said first direct current voltage exceeds said fixed voltage and renders said sec-l ond diode conductive, and means for impressing the direct current voltage across said second load resistor between the input electrodes of said signal amplifier in a sense to reduce the amplifier gain as the received signal amplitude increases above a predetermined amplitude.

3. In a receiver of the type provided with a signal amplifier and a detector, an automatic volurne control circuit of the delayed type, said control circuit including a diode rectifier network coupled to a point in the receiver for the im pression of signals on the rectifier network, a load resistor in the rectifier network adapted to develop a direct current voltage from the'rectified signal current, a second diode, a source of fixed direct current voltage connected lin series withV said load resistor between the electrodes of the second diode, the two direct current voltages being in polarity opposition and rendering said lsecond diode nonconductive for signals of less the second diode and adapted to develop a direct current voltage thereacross when said first direct current voltage exceeds said fixed voltage and renders said second diode conductive, means for impressing the direct current voltage across said second load resistor between the input electrodes of said signal amplifier in a sense to reduce the amplifier gain as the received signal amplitude increases above a predetermined amplitude, and additional means for amplifying the signal energy prior to impression upon the first diode rectifier network.

4. In a receiver of the type provided with a signal amplifier and a detector, an automatic volume control circuit of the delayed type, said control circuit including a diode rectifier network coupled to a point between said amplifier and detector for the impression of Vamplified signals on the rectifier network, a load resistor in the rectifier network adapted to develop a direct cursecond diode, a source of fixed direct current voltage connected in series with said load resistor between the electrodes of the second diode, the two direct current voltages being in polarity opposition and rendering said second diode nonconductive for signals of less than a predetermined amplitude, a second load resistor disposed in the space current path of the second diode and adapted to develop a direct current voltage thereacross when said'first direct current voltage exceeds said fixed voltage and renders said second diode conductive, means for impressing the direct current voltage across said second load resistor between the input electrodes of said signal amplifier in a sense to reduce the amplifier gain as the received signal amplitude increases above a predetermined amplitude, and means for adjusting the magnitude of said fixed direct current voltage thereby to delay the development of said gain control voltage.

5. In a receiver of the type provided with a signal amplifier and a detector, an automatic volume control circuit of the delayed type, said control circuit including a diode rectifier network coupled to a point in the receiver for the impression of signals on the rectifier network, a load resistor in the rectifier network adapted to develop a direct current voltage from the rectified signal current, a second diode, a source of xed direct current voltage connected in series `with said load resistor between the electrodes of the second diode, the two direct current voltages being in polarity opposition and rendering said :second diode nonconductive for signals of less than a predetermined amplitude, a second load resistor disposed in the space current path of the second diode and adapted to develop a direct (current voltage thereacross when said first direct lcurrent voltage exceeds said fixed voltage and renders said second diode conductive, means for impressing the direct current voltage across said :second load resistor between the input electrodes of said signal amplifier, and means in said gain (control connection for filtering the pulsating voltage components from the gain control voltage.

6. In `a receiver of the type provided with a signal amplifier and a detector, an automatic volume control circuit of the delayed type, said `control circuit including a diode rectifier network coupled'to a point between said amplifier :and detector for the impression of amplied signals on the rectifier network, a load resistor in the rectifier network adapted to develop a direct current voltage from the rectified signal current, a second diode, a source of fixed direct current voltage connected in series with said load resistor between the electrodes of the second diode, the two direct current voltages being in polarity opposition and rendering said second diode nonconductive for signals of less than a predetermined amplitude, a second load resistor disposed in the space current path of the second diode and adapted to develop a direct current voltage thereacross when said first direct current voltage exceeds said fixed voltage and renders said second diode conductive, means for impressing the direct current voltage across said second load resistor between the input electrodes of said signal amplifier in a sense to reduce the amplifier gain as the received signal amplitude increases above a predetermined amplitude, and means for adjusting the signal input to the control circuit in relation to the input to said detector.

7. In combination with a signal transmission tube, a vsource of signal waves connected to the input electrodes of the tube, a signal utilization network' coupled to the output electrodes of the tube, a signal rectification network, responsive to waves impressed upon said transmission tube, for producing a direct current voltage varying in magnitude with the signal wave amplitude, a device of uni-directional conductivity, a source of direct current voltage connected in circuit with said device and the rst direct current voltage, the two direct current voltages being of opposite polarity and rendering said device non-conductive for signals of less than a predetermined amplitude; and an impedance in circuit with said device which is adapted to have a direct current voltage developed thereacross when said first direct current voltage said device becomes conductive, means for impressing the third direct current voltage upon said transmission tube as a gain control voltage, said rectication network including an impedance across which said first direct current voltage is developed, and said device comprising a diode having its cathode connected to a point on said last impedance.' v

RENE A. BRADEN.

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