US2206181A - Remote control for radio receivers - Google Patents

Description

July 2, 1940- w. E. GILBERT 2,206,181

REMOTE CONTROL FOR RADIO RECEIVERS V Filed latch 14, 1938 3 Sheets-Sheet 1 July?, 1940. w E BERT 2,206,181

REMOTE CONTROL'` FORy `RADIO RECEIVERS Filed latch 14,. .1938 v 3 Sheets-Sheet 2 'MIM July 2, 1940 w. E. GILBERT 2,206,181

REMOTE CONTROL FOR RADIO RECEIVERS Filed March 14. 193s s sheets-sheet :s

`ies

tune-dto any desired signal. `understood by those skilled in the art, the signal Patented July 2, 194@ UNITED sTATEs VPAT ENT ,ioFFicE REMOTE ofoNrRoLFoR 'immo aEcEIvEas Walter E. Gilbert, Phi

ladelphia, i Pa., assigner,

`by mesne assignments, `to `Pennsylvania `Patents, Inc., Carson Cit Nevada tain complete control of his receiver while comfortably seated or located at a distance therefrom.. That the Volume level `may also be remotely controllable is of particular importance since it permits of` an accurate adjustment in accordance with the strength of the signal being received and the distance of 'the listener yfrom the receiver. It is further desirable that the remote control `unit incorporating these `features be capable of installation on all `types of `standard receivers with a minimum of laboran'd With- `out` the `necessity of making changes in fthe `receiver circuits.

Preferably the present invention is embodied in a remote control unit of the type employing a frequency converted, including, for example, a tube knownin the art asa detector-oscillator. Signals from a suitable antenna'may be supplied tothe converter through a transmission line. The` input section of the converter may thenlbe In a `manner well generated by `the oscillator section rof the converter heats with the desired incoming signaL. producing a signal whose frequency equals the sum, or preferably the diiference frequency, which may then be transmitted `'tothe receiver. `By properly designing the `circuit constants, this difference frequency maybe maintained substantially constant for any station tuned in. By previously adjusting `the timing of thelreceiver to this diiierenceafrequency, all tuning may be accomplished at the remote 'point bymeans'of `such converter system.

Owing to the automatic volume control systems generally usedat present, ordinary 'methods of `n'ianual volume control cannot be successfullyiemployed to control the volume level of a receiver from a remote` point.' Attemptsto fob- `tain such control solely byireducti'onof gain fat `the remote point are impractical, for thehighly ent automatic volume control means, "with :vtv i modern receivers Lareequipped,autorriati- `cally tend to counteract :any reduction infap- 4plied signal by an equivalent increase? inthe gain y, Nev., a corporation of issafseriai No. 195,395 (ci. 25o-+20) `or 'amplicationof the receiver. For example, inreceivers so equipped, it is not at all unusual for a 500:1 'change in input signal strength `t0 produce less than `a 2:1 changein detector output. lUnder these circumstances, ordinary remote operation would be highly unsatisfactory, "for, with a receiver operating at full sensitivity asa result of 'a weakenedsignal'at its input terminals, the noise pick-up `and inherentampliiier disturbance wouldbe highly objectionable, `and in extreme cases might be so great as to make reception ati low volume levels impossible..

Accordingly, the 'principal object of this invention is to provide a simple but highly eilicient and convenient remote control unit for radioreceivers,`l"which` embodies novel volume `control means `free of `the defects above-mentioned.

invention will be apparent from the following description and the accompanying drawings, in which:

Fig. 1 is a diagrammatic *illustration of a remote control unit `embodying the invention;

Other objects and other novel featuresl of this `Fig. 1A is a similarillustration of areceiveri with which the employed;

`Fig. 2 is a detail sectional `View of `a connector employedbytheinvention;

Fig. 43 is Aadiagrammatic illustration of a modification; and

Fig. '1 4 shows certain explanatory curves.

. Referring novvto Figs. -1 and 1A of thepdrawings, in Fig. `1 thereis shown a `remote control remote control unit may be lunitanda transmission vline for supplying sig-` nals to, and for controlling, a radio receiver such `as shown infFg. 1A.` For `'simplicity the several `*voltage @supply sourcesare indicated, but notv 'shown 4in detail. "However, the remote control unit, as Well vasth'e `radio receiver, may include @an individualvselfecontained power Asupply unit, as well understood. Incoming `signals from the `antenna I are impressed `on theiriput control lgrid of A"the converter `tube :2 through an antenna transformer 53. Frequency converter tube `2 includes` ianescillator -anodic lgrids and an oscillator control grid 5. Thevoscillator" circuits, and the *manner `in 'which the incoming frequency Iand local oscillatorl frequencyare mixed,

`therebygiving riseito -an output frequency equal 1 to fthe vdifferencerofl the first two frequencies, arewelllknjownfto .those skilled inthe art and vdetailed :description :is wdeerned unnecessary. The .secondary winding .of transformer `3, and the oscillator grid inductancet '.1 are ituned "by `1 condenser gang 1, controlling condensers 8 and 9, and these circuits are so designed as to give rise to a constant difference frequency which is substantially independent of the setting of condenser gang l. This difference frequency is preferably adjusted to the lower end of the radio receivers frequency range. A suitable frequency is 530 or 540 k. c., since at present these" are littleused channels at the low frequency end lof the range receivable on the standard broadcast band of most modern receivers. In the con'- verter plate circuit, there is 4provided a resonant load circuit I which is tuned to this difference frequency. A suitable transmission line Il is modulation of the signal being received, is applied over the transmission line -tofthe input terminals l2 which may be connected lto tlieterminals I3 of the radio receiver (see'Fig. 1AI).'

The radio receiver, ,having been vtuned previously to the difference frequency employed, will amplify and detect themodulated differencefrequency signal in the normal manner. In accordance with the present invention, remote control of the volumelevel at the receiver is provided by means which produce a reduction in gain both at the remote control unit and in the radio receiver itself. lt is furthermore a highly important feature of this invention that the manual volume control means is arranged cooperatively with the receiverls automatic volume control system to provide eflicient` remote manual volume controly and automatic volume control of the received signals. A voltage dividing network, consisting of resistor I4 and potentiometer l5, is connected across a source of bias voltage at the remote control unit. A manually adjustable bias voltage is supplied to lead I6 by means of the movablerpotentiometer arm. `When switch Il is closed, as shown, thecontrol bias is applied over conductor i8 and through resistor I9 to the control grid `of `an amplier tube 20 vlin the remotelyl located radio receiver,` as will be hereinafter more fully explained. The control bias may be supplied to any amplifier tube in thereceiver which normally obtains its biasfrom the receiver AVC system.` Preferably the bias may be applied to the first amplifier in the receiver.

In the ,drawings the 'remote control unit, embodying the present invention, is shown in connection with a representative form of superheterodyne radio receiver in a manner which will permit ofl highly-satisfactory remote operation,

,both as regards 'station selection and volume level control. f

A typical receiver may comprise a tunedv radio frequency stage employing a variable mu pentode tube 20 followed by a` combination detectoroscillator stage which may includea frequency converter tube 2l. The intermediate frequency output of this stage may be amplified by an intermediate frequency ampliiier which may employ The output of triode detector andA audio amplifier 23. One

`diode section including diode `plate 24 may be used as the second detector, while the other diode section which includes diode plate '25 may-be used inthe AVCl circuit. The triodesection including grid 26 and plate 2l may serve as an audio frequency amplifier, amplifyingv the audio output Athe several amplifier tubes. because no rectified current will flow in the AVC :characteristic curve.

have a small bias, the receiver gain will be maxi- Considering the AVC system, and disregarding the remote control unit for the moment, it will be seen that the path for the rectified current is as follows: from diode plate 25 through resistors 3| and 32, and the bias battery 33 to the point designated as ground, thence to the cathode 34 and back to the diode plate 25. The total AVC voltagewill appear across resistors 3l and 32, and a'certain'fraction of the total AVC voltage may be obtained by tapping olf from the junction 36 of the resistors 3| and 32. The AVC diode plate equal to the voltage of battery 33 which provides a suitable delay voltage and minimum bias for The .delay obtains circuit until the peak voltages impressed on plate `25 exceed the initial negative bias or delay voltage. When such a delayed AVC system is employed, no further self-bias is neededA for these Vtubes and their cathodes may all be connected directly to ground. In the receiver of Fig. 1A, all of the developed AVCv voltage may be utilized in controlling the gain of the radio frequency amplifier tube and the first .detector tube. ln the case curves S2 F2 and S3 F3 show the operation for .medium and large voltages, respectively. ln Fig.

4, the ordinate indicates the detected output voltage as measured, for example, at the detector and the abscissa indicates the input signal from the antenna. It will be understood, of course, that the curves are only qualitative as the tremendous voltage range of the input signal cannot be indicated conveniently on a linear scale.

As the arm of the potentiometer l5 is moved plied to the receiver will become more negative.

lThe resistors I9 and 3l', 38 of the AVC system act as a voltage dividing network and hence the several grids of. the receiver amplifier tubes as well asthe anode of the AVC' detector will become T50 more negative. Hence, the magnitude of the delay voltage will increase. Now considering the curve .Si F1, for small input signals, insuicient to overcomethe delay voltage, the input v. out- `25 is initially biased negatively by an amount l, away from the ground end, the D. C. Voltage supput curve will be given by the portion Si of the f Since the amplifier tubes mum and vhence the slope of S1 will be steep. In normal operation the signal at the detector will rectified voltage to be produced sufficient in magnitude to bias back the amplifier grids to maintain the output at a level F1 substantially independent of the amplitude of the input signal. If the voltage from the control point is increased, the starting bias on the amplifier tubes is made morefnegative. Hence the small-signal gain of the receiver is lessened as indicated by the less steep slope of. S2, and even though a larger signal lexceed the delay voltage and hence will cause all is required at the detector to overcome the del lay voltage, nevertheless the curve fiattens out'at .a lower output level such as F2. For larger control voltages the slope becomes even less steep as indicated by S3 and flattens out at an even lower amplitude wllvbe well in excessof the knee of Hence, good AVC action obtains.

It will be further appreciated that the usual stroyed, and that it will continue toieXert a compensating inliuence upon undesired fluctuations in recevedsignal strength. The effect of the remote manual `volumr': level control is` to establish thele'vel about which the AVC system functions,

`and to determine the maximum gain.

In addition to its function as part of a voltage which are present about lead I8 would serve to detune the resonant circuit lassociated with. the

control grid to which connection has been made.

Returning now to the voltage dividing network consisting of resistor I4 and potentiometer I5, it should be noted that the prsence of resistor I4 establishes a definite minimum bias which is lavailable to the lead i3 through the potentiometer arm. This is of particularimportanceinre- `ceivers whose controlledtubes are not supplied with some degree of `self-bias. The receiver of Fig.\1`A illustrates this condition. It will be apparent to one skilled in the art that if the potentiometer arm were permitted to move to a positionof Zero Voltage, the bias` effective on the grids of the controlled tubes for a condition of very weak signal `inightfall so low as to permit of the `flow of excessiveplate current, with a con.-

sequent reduction of "tube life, and in addition would `bias the grids 4to an unusable portion of theirlcharacteristic where cross modulation obtaillis;

In order :to accomplish-a more complete and satisfactory `controlfof the volume level, and to "aidfthe reoeivers AVC system in this function, it

is "provided, in accordanoewith this invention, `that a portion of the bias appearing at the potentiometer arm be employed to control the gain ofthe frequency converter tube 2. For` this purpose there is provided a voltage divider circuit comprising resisto-rs 39 and 4G serially connected shown.

`between the potentiometer arm and ground. 'Ihe "voltage appearing `at the junction of resistors 39 and 40 is applied through the secondary of transformer "3 to the input control grid of frequency converte-r tub-e 2. By a proper selection of values for theresistors 39 and 4l) it is possible to *eliminate any possibility of over-biasing the fre- "quen`cy oonvertertube '2. On the other hand, to

insure thatfthe frequency converter tube is at no timeunder-fbiased, thereis provided a self-b1-A *asing resistor '4I shunted by the bympass condenser '42.

fWhen tuning `a radio receiver by means of the remotecontrol unit the switches Il, 43 and 44, preferably ganged together, are `adjusted as are thrown :to their other positions. The result isitodisconnect the output winding of the remote Vcontrol unit from the transmission line Il and toconneot the antenna I directly to the transmission line through lead 45, `and also to opencircuit the lead I8. It is further desirable that 'the power supply switch of the remote control adapter, `placing socket trl on the grid cap and :placing the larger end of the adapter 52 in the "to the receiver without changing `or disturbing When, however, it is desired to operate "the `'receiver in the normal manner, these switches `resistors in the cathode circuits. glar lform chosen for illustration, bias for both `the detector-oscillator `tube 4l l and `the intermen sist, for example, `of a coaxial cable, or a twisted Wire line with `or without a metallic braid covering. It may include, if it is desired, in addition `cate the le`ad-inof antenna I near the radiating receiver,"a.nd to connect it to switch 43 by means of a. feed linedisposed in cable 45. Where this procedure is followed, it` is possible to make all external connections to the remote control unit through cable 46, thusproviding for increasedf/Ifg simplicity and portability.

In Fig. 2, there is shown a novel connector by means of which the conductor I8 and resistor I9 maybe `removablyconnected to the grid cap of tube 28. The grid cap is shown at 4l on the top of the tube. The resistance element I9 is disposed in fa hollow tubular extension 48 of an insulating socket housing 49. Within this nousing there are sockets 50 and 5I which are. connected together and to resistor I9. One socket receives the grid` `cap while the other receives the `adapterf52. The usual grid clip 53 is placed over the adapter. Thus the conductor i8 together with resistor I9 and usual input conductor 54 are removably connected to the grid ca-p lll.

The connector and adapter are constructed so thatthey maybe used with tubes having differn ent sized grid leaps, of which there are generally two sizes. By reversing the. connector and socket 50, the device may b-e used 'with tubes having smaller 'grid caps.

It will be seen that the invention thus provides for the `connection of the remote control unit l.

the receiver connections. By merely connecting the conductors l2 to the input terminals of the receiver, removing the grid clip of tube 2li, and making -theremovable connections shown in Fig. 2, the remote control unitmay be `easily con-- nected to the receiver.

n lan 'experimental remote control unit using a type lSABGT tubeias frequency converter, the following values were found to be satisfactory:

"50 Resistor 392333,00@ `ohms Resistor 4ll=25ll,000 ohms `liesistor I4=5,il(l0` ohms Potentiometer l5 -`2`5,0(l0` ohms A l Resistor `I9`=2,`000,000 ohms "55 Resistor 41:20u ohms sirable. Typical values for the resistors in the AVC system of the receiver shown in Fig. 1A

are `as follows:

Referring now to Fig. `3, there is shown a somewhat diiferent form of radio `receiverwherein the vcontrolled tubes are self-biased `by means of In the particudiate frequency amplifier 48 is obtained by means of the voltage drop across a single resistor 49 which is common to the cathode circuits of both tubes. The normal AVC voltage is developed across the resistor 50 which is connected between AVC diode plate 5I and ground. A desirable delayed AVC action is obtained by placing a small positive bias, or. delay voltage on the cathode 52 of the double diode-trodetube 53. When the remote control unitI is in operation the voltage effective in controllingthe gain of tubes 41 and 48 will have so-me value between the AVC' voltage developed across resistor 50 and the control voltage appearingl on lead I8. The magnitude of this voltage is dependent upon the relative magnitudes of the resistors I9 and 54 which form a voltage divider serially disposed between the two control voltages. i

In general, operation will be similar to that already outlined in connection with the explanation of Figs. 1 and 1A. The chief difference occurs in the reception of weak signals. InFig. 3 when the remote control unit is not in use, and for a condition Where signals are less than the minimum required to produce operation of the receivers AVC system, the control grids of the tubes 41 and 48 are at ground potential, and the effective bias is only that which is developed across the resistor 49. In this condition, sensitivity of the receiver is at a maximum. `Now when using the remote control unit with this receiver it would be most advantageous. to so design the bias supply vof the remote -control unit that the voltage brought out by lead I8 could be reduced to zero and thus permit the control grids to fall to ground potential, thereby securing the maximum receiver sensitivity. However, for reasons alreadypointed out in connection with Fig. 1, it vwould be distinctly disadvantageous to permit lead I8 to fall to zero bias where there is any possibility that the remote control unit might be used in conjunction with a receiver of the general form shown in that figure. Therefore, the minimum negative bias may preferably have a. value greater than zero but not so large as to appreciably reduce the sensitivity of receivers of the form shown in Fig. 3. A minimum negative bias of 1 volt has been found to be satisfactory. This value may be fixed by suitably selecting the magnitude of the resistor I4 of Fig. 1. There is thus provided a remote control unit which can be successfully used in conjunction with practically any standard type of commercial radio receiver for home use.

In a radio receiver constructed according to Fig. 3 both the resistors 50 and `5II had a value of 1 megohm, and the bias across resistor 49 for condition of no signal was about 3 volts.

While certain forms of the invention have been described for purposes of disclosure it will be apparent to one skilled in the art that the invention is by no means limited to the particular circuits herein described, but that modifications may be made without departing from the scope of the invention as set forth in the appended claims.

I claim'. i

1. A remote control unit for use in connection with a radio receiver having at least one space discharge amplifier including a control grid and an automatic volume control system connected to said control grid, said control unit comprising means for providing a manually variable control voltage source, a radio-frequency isolating resistance having one end connected to said source, and means for directly connecting the. other end of said resistance to said grid, whereby the gain of said amplifier may be varied Without deleterious effects from the automatic volume control system.

2. A remote control unit for use in connection lwith a radio receiver having at least one space vfor removably connecting the other end of said resistance to said grid cap, whereby the gain of said amplifier may be varied without deleterious effects from the automatic volume control system.

3. A remote control unit for use in connection with a radio receiver having at least one space discharge amplifier including a control gridand an automatic volume control system connected to said control grid, said control unit comprising means for providing a manually variable control voltage source, a space discharge device Ihaving a gain control electrode, means for supplying at least part of said control voltage to said control electrode, a radio-frequency isolating resistance having one end connected to said variable voltage source, and means for directly connecting the other end of said resistance to said grid, whereby the gain of saidl amplifier may be varied without deleterious effects from the automatic volume control system.

4. A remote control unit for use in connection with a radio `receiver having at least one space discharge device amplier including a. control grid and cap therefor and an automatic volume control system connected to said control grid, said control unit comprising means for providing a manually variable control voltage source, a space discharge device having a gain control electrode, means for supplying at least part'of said control voltage to said control electrode, a resistance connected to said source, and means for removably connecting said resistance to said grid cap, whereby the gain of said amplifier may be varied without deleterious effects from the automatic volume control system.

5. In a remote volume control system, a radio receiver including a signal amplifier Whose gain may be controlled by varying the bias voltage of at least one space discharge device in said amplifier, a diode having an anode and a cathode, means for supplying output signaly energy from said amplifier to said anode and said cathode, a resistance connected to the anode and cathode of said diode, circuit connections for supplying at least part of the voltage across said resistance to said amplier to provide a bias for the space discharge device, means for minimizingvoltage fluctuations in said bias voltage, a remote control unit, means at said unit for forming a manually variable control voltage source, a second resistv ance, and circuit connections between said unit and said amplifier for connecting in series relation the said second resistance, at least part of said first resistance, and said control voltage source, the polarity of the control voltage being such that the diode is rendered inoperative until the sig-nal energy supplied thereto'exceeds a threshold voltage and the gain of the amplifier is decreased by said control voltage as said threshold voltage increases.

l 6. In a` remote volume control system, a radio i receiver including a signal amplifier whose gain may be controlled by` varying the bias voltage of at least one space discharge device in` saidfampllfier, a diode having. an anode anda cathode, means for supplying output signal energy from saidamplier, to said anode and said cathode, a resistance connected to the` anode and cathode oisaid diode,` circuit connections for supplying at least part: of the` voltage across Said resist- `ance to said amplifier` to provide a bias forthe .and said control voltage source, the polarity of the control voltagebeing such that the diode is rendered inoperative until the signal energy supplied` thereto exceeds a threshold voltage and the gain of the ampliiier is decreased by said control voltage as said threshold voltage increases.

7. In a remote volume control system, a radio receiver including a signal amplifier whose gain may` be controlled by varying the bias voltage of for supplying at least part of the voltage across said resistance to said amplifier through said filter to provide a bias for the space discharge device, a remote control unit, means at said unit including a potentiometer and source of unidirectional current for forming a manually variable control voltage source, a second resistance, and circuit connections between said unit and said amplier for connecting in series relation the said second resistance, at least part of said rst resistance, and said control voltage source, the polarity of the control voltage being such that the diode is rendered inoperative until the signal energy supplied thereto exceeds a threshold voltage and the gain of the amplifier is decreased by said control voltage as said threshold voltage increases;`

8n In a remote volume control system, a radio receiver includingia signal amplifier whose gain may be controlled by varying the bias voltage of at least one space discharge device in said amplifier, a diode having an anode and a cathode, means for supplying output signal energy from said amplifier to said anode and said cathode,

a resistance connected to the anode and cathode of said diode. circuit connections for supplying at least part of the voltage across said resistance to said amplifier to provide a bias for the space discharge device, means for minimizing voltage fluctuations in said bias voltage, a remote control unit, means at said unit including a serially connected potentiometer, fixed resistance and source of unidirectional current for forming a manually variable control voltage source, a second resistance, and circuit connections between said unit and said amplifier for connecting in series relation the said second resistance, at least part of said first resistance, and said control voltage source, the polarity of the control voltage being such that the diode is rendered inoperative until the signal energy supplied thereto exceeds a threshold voltage and the gain of the amplifier i is decreased by said 4control voltage as said threshold voltage increases.

`9. In a remote volume control system, a radio receiver including a signal amplifier Whose gain may be controlled by varying the bias voltage of atleast one space discharge device inA said amplifier, a'diode having an anode and a cathode, means4 for supplying output signal energy from said ampliiier to` said anode and said cathode, a resistance connected to the anode and cathode of said diode, circuit connections for supplying at least part of the voltage across said resistance to said amplifier to provide a bias for the space discharge device, means for minimizing voltage fluctuations in said bias voltage, a remote control unit," including a detector-oscillator space discharge device having a gain control electrode,

frequency converter circuits connected to said device, circuit means for supplying heterodyne` signals from said unit to said receiver, means at said unit for forming a manually variable control voltage source, a second resistance, and circuit connections between said unit andsaid amplifier for connecting in series relation the said second resistance, at least part of said first resistance, and said control voltage source, the` polarity of the control voltage being such that the diode is rendered inoperative until the `signal energy supplied thereto exceeds a threshold voltage and the gain of the amplieris decreased by said control voltage as said threshold voltage increases, and means for supplying at least part of said control voltage to said gain control electrode.

10. In a remote volume control system, a radio receiver including a signal amplifier whose gain may be controlled by varying the bias voltage of at least one space discharge device in said amplifier, a diode having an anode and a cathode,

means for supplying output signal energy from.

to said amplifier to provide a bias for the space discharge device, means for minimizing voltage uctuations in said bias voltage, a remote control unit including a detector-oscillator space discharge device having a gain control electrode, selfbiasing means for said last-mentioned discharge devicefrequency converter circuits connected to said device, means for supplying heterodyne signals from said unit to said receiver, means at said unit for forming a manually variable control voltage source, a second resistance, and circuit connections between said unit and said amplifier for connecting in series relation the said second resistance, at least part of said first resistance, `and said control voltage source, the polarity of the control voltage being such that the diode is rendered inoperative until the signal energy supplied thereto exceeds a threshold voltage and the gain of the amplifier is decreased by said coni source, and means for connecting said source between said grid and said cathode in shunt rela- -tionwith said automatic volume control connection.

f l2. A remote control unit for use in connection with -a radio receiver having at least onespace discharge amplifier including a control grid and cathode, and an automatic volume control con'- nection extending between said grid vand said cathode, said control unit comprising means for providing a manually variable control voltage source, a resistance, and means for serially connecting said resistance and said source between said grid and said cathode in shunt relation with said -automatic volume control connection.

f 13. In a remote volume control system, a radio receiverv including a signal amplifier whose gain may be controlled by varying the bias voltage of at least one space discharge device in said amplier, a signal rectifier, means for supplying output signal energyA from said amplifier to said rectifier, a load resistance in circuit with said rectier, connections for supplying at least part of the voltage across said resistance to said amplifier to bias the ampliieraccording to the signal amplitude, means remote from the receiver for forming a manually variable control voltage source, and connections between said receiver and saidsource for connecting the source to said amplifier in shunt relation with said resistance, -whereby said control voltage is applied `to said rectifier, the polarity of the control voltage being such that the rectifier is rendered inoperative until the signal energy supplied thereto exceeds a threshold voltage and the gain of the amplifier is decreased by said control voltage as said threshold voltage increases. v14. `In a remote volume control system, a radi receiver including a signal amplifier whose gain may .be controlled by varying the bias voltage of at least one space discharge device in said amplifier, `a signal rectifier, means for supplying output signal energy from said ampliiier to said rectifier, a load resistance in circuit with said rectier, connections for supplying at least part of the voltage across said resistance to said amplilier to bias the amplifier according to the' signal amplitude, a detector-oscillator space discharge device havinga gain control electrode, located remotely with respect to said receiver, frequency converter circuits connected to said last-mentioned device, circuit means for supplying heterodyne signals from said last-mentioned devicevto said receiver, means at the remote location-for forming a manually variable control voltage source, means for supplying at least part of said control voltage to said gain control electrode to control the gain of said detector-oscillator device, and connections between said receiver and said source for connecting the source to said ampliiier in shunt relation with said resistance, whereby said control voltage is applied to said rectiiier,

the polarity of the control voltage being such that the rectier is rendered inoperative until the signal energy supplied thereto exceeds a threshold voltage and the gain of the amplifier is decreased by said control voltage as said threshold voltage increases.

' WALTER E. GILBERT.

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