US2165596A - Automatic regulation in radio receivers - Google Patents

Description

July 11, 1939. 3 w i I 2,165,596

AUTOIATIC REGULATION IN RADIO RECEIVERS Filed July 22, 1936 2 Sheets-Sheet 1 i w I w *5 I u E! S 3!: g: m I Q Q5 ml E' Q1 III .4: I

2 w? a sq I i I E I N g D I Q Q Q g k l l K 5 $1 v E Y E g Q R s 070/!51-78/11 m k R Q E Q 9170/1 All 4001/10 SIDNEY Y. WHITE INVENTOR ATTORNEY July 11, 1939 w n- 2,165,596

AUTOMATIC REGULATION IN RADIO RECEIVERS Filed Jul 22', 19se 2 Sheets-Sheet 2 l 000 JFEJKEE SIDNEY Y WHITE, INVENTOR ATTORNEY Patented July 11, 1939 UNITED STATES AUTOMATIC Sidney Y.

REGULATION IN RADIO RECEIVE White, Chicago, 111., assignor to Victor S. Johnson, Chicago, Ill.

Application July 22, 1936, Serial No. 91,945

20 Claims.

This invention relates to improvements in high-frequency signaling systems, and more particularly to systems employing modulated highirequency signals for the transmission of intellirs gence. More specifically, this invention relates to improvements in radio broadcast receivers or other types of radio receivers adapted to respond to a particular desired signal.

An object of this invention is to provide an improved type of noise suppression means for radio receivers.

An additional object of the invention is to provide means for preventing appreciable response from a radio receiver in the absence of an input 5 signal having a predetermined initial minimum amplitude.

Still a further object of the invention is to provide means for maintaining the output signal of a radio receiver substantially constant as the amplitude of an input signal, the initial amplitude of. which exceeds a predetermined minimum value, varies over a wide range.

Another object of this invention is to provide a definite and'widely adjustable threshold value for the input signal to a radio, receiver below which no response is obtained, and above which the response is maintained substantially uniform at a desired level.

Radio receivers which are equipped with means for automatically controlling their ampliflcation inversely in accordance with the strength of the input signal have the disadvantage that the amplification automatically rises to a maximum degree when the receiver is not tuned to a signal of at least-moderate strength.

, This rise in amplification or sensitivity produces a great increase in the response of the receiver to signals too weak to be useful, and to extraneous disturbances, so that the listener may hear harsh and unpleasant sounds as the receiver is tuned from one strong signal to the next. This difficulty is not overcome by retarding the manual volume control of the receiver, since to do so also reduces below audibility the response to the signals from which a selectionis to be made.

Various types of noise gates and sensitivity controls have, in the past, been employedto minimize this inherent disadvantage of receivers having automatic amplification control means. The first-mentioned devices all depended upon the amplitude of the input signal for operation, so that only relatively strong signals were able to get through the gate and produce a response. Likewise, however, extraneous disturbances such 5 as static and other undesirable electrical noises were able to get through the gate and produce a response, provided they were of sufficient strength. Noise gates had the further disadvan- 'tage of producing serious distortion of an input signal having an amplitude close to the threshold value for which the gate was initially adjusted. For these reasons, noise gates are not a satisfactory solution of this problem in a receiver having automatically controlled amplification.

Another attack on this problem was made by employing means for limiting the maximum sensitivity of the receiver. In this way, the performance of the receiver on relatively powerful input signals was not appreciably affected, but its response to relatively weak undesired disturbances was substantially reduced. Although reducing the maximum sensitivity of the receiver was partially successful in preventing signals too weak to be useful and extraneous noises from being objectionable, this method had the distinct disadvantage of greatly decreasing the ability of. the receiver to cope with fading of desired signals. As the signal faded, its amplitude would fall below the threshold value for which the sensitivity control had been adjusted, and nousefulresponse would then be obtained. To keep extraneous response at a minimum between the desired signals, the threshold value had to be set relatively high,. so that even strong input signals frequently fadedbelow the threshold value. Providing means for regulating the maximum sensitivity of a receiver, therefore, is asatisfac tory method of preventing any appreciable response only when the input signal initially eX- ceeds a predetermined minimum value and never fades below that value.

The arrangements of the present invention, however, provide an improved form of noise suppression system which discriminates between in put noise and input signals and. which is automatically unlocked only by an input signal having a predetermined initial minimum amplitude, Furthermore, the presence of such an input signal automatically lowers the threshold value in such a manner that the signal after being received may vary over a wide range, due to fading or other changes in transmission, without falling below the new threshold value of thereceiver. An additional advantage of the arrangements herein disclosed is that the adjustable threshold value is definite and distinct and there is no distortion of a signal having an amplitude only slightly greater than the threshold value.

Although the arrangements of the present invention are adapted for use in conjunction with any type of radio receiver, they are particularly useful in receivers which are provided with means for automatically regulating their amplification, and 'are especially suitable for use in receivers also employing automatic resonanceadjusting means, such, for example, as disclosed in my co-pendlng application for Letters Patent, No. 29,244, filed July 1, 1935. When used in conjunction with such a system, the arrangements herein disclosed: are capable of providing \a radio receiver which responds only to signals of a predetermined initial minimum amplitude, to which the receiver is automatically correctly tuned.

- It is a further object of the present invention, therefore, toprovide a noise suppression system particularly adapted to and having especial ad vantages when used in conjunction with systems.

including automaticresonance adjusting means;

' When used in conjunction with such systems, the arrangements of the present invention permit adjustment of the minimum signal to which the automatic resonance adjusting means will respond, and-thus eliminate the unpleasant sounds which might otherwise be produced due to the high amplification existing when the receiver is adjusted to relatively weak signals.

Additionally, and of even greater importance in conjunction with systems incorporating automatic resonance adjusting means, when the various. components of the arrangements about to be describedare chosen to have the proper values,

the automatic resonance adjusting 'means is caused to relinquish its control with respect to any particular signal when the tuningmeans has been advanced a predetermined amount beyond the point corresponding to resonance'with that signal.

will relinquish control when the tuning means. has been advanced a reasonable amount beyond the signal. When the receiver is tuned from one reasonably strong signal directly into another reasonably strong signal, this problem is not difllcult of solution; but when tuning proceeds into a region in which there is no signal or only a very weak one, the automatic resonance adjusting means does not relinquish its control as it should,

- mployed.

but continues to maintain the receiver tuned to the signal to which it was previously-adjusted.

It will be understood, therefore, that my present invention contemplates several highly advantageous improvements applicable to automatic resonance adjusting systems, but also having utility of a high order in receivers in which no automatic resonance adjusting means are The above and other objects of my invention will more clearly appear by reference to the accompanying drawings illustrating a preferred embodiment thereof, in which: Fig. lis a schematic diagram of one form of the inv'entionfincorporated in a portion of a radio receiver:

Fig. 2 is a schematic diagram of a superheterodyne radio receiver incorporating both the arrangements of the present invention and the automatic resonance adjusting means disclosed in my co-pending' application for Letters Patent, No. 29,244; 1

v I is grounded.

3 is a graph showing the input vs. output "characteristic of the radio receiver. a portion of tuned circuit 2 and its plate or output circuit including tuned circuit 3. Cathode 4 of vacuum tube I is grounded. By-pass capacitors I and 5a ofier a low-impedance path for high-frequency currents in the input and screen-grid circuits, respectively, of vacuum tube I.

Tuned circuit 3 is coupled to tuned circuit 6, which is included in the grid or input circuit of vacuum tube I. The input circuit of vacuum tube I also includes, filter resistor 8 and volume-control potentiometer 9. Cathode ID of vacuum tube Capacitors II provide low-impedance paths for the high-frequency currents in the input circuit of vacuum tube I; The plate circuit of vacuum tube I includes load resistor I2 and the screen-grid I3 of vacuum tube I is by-passed to ground by means of capacitor ll. Plate II of vacuum tube I may be connected to screen-grid II by means of switch I6. Screengrid 23 of vacuum tube I is connected to plate I5 of vacuum tube I.

Vacuum tube I I includes two diode-rectiflers and a triode audio-frequency amplifier. The grid I8 01 the triode portion is connected through capacitor I9 to the movable arm of volume-control potentiometer 9. The plate circuit of the triode portion of vacuum tube I I includes load resistor and filter resistor 2|. Cathode 22 of vacuum tube I1 is grounded.

Resistors 24, 25 and 26 are connected in series between a source of negative voltage E and plate I5 of vacuum tube I. Grid-leak 21 is connected between the junction of resistors 24 and 25 and grid I 8 of vacuum tube I'I. Capacitor 28 bypasses the junction of resistors 2! and 25 to ground. The junction of resistors 25 and 28 is connected to diode anode 29 of vacuum tube II. Resistors 30 and 3| are connected in series between source of negative voltage E and ground. Resistor 30 is shunted by potentiometer I2 and resistor 33 in series.

Control-grid 34 of automatic amplification con trol amplifying vacuum tube II is connected to control-grid 36 of vacuum tube I.

vacuum tube 3! includes the primary winding of transformer 38. One terminal of the secondary winding of'transformer 3| is connected to diode anode 39 of vacuum tube II. The other terminal is connected through resistors ill and 4| in series to the movable arm of potentiometer 32. The

low-potential terminal of tuned circuit 2 is connected through resistor 42 to the junction of recapacitor 43. s

In operation, let it first be assumed that no signal voltage is present across tuned circuit 2. The control-grid l6 oi vacuum tube I has sub- Cathode 31 .of

vacuum'tube 35 is grounded. The plate circuit of 66 v stantially the same potential as does cathode III of vacuum tube I, and therefore this ,tube draws a relatively high,plate current. The high plate current oi vacuum tube I flowing through re- 3 plate II of tube I.

sistor I2 produces a large voltage drop, so that vacuum tube I may have a positive potential of only about 25 volts. Since screen grid 22 of vacuum tube I is connected to plate ii of vacuum tube 1, the amplification of intermediate-frequency amplifying vacuum tube I is terially reduced due to the very low screen-grid voltage. I tube I is determined by the-adjustment of potentiometer 32, which alters the direct-current potential applied to control-grid 3B- of vacuum The values of resistors 24, 25 and 26 are so chosen that, inthe no-signal condition, a negative bias potential suflicient to block the tube is applied to grid II of the triode portion of vac-- uum tube I1. With current of the tricde portion of vacuum tube I1 is substantially zero and' the tube is therefore incapableof amplification.

.Now let it be assumed that a relatively strong signal voltage appears across tuned circuit 2. Even though theampliflcation of intermediatefrequency amplifying vacuum tube I is low, sufllcient signal voltage appears across tuned circuit 6 to develop a potential drop across.resistor I and potentiometer 9 in series because otherwise unbiased control-grid 4.5 of vacuum tube 1, with cathode It), .acts as a diode rectifier. This voltage drop causes control-grid 46 of vacuum tube I to become negative with respect to cathode III, producing a substantial decrease in the plate current of vacuum tube I. This in turn reduces the potential drop across resistor I2 and increases the voltage applied to screen-grid 23 of intermediate-frequency amplifying vacuum tube I, with a resultant increase in the amplification of vacuum tube I.

This further increases the signal across tuned circuit 8, which again decreases the plate current of vacuum tube I, resulting in a cumulative unstable condition which continues until full am-' pliiication has been restored to vacuum tube I,

and the threshold has in eflect disappeared. The

rate at which the amplification is restored depends'upon the value of capacitor a. a

To re-establlsh this threshold, it is necessary that the input signal voltage decrease, either by detuning or due to severe fading, to a value where the signal, despite full gain in vacuum tube 'I,

is too weak to maintain a substantial negative bias on control-grid 46 of vacuum tube I. Under these conditions, the plate current of vacuum tube 1 increases, with consequent greater voltage drop in resistor l2, thus decreasing the voltage on the screen-grid of vacuum tube I, further decreasing the signal, and another cumulative unbalance occurs, ending in the original threshold being re-established.

The increase in the potential of plate I5 of vacuum tube 1 causes diode anode 29 to become positive with respectto cathode 22 of vacuum tube I1. Since the internal resistance of "the diode rectifier is now small compared'with the values of resistors 24, 25 and 26, the junction of resistors 25 and 26 is effectually grounded. The

values of resistors 24 and 25. are so chosen that the proper negative bias voltage is applied to grid I8 of the triode portion of vacuum tube I1 for normal operation as an audio-frequency amplifier. The rate at which vacuum tube I1 is restored to normal operation is determined by the value of capacitor 28.

The audio-frequency component of the signal appears across'volume-control potentiometer 9,

The actual amplification of vacuum this bias voltage, the plate grid 45 of vacuum tube 1 operates simultaneously and as the control-grid as the anode of a 'diode v of atetrode, the tetrode functioning to amplify the 'direct current potential variations which appear across resistor 8 and potentiometer in series.

By making the time required for unblochns the audio-frequency amplifying vacuum tube' I'l considerably greaterthan that required to restore the amplification of intermediate-frequency amplifying vacuum tube I, the'system is made unresponsive to bursts of static'and other electrical disturbances of shortduration regardless of their amplitude. A continuous signal of predetermined initial minimum amplitude, however, first restores the amplification of intermediate- 'frequency amplifying vacuum tube I'and then unblocks the audio-fretfuency amplifying portion of vacuum tube II. This definite order oi opera-' tion has the additional and important advantage of preventing response in the to of an output signal across resistor until the trength of the input signal is well above the threshold value, thuspreventing any distortion which might occur at or near the threshold value from being present in the output of the system.

Ths threshold distortion is of two kinds. If

change its plate current. Since vacuum tube I1 is blocked until a definite amount of change of plate current of vacuum tube 1 occurs, the receiver produces no sound while the signal is breaking down this threshold.

The other kind of threshold distortion occurs I during the period in which the grid bias ofvacuum tube I1 is decreasing from the blocked to the operating condition. cumulative snap interaction of vacuumjtubes I and 'l', the grid-bias'voltage' on vacuum tube I-l is changed so rapidly passed through and consequently is not noticeable.

However, due to the that this distortion is in a small fraction of a second,.

Since the amplification of intermediate-frequency amplifying vacuum tube I is substantially increased by the presence of an input signal having an amplitude above a' predetermined initial minimum value, ered as a result. Thus. there'is a'reserve of ampliflcation to aid in counteracting fading'or other changes in the strength of the input signal. This automatic lowering of; the threshold value is an important feature of the present invention.

Theivoltage across tuned circuit 2 is also applied to control-grid 34 of automatic-ampliflcation control amplifying vacuum tube .35. After being amplified by vacuum tube 35, the signal the threshold value is low.-

voltage is applied to diode anode 39 of.vacuum tube II by means of coupling transformer 38. Diode anode 39 is initially negative with respect to ground by an amount dependent upon the setting of potentiometer 32. when the signal voltage is sufficiently high to overcome this initial negative bias, a direct-current voltage is developed across diode load resistors 40 and II in series. This voltage is utilized to provide automatic amplification control of the vacuum tubes preceding vacuum tube l in the receiver, series resistor 44 and shunt capacitor 45 preventing l and 43a. Thus vacuum tube I is regulated to a lesser extent than are the preceding vacuum fluctuations due to modulation of the signal voltage from reaching the controlled vacuum tubes, the control-grids of which maybe operatively connected to the junction of resistor and capacitor 45 at AVC.

The control voltage which is developed across resistor ll is applied to the control-grid of intermediate-frequency amplifying vacuum tube I, the fluctuations due to modulation being removed by series resistor 42 and shunt capacitors tubes by the automatic amplification control means, but at the same time contributes toward the over-all effectiveness of this control means.-

In no event, however, is the amplification of this vacuum tube'reduced by the automatic amplification control means to an extent sumcient to deprive vacuum tube 1 of the signal voltage necessary to maintain its plate current at a low value. The fact that the present invention discloses an automatic noise suppression system which does not interfere wit or appreciably limit the range of operation 'of t e automatic amplification control means is another of its important features.

The setting of potentiometer 32 determines the initial negative bias voltage which is applied to the control-grids of the preceding vacuum tubes as well as that applied to control-grid 36 of intermediate-frequency amplifying vacuum tube I. This potentiometer functions therefore, both as means for adjusting the maximum sensitivity of the receiver and as an adjustment ofthe SI" initial threshold value of input signal to which the receiver is'responsive. For example, moving the arm of potentiometer 32 toward the terminal which is connected to resistor 33 decreases the maximum sensitivity of the receiver and simultaneously raisesthe threshold value, adapting the receiver to receive strong signals with a minimum of extraneous noise between signals. Moving the arm of potentiometer 82 toward the Junction of resistors SI] and II, however, increases the maximum sensitivity of the receiver and at the same time decreases the threshold value, thus adapting the receiver for the receptionof. rela-' tively weak signals. An important feature-of the present invention is that only a single control is required to adjust the system for any particular condition of operation, thus greatly simplifying the construction and operation of the complete receiver. The entire noise suppression system is rendered inoperative merely by throwing switch It to the lower position, and this switch may readily be arranged so that it is operated by a slight movement of the arm of potentiometer I! at the high-sensitivity end. The

switch connects plate I! of vacuum tube 1 and screen-grid 23 of vacuum tube l to the screensupply point B, restoring full amplification to .vacuum tube I and ensuring correct bias for vacuum tube l1.

It will be understood that the noise suppression system Just described differs in fundamental particulars from the various noise gates and "sensitivity contro of the prior art, in that it comprises asequential series of automatic operations, this series being inherently nbn-reversible. It will also be understood thatthis noise suppressiori system, in and of itself, has highly advantageous application not only in radio receivers but equally as well in any device or apparatus'in which it is desired to establish oper- --at ive conditions in response to initial conditions,

without regard to subsequent fluctuations. Also, in radio receivers, it is possible by suitable choice of constants to cause the neise suppression system-to operate in response to relatively small changes in signal level at the demodulator, such as might result if the receiver is tuned only slightly away from resonance with a desired signal. Such an arrangement prevents any response from theloud speaker. under conditions which might otherwise produce seriousdistortion,

and operates to give a\res ult similar to that secured by automatic resonance adjustment.

Fig. 2 is the schematic diagram of a multiband superheterodyne radio receiver which incorporates not only the improved noise suppression system which is shown in Fig. 1, but also the automatic resonance. adjusting means which is disclosed in my copendi'ng application for Letters Patent, No. 29,244. This receiver. employs thirteen vacuum tubes arranged as follows: a

radio-frequency amplifying vacuum tube 46; a

modulator vacuum tube- 41; a decal oscillator vacuum tube 48; a first intermediate-frequency amplifying'vacuum tube I; a combined demodulator, second intermediate-frequency amplifying and direct-current amplifying vacuum tube 1; a combined first audio-frequency amplifying automatic amplification control rectifying and bias voltage determining vacuum tube ll; a second audio-frequency amplifiying'vacuum tube 49;. a push-pull audio-frequency output stage consisting of vacuum tubes 50 and 5!; a power.

rectifier vacuum tube 52; an automatic ampli-- fication control amplifying vacuum tube '35; a

director vacuum tube 53; and a corrector vacuum-tube 54'. With the exception of the novel features herein disclosed and those disclosed in my co-pending application for Letters Patent, No. 29,244, the receiver is of conventional design 5 and operates in the usual manner,

As between Figs. 1 and 2, corresponding parts are designated by the same numeral. It is unnecessary, therefore, to repeat a detailed description of the novel noise suppression system-herein disclosed as to its operation in the receiver of Fig. 2, except insofar as the results obtained differ from those stated in the discussion of Fig. 1. In addition to functioning as a rectifier and as a direct-current amplifier, vacuum tube 1 serves as an intermediate-frequency amplifier. The plate circuit of vacuum tube I includes series inductor 55a and-coils 55 which are coupled with coils 56a. and SBb-associated with vacuum tube 53 to form the director portion of the automatic frequency adjusting system. The director is arranged to supply a direct-current voltage to control-grid 51 of vacuum tube 54, which is arranged to function as the corrector portion of the automatic frequency adjusting means. The

input capacitance of vacuum tube 54, in series with capacitor 58, is in shunt with the plate load of oscillator vacuum tube 48.

In the presence of no input signal or of a signal having a value lower than the predetermined threshold value for which the receivei may be solely by the potential at tap A on the powersupply divider network. The value of the voltage at tap A is so chosen that the input capacitance of vacuum tube 54 is approximately at its midpoint. g I

when the initial amplitude of the input signal exceeds the threshold value, the amplification of vacuum tube I is greatly increased by the action of the automatic noise suppression system, with the result that a strong signal current flows through coils II in the plate circuit of vacuum tube I. Since coils 56a and 58b are tuned respectively to a frequency slightly lower than the intermediate frequency of the receiver and to a frequency slightly higher than the intermediate frequency, the polarity of the direct-current voltage which is supplied by the director to the corrector depends upon whether the frequency of the signal current in coils is higher or lower than the nominal intermediate frequency of the receiver. If the signal frequency is exactly corrector-depend respectively upon the amplitude and the polarity of the voltage supplied by the .voltage from reaching vacuum tube. 53.

director.

The high-frequency amplification of the re-.

ceiver is regulated automatically by a system which includes amplifying vacuum tube 35 and diode anode I) of vacuum tube II. A direct-current control voltage is develo'bed across diode load resistors I. and ll in series, when the signal voltage across the secondary winding of trans-'- former 3| exceeds a minimum value determined by the setting of potentiometer 32. The full control voltage is applied to the control-grids of vacuum tubes 46 and 41, and a portion of the control voltage is applied to the control-grid of vacuum tube i. The performance of the delayed, amplified automatic amplification control is such that the signal voltage across tuned circuit t remains substantially constant as the input signal voltage varies over a wide range.

When switch it is thrown'to the lower position, not only is the automatic noise suppression system rendered inoperative as explained above in connection with Fig. l, but also the automatic frequency adjusting means is no longer in operation, since anegative-voltage is applied to screengrid l3 of vacuum tube I, rendering it incapable of amplification and thus preventing any signal With switch II in the lawer position, the receiver functions just as does any standardsuperheterodyne, but withthe switch in theupper position, the normal operation of the receiver is supplemented by the automatic frequency control system and by the automatic noise suppression system which are associated with it.

As has already been pointed out, a major difficulty in the design of automatic resonance adlusting systems lies in so arranging the system that it will relinquish a station once it has been tuned in. The intermediate-frequency amplifier inherently tends to stay locked on the station as the tuning dial of the receiver is turned, until the limit of oscillator frequency control has been reached, when the oscillator frequency is again controlled by the dial movement. If this dial movement be further continued, the oscillator is forced so far out of correct relation with the signal that the selective frequency cut-off of the intermediate-frequency amplifier is approached,

I a limiter tube.

'cel quite completely.

re-ali'gning the oscillator with the dial. s mg. During this process, however, the signal h departed widely from true resonance with the intermediate-frequency amplifier, and serious. distor-' 1. The amount of control on the oscillator must be limited and must be uniform throughout the band. This is necessary in order that a reasonable amount of movement of the dial will exhaust the control possibilities of the system, and restore control to the dial.

2. The director voltage must belimited, and must be relatively independent of signal strength; This is accomplished in a two-fold manner, first by the automatic amplification control and second by operating the tube driving the director as 3. The voltage delivered by the director ordinarily contains transients of considerable mag- Across each diode load resistor there will also appear audio-frequency voltages and sy-llabic modulation transients. When the intermediatefrequency amplifier is in accurate relation with the signal, these audio-frequency and transient voltages are in phase opposition and are substantially equal to each other and therefore can- I However, as soon as the oscillator departs from correct relation with the signal, the audio-frequency and transient volt- I ages produced by one diode will be substantially greater than those produc'ed'by the other and' complete cancellation will no longer occur. There will therefore be superimposed on the steady di-- rector voltage a series of transients of substantial amplitude. These transients I do not filter out. When control tube 54 is at the limit of control due to theisteady direct voltage applied to its grid 51, any substantial relaxing of the control voltage will allow the oscillator to snap back into alignment with the dial setting, which may be for instance 15 kilocycles to one side of the setting exactly corresponding to the frequency of the signal. Thetransients 'of random amplitude increase as the oscillator is mistuned, and finally reach an amplitude sufficient to partly cancel the steady direct-current component of the director output voltage, If desired, these transients may be separately amplified and re-introduced to inthus loses control, the oscillator frequency changes to correspond withthe dial setting, and

,the signal has thus been relinquished.

The effect of the audio-frequency transients in causing the corrector to relinquish control .of the oscillator when'the receiver is detuned, is made more pronounced by resistor 59a,- shunt'ed by capacitor 59, and resistor 59!; connected in series from the director unit to ground. It will be noted that resistorsisa and 591 form a potena1- dividing system, the corrector to their junction. The values of these two resistors are preferably so chosen that about one-' fourth of the direct-current voltage delivered by the director is applied to the corrector. As the receiver is mistuned, the direct-current voltage across resistors 59a and 59b rises, and then, say when the tuning means is about 12 kilocycles from resonance, begins to fall. During the-process of tuning awayfrom the signal, the audiofrequency transients have been gradually increasing. By virtue of capacitor 58, which is preferably of a fairly large value, the audio-frequency transients are 'by-passed across resistor 59a and generate varying voltages only across resistor The effect of the combination of resistors 58a and 59b and capacitor 59 connected as shown in Fig. 2, therefore, is to increase the effectiveness of the transients in varying the voltage applied to grid 51 of corrector vacuum tube 54 in such a manner that as soon asthe directcurrent voltage delivered by the director begins to fall, a condition of cumulative instability occurs-and corrector vacuum'tube 54 relinquishes control of the frequency of oscillator vacuum tube I I.

Additionally, the arrangement of resistors 59a and 59b and capacitor 59 is'such that-when the voltage delivered by the director fails, capacitor 59 discharges not only through resistor 59a which is of a very large value-"but also through resistor 59b, whichis of much smaller value, and

the current through resistor 59b is in a direction opposite to that which flows when the direc-.

tor is-delivering appreciable voltage. In the interval during which capacitor 59 is discharging, therefore, there is an actual reversal of potential at the junction of resistors 59a and 59b withrespect to'ground. This reversed voltage is applied to grid 51 of corrector vacuum tube 54 and produces a change in the frequency of the oscillator such as to immediately cause it to' bring into approximate resonance with the intermediate-frequency amplifier the signal in the adja cent channel, if such signal is present.

An analysis of the behavior of r tors 59a and 59b and capacitor indicates that the same effect which produces actual reversal of potential .on grid 51 of vacuum tube 54 when the voltage from the director fails or falls to zero, operates to enhance the condition of instability'in tlie region between one signal and another in such a way as tocause an instantaneous .release of one signal and capture of the next and to narrow theregionin which this exchange occurs so that the operation of the automatic frequency adjusting means :becomes entirely definite and all-tendency to continue to maintain resonance with the undesired signal disappears.

By suitable choice of values for resistors 59a and 59b and capacitor 59' it is possible 'to so emphasize the effects above described that ralease occurs just as close to the position of exact resonance as does the initial operation of the automatic frequencyiadjusting means as thereceiver is tunedinto the signal. I havefound,

however, that it is preferable to so choose these values that release. does not occur quite so close to the position of resonance as does .initlal capture (if the signal.

If the receiver is tuned into from 1 aboveand also from below the frequency of resocurs are noted.'a "capture band wid nance, and the frequenciesat which capture oc- M be determined. Similarly, if'the receiver is tuned away from the signal, iirstby increasing the resonant frequency of the receiver and then by decreasing it. andthe frequencies at'which release occurs are noted, a release band width" may be determined. I have found that the best performance is obtained when the"capture band width" is about 85% of the "release band width". It will be understood, however, that by suitable choice of constants, at the discretion of the designer, various relations of these two band widths can be achieved, including the condition in which they are substantially equal.

In thetabulation of constants which I give below, asan illustrative embodiment of my in vention; the values of resistors 59a and 59b and capacitor 59 are such as to give approximately 'the 85% relation of the capture and release band widths described above as preferable.

It will be understood that when in the specifl-. cation and claims I refer to capture of a signal, V

I use this phrase to identify the operation by which the automatic frequency adjusting means acts to change the oscillator to a frequency ap-.

proximately correct toproduce thedesired intermediate frequencyfor that particular signal. Similarly. I refer to release of asignal when I wish to describe the operation by which the automatic frequency adjusting means ceases to maintain the frequency of the oscillator at approxi- I mately the correct value to produce the desired intermediate frequency for that particular signal.

It will be understood that by suitable choice of constants, the noise suppression syste'rn' first described maybe made to operate at .positions closer to the position of exact resonance than they release .system just. described. 'With the 'constants given below, the release system will operate closer' to the position of resonancethan' will the noise suppressiofi system, its mode of operation being through the automatic resonance adlusting system. With other constants, however, the noise suppression system will operate closer to the point of resonance, its-mode of operation being to decrease'the sensitivity of the receiver and at the same time .render the audio-frequency output system inoperative When-the noise sup pression system operates in the manner just. de.-'

scribed, the automatic'resonanceadjusting sysimmediately'relinquishes its control of the frequency of'the oscillator; due to failure of the 1 signal voltage at the'demodulator grid.

'Rhe value ofcapacitor is of considerable comeoperative and a sound. will be heard. If-

capacltor 28. is made very large, on the other hand, the grid bias voltage on vacuum tube. I! will change so gradually that when a desired 'signal is first heard it will be relatively faint, and it will increase in loudness over an appreciable interval before it reaches its final level. Under these circumstances, rapid operation of the tuning means maycause the receiver to pass completely through a signal of sufficient initial intensity to otherwise-be heard, without pro- Under these.

ducing an audible sound from the loudspeaker.

Thus rapid operation of the tuning means will produce a condition of "silent tuning" in which,

unwanted signals may passed over without being heard, but as soon as the vicinity of a desired station is reached, a slower operation or the tuning meanswill ensure hearing it.

that this is not to'be taken as in any way limiting .my invention, since other types and values of components may be employed in a circuit arrangement identical with that shown in Fig, 3,

or in modifications thereof.

' I Reference numeral 'iypeor value Vacuum tube 1, 7, 35, 46 0117. V tube 17 1 BQI.

83-V.. V tube 53 H6. Resistors 8, 2i, Z4 .060 negohm P 9-- .25 megohm. Ca f 11- 200 micromicroiaradsa -R" .023

Capacitor i4 .025 microfai-ad. Capadtors 10,28, man. .05 microisrad. Resismrm 1 .100 Resistor 25. .015 mogobm. Real-swim .lccmegobm.

.600 .250 n 'iegohm 0.1 mlcroiarad. 2.0

volts.

Fig. .31shows graphically the performance of a radio receiver, a portion of the circuit of which '40 is shown in Fig. 1. .Cu'rve A represents the relation between input and output voltages which is 45 the output voltage begins to. increase gradually as the input voltage increases and tends to level on at the high-input end as the automatic ampliflcaflon control"system becomes me sly et- Its advantages may be realized in receivers in which {the high-frequency amplification takes fective. Small input voltages produce an appre- 60 ciable response. time B shows the performance a predetermined threshold value, but when the 1 The arrangements of h present invention are of the system when the automatic noise suppression system is in operation. that is, with switch Ii of Fig. 1 open. In this case, no output voltage 4 is obtained untiLthe l nitial input voltage reaches input voltage increases even slightly above this value the output voltage rises to a point which imum' input voltage which will produce a response. Curve 0 indicates the effect of lowering the threshold value, and curve D indicates the performance obtained when the threshold value is raised, By properly adjusting potentiometer 32 of Flg.,1, the threshold value may be made just high enough to permit response to signals of desired initial amplitude without allowing weaker signals and extraneous disturbances to produce a response.

so described hereinabove.

Referring to Fig. 4, triangles 80 -85 represent initial input signals of various amplitudes. will be understood that the input signals are thus only symbolically represented in this figure. The signal voltage at the input to the demodulator is represented by rectangles 66, 61 and B0, corresponding with input. signals 60, 83 and 65, re-.

spectively. Input signals 6!, 62 and 64, being below thethreshold value for which the receiver is adjusted, do not produce any response, due to the action of the noise suppression system. Al though input signals-60, 63 and vary considerably in amplitude. the amplitude of the corresponding output signals 66, '61 and 68, respecequipped with the automatic noise suppression system ofthe present invention and with the automatic frequencyvadjusting means disclosed in my co-pending application for United States Letters Patent, No. 29,244, produces a response only to input signals exceeding a predetermined initial minimum amplitude and only when. the

.sponds to one relatively strong signal after another with substantial sllence between these signals. Moreover, no signal is'heard unless the receiver'is automatically in correctadjustment for the reception of thatsignal.

It will be understood that Figs. 3 and 4 are not intended .to show graphically the automatic decrease of the threshold value upon the reception of an input signal having a predetermined initial minimum amplitude, which is one of the important features of thepresent invention and, is

The application of the present invention is not limited to any particular type of radio receiver.

place at the frequency of the incoming signal as well as in receivers of the superheterodyne type, in. which the greater part .of the amplification before final demodulation is accomplished at a relatively low constant intermediate frequency.

not limited to use in radio receivers. They may be applied-with equal success to other systems which are to operate in such a way as to be initially unresponsive to ihput'voltages below a predetermined minimum value. Additional advanamplifier employing a vacuum tube having ascreen-grid, an audio-frequency amplified employing a vacuum tube having a control-grid,-a

loud speaker, and meansfonpreventing response.

from said loud speaker until a signal voltage having an initial amplitude exceeding ap're'detere mined valueis impressed upon said means, said means including a rectifier connected to rectify said signal voltage, a resistor across which there is developed a positive voltage of such value as to maintain said high-frequency amplifier at low amplifying efiiciency, a connection by which said positive voltage is applied to said screen-grid, a resistor across'which there is developed a negative voltage sufficient to maintain said audi-frequency amplifier completely inoperative, a connection for applying said negative voltage to said control-grid, and a time-delaying network for sequentially changing said positive and negative voltages, first on 'said screen-grid and then on said control-grid, as soon as said signal voltage exceeds said predetermined value, to values which increase the amplifying efiiciency of said highireque'ncy amplifier and render said audio-Irequency amplifier operative.

2. A radio receiver including high-frequency and audio-frequency amplifiers; a loud speaker; and automatic regulating means for reducing the amplification of said amplifiers to prevent re' spouse from said loud speaker whenever the ampiitude of the signal impressed across said means is less than a certain value, for maintaining said amplifiers in said condition of reduced amplification until said amplitude exceeds a substantially higher value, for sequentially restoring the amsaid amplifiersn. i

plification of 'said high-frequency'amplifier and said audio-frequency amplifier as soon as said amplitude exceeds said higher value, and for maintaining said amplifiers in said condition oi restored amplification until said signal falls below said first-mentioned value; said automatic- 3. A radio receiver including a high-frequency amplifying vacuum tubehaving a screen-grid; an audio-frequency amplifying vacuum tube having a control-grid; a loudspeaker; and automatic regulating means for preventing response fromsaid loudspeaker whenever the amplitude of the signal impressed across said means is less than a certain value and until said signal exceeds a substantially higher value, andfor permitting response from said loud speaker as soon as said signal ex'ceeds said higher value; said automatic regulating means including 'a' rectifier, a net- 'work associated with said rectifier and connected to sources of positive and negative potential, connections from said network to said screengrid and said control-grid, and'tirneedelayi ng elements in said connections.

4. A radio receiver including ahigh-frequency amplifying vacuum tube having a control-grid,

a screen-grid, and an output circuit; a second vacuum tube arranged to function both .as a recti-' fler to produce a; direct-current voltage and as, an amplifier of said direct-current voltage, saidrectifier being coupled to the'output circuit of said high-frequency amplifying vacuum tube and said amplifier having output circuit includ: ing a resistor; a third vacuum tube arr'angedto function as an' audio-frequencyamplifier and having a control electro'de;"a direct-current con; nection fromsaid screen-grid to the low-potene tial' terminal of said resistori a. network consisting of a second and a third resistor in series'co'nnected between said low potential' terminal and,

a source of negative potential; and a time-delay- -ing connection between the junction 01' said sec-- 0nd and third resistors and said control electrode. v

5. A radio receiver including in combination,

tuning means, automatic resonance adjusting means for bringing said receiver into substantially' exact resonance with a high-frequency vsignal selected by approximate setting of said tuning means, and means for causing said auto matic resonance adjusting means to release said signal when said tuning means reachesa point displaced a predetermined amount from the setting which would correspond to exact resonance with said signal; said resonance adjusting means including a director unit and a corrector unit,

tuning means, automatic resonance adjusting means for bringing said receiver into substantially exact resonance with a high-frequency signal selected by approximate settingv of said tuning means, means for preventing response from said receiver until said signal has aninitial amplitude exceeding a predetermined minimum value, and means independent of said responsepreventing means for releasing said signal when said tuning means reaches a point displaced a '30 predetermined amount from the setting which would correspond to exact resonance therewith,

including a potential-dividing and polarity-reversing network which renders said resonance producing means increasingly unstable as said tuning means is moved away from said setting of exact resonance.

7. The method of preventing undesired sound reproduction in a radio receiving system pro- .viding high-frequency and audio-frequency amplifications and tunable to selectively receive any one of a number of different carriers throughout a wide range of frequencies, which comprises restraining said amplifications when the ampli-.

tude of the selected carrier is less than a'prev determined minimum value and until said amplitude exceeds'a value substantially highei than said minimum value, and removing saidirestraint of said high-frequency amplification and of said audio-frequency amplification in the order named as soon as said amplitude exceeds said higher value.

8. The method of preventing undesired sound reproduction in a radio receiving system employing a vacuum tube having a screen-grid for higha frequency amplification and a vacuum. tube hav- 'ing a-control-grid for audio-frequency amplification, and tunable to selectively receive any one of a number of different carriers throughout a wide range of frequencies, which comprises applying amplification restraining potentials on said screen-grid and on said control-grid when the amplitude of the selected carrier is less than a. predetermined minimum value and until said amplitude exceeds a value substantially higher than said minimum value, and removing said screen-grid and from said control-grid in the order named as soon assaid amplitude. exceeds said higher value.

9. .A radio receiver including in combination, j tuning means for tuning said receiver over a -ra'nge of frequencies, resonance adjustingmeans ifor automatically bringing said receiver into substantialiy exact resonance-with a first highwas frequency signal selected by approximate setting or said tuning means but for releasing said signal when said tuning means is adjusted to select a second signal, and means connected to and actuated by said resonance adjusting means for causing immediate release of said first signal. a

10. A radio receiver including in combination, tuning means for tuning said receiver over a range of frequencies, resonance adjusting means \for automatically bringing said receiver into substantially exact resonance with a first high- Irequency signal selected by approximate setting 0 said tuning means but for releasing said signal when said tuning means is adjusted to select a second signal, and means for accelerating the release of said firstsignal by said resonance adfor automatically bringing said receiver into justing means, said ireleaseeaccelerating means including a potential-dividing and polarityreversing "network so arranged as to'render said resonance adjusting means increasingly unstable as said tuning means is moved away from the setting which corresponds to exact resonance with said first signal.

11. A radio receiver including in combination, tuning means for tuning said receiver over a range of frequencies, means for automatically bringing said receiver into substantially exact resonance with a first high-frequency signal selected by approximate setting of said tuning means, means for preventing response from said receiver unless said first signal has an initial amplitude exceeding a predetermined minimum value, means'for causing said resonance \adjusting means to release said first signal when said tuning means is adjusted to select a second signal, and means independent of said response preventing means 'for' accelerating the release of said first signal by said resonance adjusting means.

12. A radio receiver including in combination, tuning means for tuningsaid receiver over a range '01 frequencies, resonance adjusting means for automatically bringing said receiver into substantially exact resonance with a high-trequency signal selected by setting said tuning means between limits above and below the setting which corresponds to exact resonance with said signal, and means connected to and actuated'by said resonance adjusting means for causing immediate release of said signal as said tuning means is adjusted from a point within said limits to a point outside said. limits.

13. A radio receiver. including in combination, tuning means for tuning said receiver over a range of frequencies, resonance adjusting means for automatically bringing said receiver into substantially exact resonance with a first modulated high-.irequency signal selected by approximate setting of said tuning means, means for preventing response from said receiver unless said first signal has an initial amplitude exceeding a certain minimum value, said resonance adjusting means being such that modulation peaks in said-first signal cause release of said first signalwhen said tuning means is adjusted, to select a second signal, and means independent or said response-preventing means for accelerating the releaseot said first signal by said resonance adjusting means. p

l4.'A radio receiver including in combination, tuning means for tuning said receiver over a range of frequencies, resonancendiusting means substantially exact resonance with a high-trequency signal selected by setting said tuning means between limits above and below the setting which corresponds to exact resonance with said signal, means i'or preventing responsetrom said receiver. unless said signal .has an initial amplitude exceeding a certain minimum. value, and means independent of said response-preventing means for accelerating the release of said signal by said resonance adjusting means when said tuning means is adjusted from a point within said limits to a point outside said limits,

said limits being so chosen that said resonance adjusting mea'nsreleases said signal beiore'distortion of said signal due to inaccurate setting of said tuning means becomes appreciable.

15. A radioireceiverincluding in combination, tuning means for'tu'ning said receiver over a range of frequencies, resonance adjusting means for automatically bringing said receiver into substantially exact resonance with a first highfrequency signal selected by approximate setting of said tuning means but for releasing said Lfirstfsignal when said tuning means is adjusted to select a second signal, means for automatically regulating the high-frequency amplificatiomoi.

said receiver inversely in accordance with the strength of said first signal, and means connected to and actuated by said resonance adlusting means for causing immediate release 0! said first signal.

16. A radio receiver including in combination, tuning means for tuning said receiver over a range of frequencies, resonance adjusting means for automatically bringing said receiver into substantialiy exact resonance with a first highfrequency signal selected by approximate setting of said tuning means but for releasingsaid first signal when said tuning means is adjusted to select a second signal, means for automatically regulating the high-frequency amplification, 01 said receiver inversely in accordance with the strength of said first signal, means for preventing response from said receiver unless said first signal has an initial amplitude exceeding a:certain minimum value, and means indepe'ndentoi said response-preventing means for accelerating the release of said first signal by said resonance ad-u justing means.

1'7. A radio receiver including in combination, tuning means for tuning said receiver overa range or frequencies, resonance adjusting means for automatically bringing said receiver into substantially exact resonance with a first highfrequency signal selected by approximate setting of said-tuning means but for releasing said first signal when said tuning means is adjusted to select a second signal, means but for preventing response from said receiver whenever the amplitude of thesignal impressed across said means is less than a certain value and until said amplitude exceeds a substantially higher value, and means independent of said response-pneventing means for accelerating the release 01 said first signal by said resonance adjusting means.

18. A radio receiver including in combination, tuning means, resonance adjusting means for automatically bringing said receiver into substantially exact resonance with a first highfrequency signal selected by approximate setting of said tuning means but for releasing said first strength of said first signal, means for preventing response from mid -receiver whenever. the amplitude of the signal impressed across said means is less than a certain value and until said amplitude exceeds 'a substantially higher value, and means independent of said responsepreventing means for accelerating the release of said first signal by said resonance producing means.

19. The method of preventing undesired sound reproduction in a radio receiving system tunable to selectively receive any one of a number of diiferent carriers throughout a wide range of frequencies and providing resonance adjustment for automatically bringing said system into substantially exact resonance with a first carrier selected by approximate tuning. thereof-hut releasing said first carrier when said system is tuned to receive a second carrier, which comprises maintaining said system in a substantially inoperative condition whenever the amplitude of said first carrier is less than a certain minimum value and until said amplitude exceeds .a value substantially higher than said minimum value, rendering said system operative as soon as said amplitude exceeds said higher value, and independently accelerating the release of said first carrier.

20. The method of preventing undesired sound reproduction in a radio receiving system tunable to selectively receive any one of a number of difierent carriers'throughout a; wide range of frequencies and providing resonance adjustment for automatically bringing said system into substantially exact resonance with a first carrier selected by approximate tuning thereof but releasing said" first carrier when said system is tuned to receive a second carrier and also pro-.

viding automatic regulation of the high-frequency amplification of said system inversely in accordance with the amplitude of said first carrier,

- which comprises maintaining said system in sub- 'stantiaily inoperative condition whenever the amplitude of said first carrier as modified by said automatic high-frequency amplification regulatln-is less than a certain minimum value and until said modified amplitude exceeds a value substantially higher than said minimum value. rendering said system operative as soon as said modified amplitude exceeds said higher value, and independently accelerating the release of said first carrier,

SIDNEY WHITE.

. CERTIFICATE OF CORRECTION. Patent No. 2,165, 596. July 11, 1959.

SIDNEY Y. WHITE. It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page L1,, secent Office 0nd column, line 27, after the word "amplifying" insert a comma; page 5-, first column, line 51, for "'lawer" read lower; same page, second column, line 27, for "well" read will; line 52,- for "direct" read director; page '2, second column, line 69, claim 1, for theword "amplified" read amplifier; page 8, first column, line 7-8, claim 1, for audi-fre'truency" read audiofrequency; page 9, second column, line 57, claim 17, strike out the word Jbut"; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in .the Pat- Signed and sealed this 22nd day of August, A. D. 1959.

Leslie Frazer (Seal) Acting Commissioner of Patents.

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