US2152482A - Noise suppressor - Google Patents

Description

March 28, 1939.

J. K. JOHNSON NOISE SUPPRESSOR Filed Nov. 21, 1932 a sneet -sheet 1 INVENTOR ATTORNEYS March 28, 1939. JOHNSON 2,152,482

NOISE SUPPRESSOR Filed Nov] 21, 1932 4 sheets-shes; s

o m I Q I 8 k 9 l E g Q 2 E Qn Q INVENTOR JOHN KELLY JOHNSON ATTORNEYS NOISE SUPPRESSOR Filed Nov. 21, 1932 4 Sheets-Sheet 4 k INVENTOR QS JOHN KELLY JOHNSON ATTORNEY Patented Mar. 1939 UNITED. STATES PATENT OFFICE. Y

m... nu, rpm, an, s. r., saline: to Hueltine Oorporation, Jersey City, N.- 1., a corpora'tion of Delaware Application Novemberzl, 1932, Serial No. 643,852

v(H'llaims.

The present invention relates to signal transmission, and more particularly to a noise suppressor" for automatically controlling the transmission through a signaling system to prevent the reproduction of noises,- whether occasioned by atmospheric conditions or produced within the system, whenever the noise intensity is greater than the signal intensity, as for instance, in a radio receiver when tuning from station to sta- 1 tion. c

, This application is a continuation in part 0! applications Serial No. 603,489, filed April 6, 1932, and Serial No. 603,490, filed April 6, 1932.

The suppression of fnoise signals is accom- 15 plished, in accordance with the present invention, by a noise-suppressor arrangement in which the transmission of signals through some portion of the system is controlled in such a manner that when the average signal intensity falls below a predetermined amount, the portion thus controlledwill be rendered partially or completely inoperative and thus prevent transmission of the signals therethrough.

The features or this invention are preferably, though not necessarily, incorporated into radio receivers of the usual automatic volume control type, wherein the radio-frequency amplification is automatically controlled in accordance with the average signal intensity.

with a receiver having the usual automatic volume control arrangement, when operating with the manual volume control so adjusted as to give the maximum volume, the sensitivity of the receiver is greatly increased when no signal is being received, as when tuning from station to station. This results in amplifying static and the effect of other atmospheric disturbances to a high degree, causing unpleasant sounds to be emitted from the loudspeaker.

Also, when such a receiveris not tuned to a channel disturbances, it is being operated so that the total amplification is maximum, and the radio-frequency amplifier tubes, while being operated near their practical limit of amplification, cause the reproduction of disagreeable noises by the. loudspeaker.

In view of the fact that in the usual automatic volume control receiverthe sensitivity of a set is 50 decreased in proportion to the signal intensity, it

is necessary to arrange the automatic volume slgnal'channel and is therefore subject to inter nate the above-noted disadvantages oi! the automatic volume control system, to prevent the reproduction 01 noise, whether produced by atmospheric disturbances or by the actions of the tubes themselves, and to prevent unpleasant sounds as the receiver is rapidly tuned across the irequency allocations of several stations, as in tuning to a desired signal.

This and iurther objects will become apparent from the following specification taken inconnection with the accompanying drawings.

In accordance with a feature of this invention, a noise-suppressor tube, as it will be called hereinafter, is provided. There is furnished or developed in a circuit associated withthe noisesuppressor tube a uni-directional voltage representative of the received signal. This tube is so operatedthat it serves to reverse the fluctuations of the uni-directional voltage and impress the reversed direct-current voltage fluctuations upon a control circuit of some other tube of the receiver, preferably a tube following the detector, such, ior instance, as the second detector tube in a superheterodyne radio receiver, or an audiofrequency amplifier tube. Hereinafter the tube controlled by the noise-suppressor tube will referred to as the controlled tube. a

The characteristics oi the suppressor and controlled tubes are so chosen that a substantial change in the voltage of the output of the suppressor tube is obtained by a relatively small variation in the received signal intensity to vary the bias of the controlled tube between a value which will prevent the e oi! signal voltage therethrough, or will bias it to cut-oil, and a value which will permit normal gain in the system.

Thus, when no input voltage due to received sigcontrolled tube is biased to cut-oil, thereby p'reventing signal voltage impressed upon. the input thereof from becoming efiective in the output circuit. The cut-oil bias is ordinarily negative, that is, less positive than is required for normal transmission; hence, when operated in the cut-0i! condition, the entire controlled system oflers ineflect a high impedance tothe transmission of signal voltages therethrough. Conversely, when an appreciable signal voltage is present, the cut-off bias of the controlled tube is altered, and hence the impedance of the controlled system is in eflect therethrough.

It can be seen that whereas, when receiving no signal, the radio-frequency amplifiers oi the usual automatic volume control receiver will have the greatest sensitivity. due to the automatic volume control feature, a receiver embodying the present invention will, under the same conditions, have the controlled tube biased to cut-oil. and therefore will reproduce no sound. impulses lowered, thus permitting the passage of signals.

.the noise-suppressor circuit's'so that the cut-oi! bias of the controlled tube is not removed rapidly enough to let each signal through to produce sounds in the loudspeaker.

A feature is the provision of means for manually controlling the characteristics of the suppressor tube circuit so that any desired input voltage level may be required to cause the controlled tube to operate. This provides a manual control for the "cut-oil!" level, which term will be employed hereinafter to mean the minimum level at which the suppressor tube will operate to permit reproduction of signals. Thus if the level of noise is high, the cut-ofi-level control may be operated to raise the cut-oil level so that 'an input voltage greater than the general noise level will be required before the controlled tube will operate. Conversely, if there is a very low noise level, the cut-ofi-level control may be manually operated to adjust the characteristics of the suppressor tube circuit so that a small input signal voltage will be reproduced.

In order to permit an adjustment of the cutoil level which will not have to be altered while tuning through the frequency band of the receiver, uniform-gain radio-frequency circuits are preferably employed.

Having thus briefly described the present invention, attention is invited to the accompanying drawings, in which:

Fig. l is a simplified schematic diagram of a circuit incorporating the invention:

Figs. 2a," 2b, 2c, 2d, 2e, and 21 are diagrams illustrating the operation of the circuit of Fig. 1; Fig. 3 is a circuit diagram of a more practical embodiment of the present invention;

18. 4 is an overload-characteristic curve of a radio receiver for illustrating the operation of a receiver embodying this invention;

Fig. 5 illustrates another embodiment of the invention;

Fig. 6 illustrates an operating characteristic of the arrangement shown in Fig. 5; and

Fig. 7 an arrangement of Fig. 5 in a more complete receiver.

The circuit of Fig. 1 includes a radio-frequency, or carrier-frequency, amplifier II which is adapted to amplify the incoming signal and impress the amplified signal upon the input of the diode detector i2. The detected signal is then amplified by" a signal translating device which is the audio-frequency amplifier it, which is the controlled tube" in the embodiment shown, and the amplification of the detected signal is controlled, in accordance with the present invention, by means of a noise-suppressor tube II, the operation of which will be explained in connection with the following more detailed description of the circuit arrangement.

The input of the radio-frequency amplifier tube ll includes a tuned circuit II for tuning to carrier-frequency channels, and the output of this amplifier includes the output inductance 20, which is inductively related to the input inductance of the tuned input circuit 2| of the detector i2. The detectoroutput includes the resistor 22, by-passed by the radio-frequency by-pass condenser 23. The resistor 22 and the condenser 28 are so proportioned as to cause the tube [2, which is connected as a diode, to operate as a "peak detector, as has been described in the above referred to application of Harold A. Wheeler, Berial No. 603,490.-

One side of the resistor 22 is connected through resistor-4 to the tuned circuit ii of the radio-' frequency amplifier II. The output circuit of the amplifier II is completed through the highpotential source 2!, and a biasing battery 30 is provided to give the grid of the tube the proper bias, as will be explained hereinafter. The detected signals are impressed upon the amplifier l3 through the coupling condenser ii and resistor 32, the voltage drop across a portion of which is impressed upon the grid of the amplifier tube ii. The amplifier i3 is provided with a grid biasing source 35 and with an output circuit comprising output inductance and high-potential source 31. The inductance 8B is inductively coupled to the inductance 38, which, by means of the terminals 38, may be employed to supply a a sharp cut-off in its grid-voltage plate-current characteristic curve in order that a relatively small change in grid voltage will make a very decided change in the plate current. A type 221 tube is satisfactory for this purpose.

The controlled tube, in this case the audio-frequency amplifier l3, should have a vsuillciently high amplification factor that a small change in negative grid bias will bias the tube to cut-ofl and will stop substantially all plate current. A tube which has been found satisfactory for this purpose is the type'22l tube.

The condensers by which circuits I i and 21 are tuned may be electrically similar and connected to be simultaneously operated as indicated by the dotted line connecting them in Fig. 1.

In operation, the high-frequency amplifier tube I I is biased by means of the potential source so that it will have maximum sensitivity withno signal present. The presence of a high average current through the resistor 22 will cause the grid of tube i I to become more negative relative to the cathode thereof and consequently reduce the amplification of the high-frequency amplifier.

With no radio-frequency signal input to the detector i2, the grid of the noise-suppressor tube It will have substantially the same potential as the cathode thereof, and a maximum plate current will flow through the lower portion of the resistor 32. This will cause the point 34 and the grid of the audio-frequency amplifier i3 to be biased so much more negative than the normal bias voltage produced by the bias source that it will be biased to cut-oil or past, and no amplification or transmission through this tube will take place when the voltage of the point 34 is fluctuated by the detected currents passing through the coupling condenser 3i and resistor 32. It is thus apparent that when the grid of audio amplifier I3 is biased more negative under the influence of smaller received signal intensities, its mutual conductance, and hence, amplification decreases; that is to say, its effective internal impedance increases.

Thus, in the condition of no radio-frequency signal, although the amplifier tube I I is operating at maximum efilciency and the receiver is in its most sensitive condition, as, there is very little or no average fiow of rectified current. through the resistor 22, the bias of the tube l4 will be such that a substantial plate current will-be passing through the resistor 32, and the audio-frequency amplifier 13 will therefore be inoperative to transmit strayv voltage fluctuations which pass through the detector and are impressed upon the grid of the tube through the coupling condenser ii. If, on the other hand, a signal is impressed upon the detector through the amplifier H, the average current through the resistor 22, and therefore the average negative voltage at the low end thereof, will increase and the plate current of the noise-suppressor tube l4 will be decreased, thus increasing the voltage at the point 34, that is, making it less negative with respect to the cathode, and permitting the tube I3 to act as an audio-frequency amplifier of the signals impressed thereon throigh the coupling condenser 3i.

It is to be understood that, due to the automatic volume control of the radio-frequency amplifier I I, if there-is any substantial signal present at all, the average current through the resistor 22 will be sufilciently high to cut oil? the plate current in tube l4 and will leave the negative biasing voltage of the point 34 substantially constant over the range of received signal intensities required for normal operation. If, on the other hand, no signal is present, there will be substantially no current fiowing through resistor 22, and the point 34 will have a potential which will cause sufiicient plate current in tube i4 to-bias the amplifier 13 to cut-off.

The specific details of the method shown for obtaining the automatic volume control effect, constitute .no part of the present invention, it being understood that anyappropriate method may be utilized by which the amplification of the radio-frequency amplifiers and /or the translation gain of the firstdetector and intermediate-frequency amplifier of a superheterodyne are controlled. The noise-suppressor efiect may be obtalned by controlling the amplification of any tube or tubes following that which supplies the suppressor tube, such, for instance, as an intermediate-frequency amplifier, second detector, or first or second audio-frequency amplifier of a superheterodyne receiver.

For the purpose of explaining the function of the circuit just described, attention is now invited to Figs. 2a through 2!, inclusive. In Fig. 2a the ordinates represent the radio-frequency voltage in the input of the amplifier ii '(Fig. i) whereas the abscissas represent the frequency to which the input circuit I5 is tuned. Curve 40a represents the voltage impressed upon the grid of tube as the input circuit is tuned through the frequency of transmission 40 of a signal which is received with medium intensity. Curves Ma and 42a similarly represent the grid voltages of tube l I as the circuit 15 is tuned through the frequencies 4i and 42 of signals being received with low and high intensity, respectively. Curve 430. represents the voltages impressed upon the tube H due to stray atmos pheric disturbances and due to tube noises, etc., in the preceding amplifier stages.

Fig. 2b shows a curve 44b, which represents the radio-frequency amplificationin the tube H as the circuits I5 and 2| are tuned through the range of frequencies covered by Fig. 2a; The portions 431), which show substantially uniform radio-frequency amplification, indicate the amount of amplification of the ground noise, etc., shown by the curve 43a in Fig. 2a. The portions of the curve 44!) designated 40b, 4"), and 42b indicate the decrease in radio-frequency amplification as the circuits i5 and 2i are tuned through the signal frequencies 40, 4| and 42, respectively. n In Fig. 2c the curve 440 represents the peak audio-frequencyinput to the tube Hi The portions 430 are the peak input voltages resulting from the amplification of the ground noise indicated by curve 431:. The portions 400, Me and 420 are the peak voltages produced as the stations 40, 4| and 42 are received. The dotted curve 41 represents the average voltages existing across the resistor 22. Curve 41' indicates the average voltage representative of a much higher ground noise level, as will be explained hereinafter.

As explained above. whenever the voltage across the resistor 22 increases above a certain predetermined value. indicated by a decrease in the radio-frequency amplification at point 45 in Fig. 2b, assuming the circuits to be tuned from left to right, the tube l4 will be biased to cut-off, which will remove the cut-off bias potential of the grid of tube 13. This results, then, in an audio-frequency amplification by the tube 13,

which is indicated in Fig. 2d, in which the portions 40d, Md, and 42d represent the audio-frequency amplification existing asthe receiver circuits are tuned through the frequency of the signals 40, 4i and 42 respectively. It will be noted thatthe sides of the curves 4211, as represented at 45d and 46d respectively, are decidedly abrupt. This abrupt change in amplification takes place as the average detector voltage across the resistor 22 passes points which may be adjusted as desired and which are indicated at 45c and 460 respectively. Between these points on' the curve 41, indicating the average voltage across the resistor 22, a signal of relatively uniform intensity is being impressed upon the detector circuit.

In Fig. 22, the combined amplification of the tubes ii and i3 is indicated. The portions of the curve 4ile, for instance, are represented as being composed of the abrupt rise in audio amplification 40's, the dip in radio-frequency amplification, 402), between the points 45c and 46s caused by the decrease in radio-frequency amplification as indicated in Fig. 2b, and the decrease in audio amplification indicated, 40"e.

The audio-frequency output of the receiver is indicated by curves 40f, 4|} and 421, respectively, of Fig. 2!, which curves represent the overall output when tuning through the frequency of signals 40, 4i and 42, respectively.

The cut-oil" of the audio-frequency tube is determined by the average voltage across the resistor 22, and thereforvif the noise is heavy, depending upon the characteristics of the noisesuppressor-tube circuits, sufficient voltage will be built up across the condenser 23 and the resistor 22 to result in a decrease in the bias of the audio-frequency amplifier i 3, which will permit the noise to pass through although no station is being received. As above noted, the point at. which the tube 14 is biased to prevent its drawing plate current, and thus remove the blocking bias on the tube l3, may be adjusted manually to suit theparticular conditions, which audio-frequency amplification will not be obtained in the absence of a signal.

A circuit for accomplishing this result is shown in Fig. 3, which circuit is quite similar to that shown in Fig. 1, similar parts being indicated by the same reference characters. This figure differs from the above-described figure principally in the provision of a unitary power supply source, the plate and grid voltage portionsof which only are shown. In this circuit, a series of resistors isconnected between the gh-p tential inputs, the negative terminal of which is represented at and the positive terminal of which is represented at 60. The ground of the setis connected to an intermediate point ii. The bias voltage between the grid and cathode of tube II is determined by the portion of the resistor-22 between terminal 22' and the right end thereof, resistor 9 and the resistance between terminals 43. and ll. When no signal is being received, the current flowing through the resistance between terminals l9 and BI will provide the initial grid bias. The bias required for the automatic volume control feature is produced by the detected current flowing through the resistor 22. The grid bias of the tube I4 is provided by means of the entire resistor 22. A resistor i1 is included in the grid lead of the tube ll to prevent impressing radio-frequency signals on the grid of tube It, and a by-pass condenser IQ is also provided for I this same purpose. A radio-frequency by-pass condenser 55 is also connected between the lower end of the tuned circuit l and the cathode of tube H. The screen voltage potential of the tube Il may be adjusted, as desired, by means of the variable contact 52 on the resistor between terminals BI and II. The contact between the leads from the plate and grid of tubes i4 and I3. respectively, is made variable, as indicated at 34', by means of which the volume output of the receiver may be controlled. The plate circuits of tubes I4 and I3 include'resistors 23 and ll respectively, for the purpose of properly adjusting the potentials of these elements. Appropriate radio-frequency by-pass condensers 21 and 2| are also provided across these resistors.

The tap 22' on theresistor 22 may also be connectedto preceding high-frequency amplifier tubes, as desired, to permit automatic control .of the amplification of such tubes. The circuit of Fig. 3 is primarily intended to include the intermediate-frequency amplifier and second detector of a superheterodyne radio receiver, in

, which case the circuits i5 and 21 would be tuned ;to intermediate frequency,'and additional radiofrequency amplifier tubes and first detector tubes would be provided, the grid bias potentials of which would be automatically controlled by connection to the tap 22" of the resistor 22.

The high amplification factor of the screengrid noise-suppressor tube permits a small change in the average voltage across the grid-leak to give a large change in the plate current and therefore a large alteration in the bias of the audio amplifier. Thus, the required change in voltage across the grid-leak being very slight, the audio-frequency amplifier is eifectively turned on and off as the set is slowly tuned through a signal.

The operation of the noise-level cut-off control,

which is the most important particular in which this figure differs from the circuit of Fig 1, willnow be described. Referring to Fig. 2c, assume that with the' audio-cut-ofl working at a sensi-- tivity of 0.3 volt as indicated at "cf the noise level is increased above 0.3 to 41'. Then obviously the noise-suppressor tube would be biased to cutoff and the audio amplifier would operate. It is then desirable to decrease the sensitivity of the cut-oi! control to, say, 0.6 volt as indicated at "c. To do this, the screen-grid voltage may be manually increased to 2.4 volts by operation of the variable contact 52, which, assuming a grid-screen amplification factor of 8, will then necessitate that the grid be biased 0.3 volt more negative to operate the cut-off of the suppressor tube and cut in the audio amplifier. Then, although the average voltage existing across the grid leak 22 is that shown by curve 41, ground noise will be completely suppressed while tuning between signals.

In furtherexplanation of the operation of the cut-ofl level control, attention is now invited to Fig. 4, which shows an overload characteristic curve of an automatic volume control radio receiver. The ordinates are output'volts from the audio amplifier and the abscissas represent signal input to the antenna, in micro-volts. The curve 60 gives the overall characteristic of a receiver operating in its most sensitive condition. This curve is taken with the noise output filtered out. The curve 60 is that portion of the output due tointemal noise which was filtered out in curve 60. The curve 60" gives the total voltage output due to signal and internal noise plotted against input signal voltage.

The curve 6| shows the output of the audio amplifier when the set is operated in a less sensitive condition, as would be the case, for instance, with a local-distance switch" in the "local position. As indicated, the sensitivity of the receiver is then so reduced that there is no noise component in the output, attributable to internal noise.

The curve 8il-6ll"-62 shows the output of a' receiver employing a noise suppressor, constructed in accordance with the present invention, with'the cut-oif-level control set, say, for the lowest practical cut-off level. This level has been, in this instance, selected as that at which the result will give an audio output which is half noise and half signal, assuming the most ideal atmospheric condition of absolute freedom from atmospheric disturbances. This corresponds to the condition resulting in the curve 41 of Fig. 20,-

where the distance between the base line and the curve between signals represents the audio input resulting from internal noise. This condition gives a sensitivity of about 4 micro-volts. Yet whenever the signal input falls below point 63, corresponding to about 3 micro-volts, the noise suppressor will operate and the controlled tube falls below 30 micro-volts, the audio amplifier will be inoperative to transmit signals to the loudspeaker. Thus, if a 100 micro-volt signal is being received, the output to the loudspeaker will be 202 volts, as indicated at 81. On the other,

hand, with a simple local-distance switch, under the same conditions, the output .for a similar signal would be less than 50 volts, as indicated at point 88.

Similarly, it the noise level is greater, the cutoil-level control may be operated to require a signal of 100 micro-volts, indicated at 88, to operate the suppressor tube to cause operation of the reproducing apparatus. The receiver would then operate as indicated by curve 88-48.

The manual volume control is a common arrangernent in which any desired portion of voltage fluctuation produced across a resistor included in the output circuit of a detector may be utilized .to transmit the signals to the audiofrequency amplifiers. The connection from the plate or the noise-suppressor tube H to the resister ,82 may be at any desired point, such, for instance, as at the top thereof, although the arrangement shown has been found quite 'con-' venient due to the fact that the portion of the resistor 82 included in the grid circuit of the tube l8 and plate circuit of tube ll increases as the control is set to higher output volume. thus increasing the biasing cut-oil voltage on the grid of tube I 8 as increased suppression is needed.

In view of the fact that ii the input gain is not uniform the sound level would rise and fall as the receiver is tuned and it could therefore not be efllciently operated with one setting of the cut-ofi-levei control, it is, as above noted, desirable that the input circuits of a receiver embodying this noise suppression invention be of the uniform gain type. Otherwise, the cut-ofllevel control would have to be varied as thereceiver is tuned through its range.

The operation of the circuit of Pig. 3 is in all "other respects similar to that of Fig. 1, which has been described in full above.

The usual cathode heating circuits are provided, and any appropriate tubes may be substituted for those shown in both Fig. 1 and Fig. 3,

the specific details of none of these elements constituting any part of the present invention.

A variation of the invention is illustrated in Figure 5, wherein the-functions .of the detector and of the noise-suppressor tube are both performed in a single stage. In this figure, there are shown only the detector and part of the audio-frequency amplifier, these being the essential elements for suppressing the interchannel noise. The carrier frequencysignals are impressed upon the tunable circuit 88 of the detector tube 8|, and the signals are detected in the usual manner by the detector tube, which is of the three-electrode type operating as a bias type detector having a biasing resistor and a by-pass condenser in the grid-cathode circuit. The bias-resistor is constituted by the seriesarranged resistances 88 and 88, shunted by the condenser 81. The output of the detector'tube 8| is coupled to the first audio-frequency amplifier tube 82, which in turn; supplies'the grids 88 and 88 of a pair of power tubes-connected in the wellknown push-pull relation; No elements of the power tubes, other than the grids, are shown, asv

no further discussion of the push-pull action is required, since it consistutes no part of this inevention and the arrangement is well understood in the art.

By virtue of the rectifying action oi the detector, there is produced in resistors II and 88 a uni-directional current which causes the upper terminal of resistor 88 tohave a more positive potential than the lower terminal of resistor 88.

The interchannel noise-suppressing action is eii'ected by means of a lead 88 connected between a point of resistor 88 and the screen grid 88 o! the first audio-frequency amplifier 82, thereby applying a bias to the screen grid.

When no useful signal is present at the input or the detector there is little or no rectified current flowing through resistors 88 and 88, and consequently, the potential of the screen grid 88 is substantially that of the cathodes, namely, ground potential. Under such a condition, the screen grid acts as an eiiective shield between the cathode and the anode of tube 82 and substantially prevents the flow oi anode current, thereby causing the tube to act as a cut-out relay and prevent audio-frequency reproduction by preventing the audio-frequency current from being impressed upon the push-pull amplifier. In other words.

under the condition of zero or very-small signal intensity the efiective internal impedance between the input and output electrodes of the tube is very great. When, however, a carrier signal is received at the detector of the order of magnitude required for normal operation, the rectified current flowing in resistances 88 and 88 causes the screen grid 88 to become biased positively and thereby permits the tube to function in the normal manner as an amplifier. The audio-frequency signals will then be transmitted to the grids 88 and 84 of the push-pull power stage from whence.

they may be utilized in any desired manner.

A clearer understanding of the action of the audio-frequency amplifier 82 as a controlled" tube may be obtained from an inspection 0! Figure dwhich illustrates the relationship between the gain of the audio-frequency amplifier, plotted as ordinates and the screen grid bias applied to the screen grid 88, plotted as abscissas. It will be observed that when the screen grid bias is in-' creased from zero to an optimum value E, the audio-frequency gain rises from practically zero. or cut-oil, to a maximum. Consequently, when no carrier signal is present, the audio-frequency gain is substantially zero, and upon the receipt of signals strong enough to raise the screen grid bias beyond the predetermined "cut-oil", the gain rises rapidly to a maximum.

Figure 7 illustrates the lnterchannei noise-suppressing system of Figure 5 incorporated into a receiver comprising an automatic volume control system operating upon a carrier-frequency ampiifler. The complete push-pull power stage and a loudspeaker 8! connected to the output thereof are aisoshown. That portion of the receiver including the radio-frequency. or carrier-frequency, amplifier is represented by a rectangle 88. since it is unnecessary to illustrate it in detail to bring out the novel feature involved. The carrier-trequency amplifier is shown coupled to the tunable input circuit 88 of the detector 8!.

The automatic volume control action is proi Fig. 5, this change being made to provide the proper relative potentials for the automatic volume control and the noise-suppressor systems. The cathodes of the carrier-frequency amplifier should preferably be maintained approximately at thepotential of the detector cathode. It may also be desirable to isolate the anode-supply potentials of the carrier-frequency amplifier from those of the detector and other tubes. If it is desired that the cathodes of the carrier frequency amplifier be maintained at approximately ground potential, this may be accomplished by employing a separate phase-reversing tube which is connected between the lead '2 and the point it.

Such an arrangement will avoid the use of a separate source of anode potential for the carrier frequency amplifier.

Since, due to the rectifying action of the detector, the upper terminal of resistor II is more positive than any other point of resistors II and N, the potential of lead II is more negative than the cathode of the detector; hence, the control electrode at it is biased negatively (assuming the cathodes of the carrier-frequency tubes to have approximately the potential of the detector cathode) In consequence, the control electrodes of the carrier-frequency amplifier become biased more negatively the greater the received signal intensity, thereby producing the automatic volume control action over a considerable range; of received carrier-signal intensities.-

The constants of the receiver are preferably so chosen that when the maximum signal strength permitted by the automatic volume control is 're-' ceived at the detector, the bias on the screen grid '8 will hot rise materially beyond the optimum value indicated in Figure 6 at E. In case the received signal is rather weak and the carrierfrequency intensity is not strong enough to produce a normal signal current in the anode circuit of detector ll, the audio-frequency gain may be increased by moving the contact of lead II to a more positive position on resistor J 'Ihis resistor, or potentiometer thus acts both as a cutoil-level control and as a volume control.

I claim:

1. In a radio receiver, a detector and an audiofrequency amplifier comprising a tube having a cathode, anode, control electrode and a screen electrode, said detector having in its output circuit a resistance across which there is developed a uni-directional voltage dependent upon the intensity of signals applied to said detector, and a connection from a point of said resistance to said screen electrode for causing the transmission through said tube to be greatly reduced when :ld signal ,intensity falls below a, predetermined ue.

2. In a radio receiver, a detector having a cathode-resistor biasing arrangement across which there is developed a uni-directional voltage dependent upon the intensity of signals applied to said detector, an audio-frequency amplifier tube of the screen-grid type coupled to the output of said detector, an automatic volume control arrangement for maintaining the signal strength at said detector substantially constant over a wide range of received signal intensities, and a connection from a point of said resistor to the screen grid of said audio amplifier for causing said amplifier to become practically unresponsive when the intensity of signals at said detector falls below a predetermined value below said range.

occurs, means for automatically controlling the volume of the signals impressed upon said detector, whereby the intensity of signals at said detector is maintained substantially constant over" a wide range of received signal intensity, and a connection from a point of said resistance to said screen grid for rendering said screen grid less positive when the signal intensity at said detector falls to a relatively small value. a

4. In a radio receiver, in combination, a carrier-frequency amplifier, a detector coupled to the output of said amplifier, said detector being of the vacuum-tube type having a cathode, anode and grid, a second amplifier coupled to the output of said detector, said second amplifier comprising a vacuum tube having an anode, cathode, control grid and a screen grid, said detector having in its cathode-grid circuit a resistance shunted by a condenser for producing the detecting action, and a connection from a point of said resistance to said screen grid, the polarity of said connection being such that said screen grid becomes biased less positively when the signal strength falls below a predetermined value, whereby interchannel disturbances are substantially reduced.

5. A combination according to claim 4 in which the cathode of said second amplifier is grounded and said resistance is connected between said detector cathode and ground.

6. In a radio receiver, in combination, a carrier-frequency amplifier, a detector coupled to of the vacuum-tube type having a cathode, anode ,and grid, a second amplifier coupled to the output of said detector, said second amplifier comprising a vacuum tube having an anode, cathode,

'control grid and screen grid, said detector having connected in its cathode-grid circuit a resistance across which a uni-directional detected voltage is developed, automatic volume control means associated with .said carrier-frequency amplifier for maintaining the output of said detectorv substantially constant over awide range of received signal intensity, and a connection from a second point of said resistance to said screen grid for biasing said screen grid less positively and thereby rendering said second amplifier substantially unresponsive when the signal strength falls below a predetermined value below said range, said second point of said resistance being at a more negative uni-directional potential than said first point.

JOHN KELLY JOHNSON.

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