US2069854A

US2069854A - Automatic noise suppressor circuit

Info

Publication number: US2069854A
Application number: US35602A
Authority: US
Inventors: Otto H Schade
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1935-08-10
Filing date: 1935-08-10
Publication date: 1937-02-09
Anticipated expiration: 1954-02-09

Description

Feb. 9, 1937. o. H. SCHADE AUTOMATIC NOISE SUPPRESSOR CIRCUIT Filed Aug. 10, 1935 v mwm,

Patented Feb. 9, 1937 UNITED STATES PATENT OFFICE Otto H. Schade, West Caldwell, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application August 10, 1935, Serial No. 35,602

8 Claims.

My present invention relates to automatic control circuits for radio receivers, and more particularly to a novel and improved type of background noise suppressor for a radio receiver which utilizes automatic gain control.

One of the principal operating defects of most background noise suppressor circuits of radio receivers equipped with automatic gain control networks is the existence of a range of transi- 10 tion from cut-oi bias to normal bias on the noisecontrolled tube. This transition range causes, when the control is applied to the audio system, high distortion of the audio frequency signal. It has been found that where the background noise i5 suppressor circuit possesses a trigger action then the transition range effect is eliminated. The trigger action usually involves a network which is capable of sluiting the bias value of the controlled tube from cut-off to normal without any other possible stable value.

One of the main objects of my present invention may be said to reside in the provision of a background noise suppressor circuit for a radio receiver of the AVC type, which suppressor circuit possesses a trigger action characteristic in that it is capable of applying a cut-oi bias to an audio amplier without the necessity of adjusting the bias through any transitional bias value, and the suppressor circuit essentially comprising an oscillator under the control of the incoming signal, and whose oscillations are employed to produce the control bias for the audio amplifier to be controlled.

Another important object of the invention is to provide in a radio receiver equipped with an automatic volume control arrangement, a noise suppressor system which functions to regulate the operating bias of at least one audio amplifier of the receiver, and the suppressor circuit including an electron discharge tube which comprises an oscillator section and a pair of rectifier sections, one of the rectifier sections functioning to control the operation of the oscillator section, and the other rectifier section functioning to rectify the oscillatory output of the oscillator section for the purpose of producing the control bias for the audio tube which is to be controlled.

Another object of the present invention is to provide a superheterodyne receiver of the type including an automatic volume control system whose function it is tc maintain the signal amplitude at the demodulator substantially uniform over a wide range of signal amplitude variation at the signal collector, the receiver being particu- (Cl. Z50-20) larly characterized by its inclusion of a background noise suppressor arrangement, whose operation is dependent upon the decrease of the received signal amplitude below a predetermined amplitude, the suppressor arrangement compris- 5v ing an oscillator, whose oscillations are rectied, and the direct current voltage component of the rectified oscillations being used to render the reproduction of the demodulated signals ineflicient.

Still other objects of the invention are to im- 10 prove generally the operating efficiency of superheterodyne receivers equipped with noise suppressor arrangements, and more especially to provide back-ground noise suppressor circuits for radio receivers equipped with automatic volume l5 control, which noise suppressor circuits are not only reliable and efficient in operation but economically manufactured and'assembled in radio receivers.

The novel features which I believe to be char- 20 acteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken vin 25 connection with the drawing in which I have indicated'diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing: 30

Fig. 1 shows a circuit diagram of a superheterodyne receiver embodying the invention,

Fig. 2 graphically illustrates the operation of the noise suppressor network in the circuit of Fig. 1. A35

Referring now to the circuit diagram in Fig. 1, it is pointed out that this circuit is that of a superheterodyne receiver; the receiver comprising the usual signal collector A which may be of the grounded antenna type. The collector may, 40 if desired, be the usual automobile radio signal collector, a loop antenna, and even a radio frequency distribution line of a hotel or apartment house. Regardless of the nature of the signal collector, the collected signals are impressed upon 45 a tunable radio frequency amplifier I which may ybe of the pentode type, and utilize a tube of the 6D6 type. The input circuit of the amplifier is shown as comprising a single tunable circuit 2 which is provided with a variable tuning con- 5o denser 3. Of course, those skilled in the art are fully aware of the fact that the selector network preceding amplifier l may be of the band pass type wherein the band pass network comprises a pair of tunable circuits having over-optimum 55 known as a pentagrid converter tube, commercially designated by the symbol 6A7. Since this tube and its circuits are Ywell known. to those skilled in the -art at the present time, it is believed sufficient to point out that theinput signal for the converter circuit is impressed upon the tunable input circuit 4, while the circuit 6 functions as the local oscillator circuit. The variable tuning condenser 'I in the signal input circuit 4 is arranged to have its rotors mechanically unicontrolled with the rotors of condenser 3. The variable tuning condenser 8 of the oscillator circuit 6 also has its rotors uni-controlled with the rotors of condensers 3 and 1, and the dotted lines 9 represent the, mechanical uni-control mechanism employed to vary these tuning condensers in unison.

Assuming that the operating intermediate frequency of the system is chosen to be 456 k. c., although it is to be understood that any other value from 175 k. c. to a value of the order of 500 k. c. may be selected, then it will be clear that the rangeof the local oscillator circuit 6 will be from 1006 k. c. to 1956 k. c. The intermediate frequency energy is taken out of the converter network by means of the resonant circuit I Il which tuned by the fixed condenser included therein to the 456 k..c. I. F. The I. F. energy in circuit I0 may be amplified in one or more stages of I. F: amplification.

The I. F. amplifier II may be a tube of the 6D6 type, which is a pentode, and the resonant input circuit I 2 thereof is xedly tuned to the operating I. F. of 456 k. c. Circuits VII) and I2 are coupled to normally provide a sharp tuning single peaked characteristic. vThis is obtained by coupling the coils of the two circuits with slightly less than optimum coupling. In order to provide a means for manually adjusting the width of the I. F. band transmitted to the demodulator of the Y receiver, there is provided an auxiliary adjustable capacity in each of circuits I0 and I2. Each of these auxiliary capacities is adjustable over a small'range of frequencies, and the condensers are designed to vary the I. F. frequency in opposite senses.

Thus, when it is desired to widen the I. F. band width, the auxiliary capacities are adjusted so that the frequency of circuit I0, for example,

will Vary a predetermined frequency value below the I. F., and the frequency of circuit I2 will be varied to the same frequency value above the I. F. 'I'his band widthl control is provided in order to accommodate the receiver to local or Weak station reception ;k when receiving local stations high quality is desired and all the side band frequencies of `the carrier are thus included. Since the auxiliary capacities are large in size, the adjustment thereof is made by a step switch.

Itis to be clearly understood that the band width control mechanism shown associated with ,circuits I0.and I2 may be replaced by any other 'type of band width control.

In superheterodyne receivers of the so-ca-lled high fidelity type such a band width control is usually included because it is desired to have the operator select at his convenience the type of selectivity he Wishes t0 employ, and which selectivity is dependent upon the range of side band frequencies he desires tov have reproduced. The network II is followed by a network I3 which includes a tube of the 6B7 type. This type of tube comprises a pentode section and a diode section, the pentode being used for intermediate frequency amplification as in the case of network I I.

It is to be clearly understood that there may be employed between the output of tube II and the input of the pentode section of the following 6B7 tube a network exactly corresponding to that shown between networks 5 and II. The diode section of network I3 is employed in this case for AVC rectification. That is the I. F. energy may be impressed upon the diode section and rectified, and the direct current component of rectified signals is then employed as AVC bias in the Well known manner.

The pentode sectionof network I3 will, therefore, function as an I. F. amplifier, and, if desired, it may also function as an I. F. amplifier for the energy prior `to impression on the diode section of network I3. Reference is made to my applica.- tion Serial No. 673,389, led May 29, 1933, Patent No. 2,039,666, granted May 5, 1936 which shows such an arrangement wherein the AVC diode rectifier derives its I. F. energy in amplified form from the output of the pentode section of a 6B7 I. F. amplifier tube. It is to be clearly understood that the specific construction of the AVC network is not an important element of the present invention.

What is essential is that there be provided in the receiver an AVC arrangement which functions in the well known manner to decrease the gain of each control tube as the received signal amplitude rises above a predetermined carrier amplitude. In Vthis way the signal amplitude at section thereof has its strapped anodes connected 2 to the high alternating potential side of input circuit I5, whereas the cathode of the tube is connected to the low alternating potential side through the load resistor II` which is properly by-passed for I. F. currents by condenser I8. The l input circuit I5 which feeds the demodulator, or .second detector, is tuned to the operating I. F., and the audio component of demodulated signal energy is impressed upon the signal grid of the pentode; section of tube I6 through a path which includes the variable tap 20 and the condenser I9.

The amplified audio component of demodulated signal energy is then transmitted through the audio frequency transformer 2I to the following audio network. 'I'his subsequent audio network may comprise any of the Well known types of audio networks well known to those skilled in the art. For example, this following network may comprise a system of they type disclosed by me in Fig. 8 of my application Serial No. 698,407,

fifi

erodyne receiver wherein the signal amplitude at the second detector input circuit I5 is maintained substantially uniform regardless of fading effects. As is well known to those skilled in the art background noise reproduction increases in a most annoying fashion when tuning a receiver, as shown in Fig. l, between signal channels. Such noise reproduction also increases to a great extent whenever the signal amplitude received decreases below the operating minimum level. Background noise suppressor arrangements have been utilized in the past to automatically quiet, or mute, the receiver whenever the AVC system is not functioning to reduce the gain of the controlled stages, and the received signal amplitude drops below a predetermined intensity level. The principal defect of most of these noise suppression circuits has been in the existence of a range of transition Ifrom cut-off bias to normal bias on the controlled tube.

W'here the noise control bias is applied to the audio stage such a transition range causes high distortion of the audio signal. The most desirable type of noise suppressor network is one which possesses a trigger action so that any stable value intermediate the normal and cut-off bias values is eliminated. A noise suppressor arrangement possessing such a trigger action is shown in Fig. l. This noise suppressor circuit generally comprises an oscillator whose operation is under the control of the incoming signals, and whose oscillatory output is rectified and utilized for the muting action on the audio amplifier of the receiver. The suppressor arrangement is constructed so as to involve a minimum of circuit elements, and comprises tube 2| of the 6B7 type whose pentode section and two diode sections are arranged as shown in the figure.

I. F. energy is impressed upon one of the diode sections through a circuit which includes the resonant network 22; the latter is tuned to the operating I. F.; and is magnetically coupled to the tuned circuit l5. The high alternating potential side of circuit 22 is connected to the diode anode 23, while the low alternatingpotential side of the circuit is connected to a point on voltage source 24 through a path which includes the resistor R1. The voltage source 24 may be part of the general B voltage supply of the tube- 2|, and for this reason the voltage source 24 is to be understood as comprising another portion of this voltage supply. The cathode of tube 2| is connected to the positive side of the voltage supply portion 24, while the negative terminal of this voltage supply section is grounded. The point on source 24 to which resistor R1 is connected is negative with respect to the point to which the cathode of tube 2| is connected.

The cathodes of tubes 2| and |'u` are at a common direct current potential, and are connected by a lead 25. The plate of tube 2| is connected to the positive terminal of the voltage supply section 24 through a resonant network 26, the negative terminal of the voltage supply section 24 being connected to the cathode of tube 2|. The resonant network 26 is reactively coupled to the screen grid of the pentode section of tube 2| through an audio frequency transformer 2'|. There is thus provided a local oscillator for producing local oscillations which are to be rectified, and the direct current component of the rectified oscillations is used to control the audio transmission efficiency of the receiver.

The oscillations produced by the oscillator just described are impressed upon a rectifier which comprises the diode anode 28. The load resistor R3 is connected to a point on the voltage supply section 24 which is negative with respect to the point thereon to which the cathode of tube I6 is connected. The condenser C reactively couples the screen grid of the pentode section of tube 2| to the diode anode 28, and comp-rises the path through which local oscillations are impressed upon the diode rectifier including diode anode 28. The direct current Voltage developed across resistor R3 is impressed upon the signal input grid` of the pentode section of tube I6 through a path which'includes the resistor R4. and the resistor R5.

'Ihe grid side of resistor Ri is grounded through a condenser C1, and the anode side of resistor R3 is connected to the opposite side of the resisto-r R4. The direct current voltage developed across resistor R1, by rectification of the I. F. voltage from circuit 22 by the diode including anode 23, is impressed upon the first grid of tube 2|. The impression path includes the adjustable tap 3|), the resistor R2 and the resistor Re, the junction of resistors R2 and Re being connected to ground through condenser C2.

The operation of the receiving systemwill now be described, attention being directed to Fig. 2 which shows in a graphic manner such operation. Fig. 2 shows the overall selectivity including the combined selectivity of all tuned circuits inclusive of circuit 22. The I. F. voltage of circuit I4 is applied to the diode section of tube I6 for demodulation, and the resulting IS, and thereafter amplified and transmitted through the subsequent audio frequency network. The pentode section of tube I6 functions as an audio amplifier, and the requisite signal grid bias is provided for the signal input grid of tube I6 through the path which includes resistor R5, the resistor R4 and the resistor R3. The cathode of tube I6 is connected through lead 25 to a point on voltage source 24 which is positive with respect to the point on this source to which resistor R3 is connected. As long as signals above a desired carrier amplitude are being received, the operating grid bias for the pentode section of tube I6 is the potential difference between the points on voltage source 24 to which lead 25 and resistor R3 are connected.

When the received signal carrier amplitude decreases below a predetermined intensity level, and the level is such that the AVC network is not functioning to reduce the gain of each controlled tube, background noises will be greatly amplified because the controlled tubes are operating at maximum efliciency. In such a case the background Suppressor arrangement functions to produce an auxiliary, or additional, Ynegative grid bias for the pentode section of tube l5. This is accomplished by permitting the oscillator section of tube 2| to produce oscillations, and the resulting oscillations are rectified for the purpose of producing the auxiliary negative grid bias for tube |6. The oscillator section of tube 2| comprises the platev circuit 26 which is reactively coupled through transformer 2! to the screen grid electrode of the tube.

The transformer 2'! is a small iron cored transformer having a low natural frequency between l() and y20 k. c., or a suitable high frequency as 1600 k. c. Such a high frequency, which is outside the limits of the operating signal frequency range of a receiver, avoids undue time delay when starting or stopping the oscillations. As soon as the amplitude of the signal carrier energy impressed on circuit 22 falls below a predetermined intensity level, the production of local oscillations between the screen grid and plate of the pentode section of tube 2| commences. The 10 k. c. voltage of the oscillator grid coil is transmitted to the diode anode 28 through the coupling' capacity C, and there is produced a direct current voltage across resistor R3 by viitue of rectification of the locally produced oscillations. Since the signal input grid of the pentode sectionpof tube I5 is connected to the anode side of resistor R3, whenever'local oscillations are produced Vthere will be impressed upon the signal grid of tube I6 the auxiliary negative bias referred to heretofore.

This auxiliary bias is ,sulcient in'magnitude to substantially prevent the amplification of audio signals in the pentode section of tube I6. 'I'he circuit elements R4 and C1 function as a 'llternetwork to suppress the fluctuating component of the rectified current ilowingthrough resistor R3 from reaching the signal gridfof tube I6. When the received carrier amplitude rises above the predetermined intensity level the I. F. voltage on the loosely coupled, sharply tuned circuit 22 rises near resonance. This voltage is rectied in the diode rectifier circuit'which includes the diode anode 23 and resistor R1, and there is produced a direct current voltage across the load resistor R1. The grid nearest the cathode of tube 2| is connected to a desired negative point on resistor R1 through the path which includes resistors Rs and R2 and the adjustable tap 30. Therefore, it will be seen that a part of the direct current voltage developed across resistor R1 is applied through resistor R6 to the rst gridfof tube 2|, and the magnitude of this applied voltage is sufficient to cut olf the plate current flow through the pentode section of tube 2|. This results in a cessation of local oscillations; and as a consequence the rectified 'voltage across resistor R3 disappears. VThis removes the auxiliary cut-01T bias from the audio amplier pentode section of tube I6, and thus permits the normal transmission of audio signals from the demodulator to the reproducer of the system.

The primary or energizing circuit I4 is coupled Y at optimum coupling to the secondary circuit I5 which is used to feed the demodulator diode. This secondary circuit is also loosely coupled to the tertiary circuit 22 which is not coupled directly to I4. This arrangement is important as then the AVC (which is derived from I3) is actuated by a circuit passing ay wider frequency band than is passed by the additional selectivity of circuit 22 which energizes the squelch diode 23. This is important for the following reasons: If tuned to a carrier frequency, the circuit 22 will build up an I. Fand diode load voltage of a certain magnitude of certain ratio to the voltage in I5. With this ratio the squelch diode operates and unlocks the audio system. If thev circuits are, however, not tuned to a carrienthe wider noise spectrum of circuit I3 produces an AVC voltage which in relation to the squelch diode voltage produced by Y noise in the more selective circuit 22 is now considerably greater, and as the ratio is greater, Ythe squelch circuit will not unlock the audio system;

provided that kthe AVC has adjusted the voltage in I 5 to a magnitude comparable with the carrier magnitude assumed before, which is the case for sets with good AVC. This arrangement thus discriminates between relatively large noise levels and fairly weak carrier signals. The point being: eXtra selectivity on the squelch diode circuit. A

second point is that the time constant of the AVC must be shorter than for the squelch, so that the AVC has decreased the noise voltage in I5 before diode 23 had time to produce sufficient bias on R1 to stop the oscillator 2|.

The resistor Re is an alternating current load resistance on the control grid of tube 2| at 10 k. c. Its function is to increase the difference of the values e1 and e2, see Fig. 2, as the oscillating elements are both positive, and thus does not produce any self bias rfor the oscillator system. As the control grid of tube 2|` is loaded at oscillator frequency by resistor Raam alternating current voltage is generated on resistor Re due to coupling in the tube 2|. nating current voltage cause grid current to ow through the control grid of tube 2 I, and this grid.

current in turn causes a self bias on the control griddue to resistors R2, R1 and condenser Cz. There is secured in this fashion for the oscillator circuit an overlapping start and stop characteristic. The required direct current control voltage on the control grid of tube 2| is very small as in practice the second grid, or oscillator grid, is only about 10 volts positive. This makes the circuit sensitive to small changes in carrier voltage which is essential forV receivers with satisfactory AVC action. Y Y v The oscillator including tube 2| may be easily adjusted to start at a certain negative grid bias value, i. e., a definite transconductance for small amplitudes, but to continue oscillating if once started, with considerably higher bias values (self bias on resistors R2 and the portion of R1 below slider 30) as the transconductance for larger amplitudes remains higher as the starting value for small amplitudes at the Vsame grid bias voltage. Thisaction causes theoscillator associated with the plate and screen grid of tube 2| to stoposcillating if the rectified I. F. signal voltage developed across resistor R1 increases to a predetermined value, e1, (see Fig. 2) but the voltage across resistor R1 must be decreased to a considerably lower value e2 before oscillations will be produced again. The voltage e1 may be adjusted by means of slider 3Q so as to be reached near, c-r at, resonance when the I. F. amplitude rises due to Vtuning in a signal. Y

However, if the signal is tuned in, a slighty detuning or fading of the signalwill not start'the oscillator action because the value e2 is considerably lower. This is clearly shown in Fig. 2 where- The positive peaks of this alter-V in the relationships between the vo1tagese1 and e2 and the resonance voltage of circuit 22 are depicted. An impro-per bias value on the signal grid of tube I6 is not possible as a stable value, because the minimum'oscillator voltage amplitude to which oscillations build up is greater than the additional bias voltage needed to cut off the plate current ilow through the pentode section of tube I6.

The tap on R1 determines 'the amount of ,bias` Moved up (in Fig. 1) a small While the suppressor arrangementzin Fig. 1 is I shown of a highly compact type, it is to be clearly 75 access-l understood that the diode demodulator 'may be used to control the production of local oscillations in the suppressor arrangement, and in that case the oscillator may comprise a triode wherein the plate and grid are reactively coupled, and the oscillator may be of the self-bias type. Any common triode oscillator with grid leak and condenser operates Class C. The grid bias voltage is highly negative and cuts off the plate current, but the tuned circuit voltage exceeds the grid bias voltage due to flywheel action of the circuit, thus making the grid positive at the peak swing (just like a diode) thus allowing plate current to flow which in turn energizes the tuned circuit. The negative bias is a self bias (diode action) produced by the oscillation voltage, the amplitudes of which are relatively large. The operating conditions of the oscillator can be so adjusted that the transconductance at large swings is larger than at very small amplitudes. Thus, the oscillator can not start by itself at this bias value, but once oscillating, will maintain oscillations. In such a case the grid of the triode would be controlled in bias from the demodulator load resistor, and the voltage values e1 and e2 when impressed on the oscillator grid would control the production of oscillations.

Furthermore, the locally produced oscillations in such a case can be impressed for rectification, for the purpose of producing the oscillator cut-off bias for the audio tube, upon a diode which is independent of the oscillator tube. Furthermore, the frequency of the locally produced oscillations in the noise lsuppressor arrangement may vary over a range of values, the essential requisite being that this frequency be outside the operating signal frequency range, but be high enough so as to avoid undue time delay when starting or stopping the noise control oscillations.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

l. In combination with a radio receiver of the automatic gain control type, a background noise suppressor network comprising an oscillator responsive to variations in received signal carrier amplitude, means for rectifying the oscillations to produce a cut-off bias for an audio amplifier of the receiver, and additional means electrically associated with the oscillator for imparting a characteristic to the oscillator such that said cutoff bias is produced or removed without any transitional bias values.

2. In a superheterodyne receiver of the type including an automatic volume control system adapted to maintain the signal amplitude at the demodulator substantially uniform over a wide range of signal amplitude variation at the signal collector, the receiver being characterized by its inclusion of a background noise suppressor arrangement which comprises an oscillator adapted to produce local oscillations of a frequency outside the operating signal frequency range of the receiver, means responsive to received signal carrier amplitude variations for controlling the amplitude of said oscillations, and additional means for rectifying the oscillations and impressing the direct current voltage component of the rectier oscillations upon a stage subsequent to the receiver demodulator to render the reproduction of the demodulated signals inefficient.

3. In a receiving system provided with a demodulator, a signal transmission network feeding said demodulator, and means for automatically varying the transmission eiciency through said network in a sense to maintain the signal carrier amplitude at the demodulator input substantially uniform, the receiver being characterized by the inclusion of a suppressor system which comprises an oscillator arranged to produce local oscillations of a frequency outside the operatingl signal frequency range of the receiver, means responsive to an increase in carrier amplitude at the demodulator input above a predetermined intensity level for automatically stopping the production of said oscillations, and means responsive to said oscillations for automatically regulating the sound output from said receiver when said carrier amplitude decreases below said intensity level.

4. In a receiving system provided with a demodulator, a signal transmission network feed-l ing said demodulator, and means for automatically varying the transmission efficiency through said network in a sense to maintain the signal carrier amplitude at the demodulator inputsubstantially uniform, the receiver being characterized by the inclusion of a suppressor system which comprises an oscillator arranged to produce local oscillations of a frequency outside the operating signal frequency range of the receiver, means responsive to an increase in carrier amplitude at the demodulator input above a predetermined intensity level for automatically stopping the production of said oscillations, and means responsive to said oscillations for automatically regulating the sound output from said receiver when said carrier amplitude decreases below said intensity level, said first means including a device for stopping said oscillations only when said carrier amplitude at the demodulator input decreases to said intensity level.

5. In a receiving system provided with a demodulator, a signal transmission network feeding said demodulator, and means for automatically varying the transmission efficiency through said network in a sense to maintain the signal carrier amplitude at the demodulator input substantially uniform, the receiver being characterized by the inclusion of a suppressor system which comprises an oscillator arranged to produce local oscillations of a frequency outside the operating signal frequency range of the receiver, means responsive to an increase in carrier amplitude at the demodulator input above a predetermined intensity level for automatically stopping the production of said oscillations, and means responsive to said oscillations for automatically regulating the sound output from said receiver when said carrier amplitude decreases below said intensity level, said rfirst means comprising a rectifier having a signal input circuit coupled to the demodulator input.

6. In the receiving system provided with a demodulator, a signal transmission network feeding said demodulator, and means for automatically varying the transmission eiiiciency through said network in a sense to maintain the signal carrier. amplitude at the demodulator input substantially uniform, the receiver being characterized by the inclusion of a suppressorsystem which comprises an oscillator arranged to produce local oscillations of a frequency outside the operating signal frequency range of the receiver, means responsive to an increase in carrier amplitude at the demodulator input above a predetermined intensity level for automatically stopping the production of said oscillations, and means responsive to said oscillations for automatically regulating the sound output from said receiver when said carrier amplitude decreases below said intensity level, said last means comprising a rectifier re-A actively coupled to said oscillator for the impression of oscillations on said rectifier, and the output of said rectifier being impressed vupon an audio amplifier of the receiver.

7. In a receiving system provided with a demodulator, a signal transmission network feeding said demodulator, and means for automatically varying the transmission efficiency through said network in a sense to maintain the signal carrier amplitude at the demodulator input substantially uniform, the receiver being characterized by the inclusion of a suppressor system which comprises an oscillator arranged to produce local oscilla- Vtions of a frequency outside the operating signal frequency range of the receiver, means responsive to an increase in carrier amplitude at the demodulator input above a predetermined intensity level for automatically stopping the production of said oscillations, and means responsive to said oscillations for automatically regulating the sound output from said receiver when said carrier amplitude decreases below said intensity level, each of said rst and second means including a rectifier, and the electrodes of said oscillator and two rectiers being disposed within a common tube envelope.

8. In a receiving system provided with a demodulator, a signal transmission network feeding said demodulator, and means for automatically varying the transmission efficiency through said network in a sense to maintain the signal carrier amplitude at the demodulator input substantially Y uniform, the receiver being characterized by the inclusion of a suppressor system which comprises an oscillator arranged lto produce local oscillations of a frequency outside the operating signal frequency range of the receiver, means responsive to an increase in carrier amplitude at the demodulator input above a predetermined intensity level for automatically stopping the production of said oscillations, and means responsive to said oscillations for automatically regulating the sound output from said receiver when said carrier amplitude decreases below said intensity level, said oscillator including an electron discharge tube provided with a pentode section and a pair of diode sections, the plate and screen grid of said pentode section being reactively coupled for the production of said oscillations, one of said diode sections being included in said first means, and the other diode section being included in said second means and having said oscillations impressed thereon through a reactive coupling from said screen grid.

OTTO H. SCHADE.