US2135599A - Automatic volume control circuit - Google Patents

Description

Nov. 8, i938. H. o. PETERSON AUTOMATIC VOLUME CONTROL CIRCUIT Filed Sept. 14, 1935 INVENTOR HAROLD o. PETERSON B QM \M4 h m m v lk A 1 5 0- M K 1 7 qniwwk ATTO R N EY Patented Nov. 8, 1938 UNITED STATES PATENT OFFICE Radio Corporation of Delaware America, a corporation of Application September 14, 1935, Serial No. 40,568

5 Claims.

My present invention relates in general to automatic gain regulation circuits adapted for use in connection with signal amplifiers and radio receivers, and more particularly the invention relates to automatic volume control circuits utilizing diodes as signal detectors and gain control devices.

Diode rectifiers are often utilized at the present time as signal demodulators of radio receiving systems. Such .diode devices are, also, suitable for use as signal rectifiers in connection with automatic volume control circuits in order to maintain a substantially constant signal amplitude at the demodulator input circuit. In the past a common diode device has been used both for the signal detection and automatic volume control rectifier functions. However, many of these past devices have been constructed so as torequire the diode anode to be maintained normally at a negative direct current potential with respect to the cathodes of the signal transmission tubes whose gain was under control.

According to one of the main objects of my present invention, a diode rectifier device is utilized'to demodulate the received signal, and simultaneously to rectify signal energy for the production of a gain control voltage which is used to decrease the gain of the signal amplifiers as the received signal amplitude increases, the diode anode being normally maintained at a potential which does not becomenegative, and which specifically is positive, in direct current potential with respect to the cathodes of the controlled tubes.

Another important object of the present invention may be said to reside in the provision of a'radio receiving system wherein a signal transmission tube has its normal negative grid bias provided by the conjoint and opposing action of 40 negative and positive direct current voltage sources, and wherein the normal negative grid bias is increased, as the rec-eived'signal amplitude increases, by a diode signal rectifier whose anode is normally maintained positive with respect to 45 the cathode of the controlled tube.

itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection withthe drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, which shows the present invention embodied in a receiver of conventional construction, it will be observed that the numeral I designates a signal collector device. This device may be the usual grounded antenna, or it may be a loop antenna; dipole or even a radio frequency signal distribution line. The collected signals, which signals may be in the broadcast range or any of the short wave ranges, are impressed on the tunable input circuit 2 of the radio frequency amplifier tube 3.

The tube 3, shown as a screen grid tube of the indirectly heated cathode type, has its cathode 4 grounded, and it will be understood that the screen grid and plate of the tube are connected to suitable sources of positive direct current potentials. Such sources are not shown, but those skilled in the art are-fully aware of the fact that these sources may be of the battery type; or they may be embodied in the usual bleeder resistor connected across the filter output of a 60 cycle alternating current rectifier.

' A grounded electrostatic shield 5 is shown disposed between the signal collector l and the input coil of the tunable circuit 2, and it will be understood that the numeral 6 designates a tuning device which is used to tune the radio receiver through the desired operating frequency range. The signal demodulator, or detector, is a triode of the indirectly heated cathode type, which may be, for example, a tube of the 37 type, and is designated by the numeral 1. To provide a diode detector device the grid and plate of tube 1 are strapped together, and the resulting diode output electrode is connected to the cathode 8 through a path which includes the signal input coil 9, the coil ID, the condenser ll, resistor R2 and the visual indicating meter, or milliammeter l2. The condenser l3, connected between the high alternating potential side of coil 9 and ground, tunes coil 9 to the operating signal frequency which is to be detected. A heating battery A is connected across the heater element 14 of diode 1, and the positive terminal of the heating source A is connected to the ungrounded side of resistor R2 through the choke coil l5, which, of course, has finite resistance.

The audio frequency utilizing network, which may comprise one or more stages of audio frequency amplification followed by a reproducer or other signal indicator, has impressed upon it the demodulated signal energy through a path which includes the audio frequency transformer [6. The primary winding ll of transformer 16 has one side thereof connected to the junction of coils 9 and ID through a path which includes coil l8. The resistor R6 is connected in shunt with winding H, the condenser (9 being connected in series with winding I I and resistor R6. The by-pass condenser 20 connects the resistor side of coil l8 to ground, and the side of resistor R6 adjacent condenser 19 is connected to ground through a path which includes resistor R4 and resistor R5. is connected to the positive terminal of heating source A through a path which includes resistor R1 and lead 2|. to the frequency of the carrier being rectified by the detector, This affords a low impedance path for the carrier frequency. The resistance Re I 4 letters AVC to designate that this is the automatic volume control circuit, and the network comprising resistor R3 and condenser 23 functions to suppress fluctuating current components in the gain control voltage.

Signal energy is impressed between the signal grid 4' and cathode 4 of amplifier 3by virtue of the connection of the signal grid 4' to the high alternatingpotential side of input circuit 2, and the signal by pass condenser 3 connected between the low alternating potential side of input circuit 2 and ground. The desired signal energy-is transmitted to the tuned input circuit 9 of the diode demodulator I through the signal transmission network generally denoted by the numeral 30. Thisnetwork 30 may comprise one,

or more, additional stages oftunable radio fre- The receiver may be of the superheterodyne.

type. In such a case the network 30 may comprise additional signal frequency amplifier stages,

followed by the usual first detector and local oscillator circuits for the production of a desired intermediate, frequency. The first, detector output will be followed by one, or more, stages of intermediate frequency amplification, A sec-' ond converter can beused to change the first I. F. (say 300 kc.) to a second I. F. (say 50 kc), and the latter will be amplified before detection. In that case the demodulator 1 functions at in termediate frequency; Of course, the condenser l3 in the case where the receiving system is of the superheterodyne type is fixed in value to tune a the input circuit of the detector to the desired arrangement of the present type may be emp yed.

While the present circuit diagram shows the AVG voltage applied to only one of the signal transmission tubes of the receiving system, it will be clearly understood that the gain control bias The junction of resistors R4 and R5- 4 Coil l0 and condenser I l resonate may be applied to any of the additional signal transmission tubes preceding the final signal demodulator. Thos'eskilled in the. art will, in general, understand the operation of the automatic volume control arrangement shown herein. Normally,'and in the absence of received signals, the automatic volume control arrangement is not "functioning. When the desired signals are received, the AVG network begins to increase the negative biason the signal grid 4. As the signal amplitude increasesfthe gain control bias increases, and in this way the gain of the signal transmission tube 3 is decreased. In other words, duringperiods when no signals are received the gain of the signal transmission tube 3 is a maximum, and the receiver is in a highly sensitive condition. The cold electrode of the diode detector is maintained normally;that is to say in the absence of desired signals, at a positive potential with'respect to the grounded cathode 4'. This positive potential may be, for example, of. the order of 0.47 volt, and is supplied from the positive terminal of the heating source-A. The latter has a voltage of about 6 volts. This heating source not only supplies heating current to the filament of the detector 1, but, also, impresses a potential-across the resistors R1 and R2. The

approximate voltages at the'ungrounded sideof' sistance.

Asa consequence of these potential relations, the AVG action does not, in the absence of desired signals maintain the signal grid 4' highly negative;' on the other hand, it maintains the signal grid! at a relatively small negative potential with respect to the grounded cathode 4, and rather close to zeroidirect current potential, with theresultthat during periods of tim'e'whenno signals are impressed uponthe collector 'Lthe signal amplifiers are in condition to produce maximum amplification of the received signals. This follows by virtue of the fact that the cold electrode of the diode is normally maintained at a posi-'-' tive potential with respect to the grounded oath-'- ode 4.

That is to say, while the junction point of re: sistors R1 and Rs is at approximately +0.5 volt, the signal grids of the controlled amplifiers are not positive by'this amount, but in fact are at a small negative potential with respect to the grounded cathodes of the controlled signal transmission tubes. This is explained by the fact that when the cathodes of the signal transmission tubes are heated to an electron emitting condition, electrons leave the cathodes at such high velocities that they impinge upon the signal control grids adjacent them. This causes a certain currents flowing through the path including resistors R3, R4, R5 and then to ground. Normally,

this rectified grid current would maintain thesignal grids of the controlled transmission tubes at a negative potential with respect totheir cathodes in the absence of received signals. This negative potential is reduced very slightly'b y the opposition of the positive voltage of 0.5 volt existing at the junction of resistors R1 and B5.

In this condition the control grid bias of these tubes assumes a value between 0.5 and -1.2

volts.- This assumed bias happens to be of a value amount of rectification action, the rectified grid 7 at which these tubes have approximately maximum gain. As received signal strength is increased irom zero, the junction of resistors R6 and R4 decreases in positive potential, goes through zero potential, and then increases in negative potential. The bias of the controlled tubes is practically unchanged until the potential of the junction of Re and R4 reaches a negative value equal to the negative value assumed by those grids in the no-signal condition. As the signal strength is further increased the grid bias of the controlled tubes will be the same negative potential as that of junction Re and R4 since for such increasingly negative values, no current will be drawn by the grids of the controlled tubes. From this point on, the gain is progressively decreased as the signal amplitude increases. In this way the signal amplitude at the input circuit of detector l is maintained substantially uniform in spite of a wide range of signal amplitude variation at the collector I. It will be recognized that the action described above is a form of delayed AVC.

The junction of resistors R6 and R4 is at positive potential for zero signal; and is at a negative potential for signals of such strength that the direct current voltage drop in R4 is greater than the fixed bias on R5. R3 is normally between ten times and one hundred times as great as R4 and R5. Also R3 is great compared to the internal resistance between grid and cathode of the controlled amplifier tubes when the grids are operated at potentials such that grid current fiows.

For this reason the grids of the controlled stages become less negative only very slightly for changes in the potential of the junction of R6 and R4 as long as the potential of this junction is such that grid current will flow through R3. As soon as this junction becomes sufficiently negative so that no grid current flows through R3 there is no potential drop in R3, and, consequently, from then on to much more negative Values of the junction it will be found that the grids have the same potential as the junction of Re and R4.

The voltage supplied at the junction of coil l and R2 could be supplied by a tap on a bleeder resistance across a rectifier output deriving power from the main supply. Likewise, the usual alternating current power supply bleeder resistor may be used to supply the positive potential indicated at the junction of R1 and R5.

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:

1. A radio receiving system of the type including a signal transmission tube followed by a signal rectifier of the diode type, means for establishing a normal negative grid bias for said transmission tube by the conjoint and opposing action of'indep'endent negative and positive direct current voltage sources, said negative voltage source comprising a circuit in the grid circuit of the transmission tube and through which grid current flows in the absence of signals of a desired amplitude, means for preventing the rectifier anode from assuming a direct current potential which is negative with respect to the cathode of said transmission tube, and means for applying the direct current voltage output of said rectifier to the grid of said transmission tube in a sense such that the said normal negative grid bias is increased as the received signal amplitude increases.

2. A radio receiving system including a controlled electronic transmission stage, a diode detector with its anode and cathode both normally at positive potentials with respect to the cathode of the controlled electronic transmission stage, a load circuit and network connected to the diode so as to obtain modulation frequency output as Well as direct current output, the direct current output being connected in series with a fixed source of positive direct current potential and a series resistor between the grid of the controlled electronic transmission stage and the cathode of the latter whereby the grid of said stage will normally assume a substantially constant potential negative with respect to the oathode of said stage until the received signal reaches such a value that the negative direct current output of the diode exceeds the above mentioned series fixed source of positive potential.

3. In a signaling system having at least one signal transmission tube, a diode, means for normally establishing the anode and cathode of the diode at positive potentials with respect to the transmission tube cathode, means for deriving from'the rectified output of the diode a direct current voltage, means including a source of substantially constant positive potential and a source of negative potential in circuit with the transmission tube grid circuit for establishing a normal negative bias on said transmission tube, said direct voltage deriving means being connected to the transmission tube grid circuit in such a manner that said normal negative bias value is increased when the magnitude of the derived voltage exceeds that of the positive potential.

4. In combination with a signal transmission tube, means including opposed positive and negative direct current voltage sources for establishing a normal negative bias on the signal input electrode of said transmission tube with respect to the cathode thereof, said sources being independent, and the negative source comprising a network included in circuit with said input electrode and through which network current fiows for signals below a predetermined amplitude, a diode signal rectifier, means electrically associated with thefirst said means for preventing the diode anode from assuming a negative potential with respect to the transmission tube cathode, and additional means for applying the direct current voltage output of said rectifier to the input electrode of said transmission tube in a sense to increase said normal negative bias as signal amplitude increases.

5. In combination with a signal amplifier of a radio receiving tube, a diode rectifier for the output of said signal amplifier, means coupled to said rectifier for utilizing the modulation component of the rectifier output, means for deriving a direct current voltage from the rectified output of said rectifier, means for normally maintaining the diode anode and cathode at a direct current potential which is positive with respect to the oathode of said amplifier, and additional means for applying said direct current voltage output of the rectifier to the input electrodes of said amplifier in a sense to decrease the gain of the amplifier as the received signal amplitude increases.

HAROLD O. PETERSON.

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