US2128996A

US2128996A - Automatic volume control circuits

Info

Publication number: US2128996A
Application number: US88076A
Authority: US
Inventors: Dudley E Foster
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1936-06-30
Filing date: 1936-06-30
Publication date: 1938-09-06
Anticipated expiration: 1955-09-06

Description

70 NETWORK D. E. FOSTER AUTOMATIC VOLUME CONTROL CIRCUITS Filed June 50,

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' klem/@RK INVENTOR DUDLEY E. POSTED` I BY ATTQRNEY TOAF.

0 C O 0 O 0 0 0 O 0 O TOS/G/VAL SOURCE Patented Sept. 6, 1938 PATENT OFFICE AUTOMATIC' VOLUME CNTROL CIRCUITS Dudley E. Foster, Morristown, N. J., assigner to Radio Corporation of America, a corporation oi Delaware Appiication June 30,

9 Claims.

My present invention relates to automatic volume control circuits, and more particularly to automatic volume control circuits associated with plate rectification detectors of the degenerative type.

It has been proposed in the past to secure substantially linear detection up to 100% modulation of the received carrier wave, in a broadcast receiver of the high fidelity type, by employing a plate rectification detector of the degenerative type. In such a detector there is used a grid bias resistor which is not by-passed for audio frequencies whereby the detector input electrodes have an audio voltage impressed upon them in degenerative phase with respect to the useful audio output voltage. The degenerative plate rectiiication detector while acting like a peak rectier in the manner of a diode, has the advantage of drawing no input power. The useful audio voltage has been tapped oi from the cathode side of the bias resistor, the other side being at a relatively iixed potential. While a direct current voltage is developed across the detector cathode load resistor thereby providing the possibility of AVC action from the degenerative detector, such AVC operation has not been deemed feasible due to the fact that the direct current voltage polarity is in the opposite sense of that required for proper gain control.

Accordingly, it may be stated that it is one of the main objects of my present invention to provide in a signaling system of the type using a degenerative plate rectiiication detector, a circuit arrangement permitting the direct current voltage developed across the detector cathode load resistor to be used for AVC purposes.

Another important object of the invention is to provide a plate rec-tication detector, for a receiver, across whose grid bias resistor is de- 40 veloped audio and direct current voltages; an AVC network using the direct current voltage, and an audio network utilizing the audio voltage.

Another object of the invention is to provide AVC in a receiver employing a plate rectiiication detector having audio and direct current voltages developed across its grid bias resistor; the cathode side of the resistor being at a relatively fixed potential, and the opposite side being of the proper direct current polarity to employ 50 directly the direct current voltage for AVC action.

Still other objects of the invention are to improve generally the simplicity and eiciency of automatic volume control circuits adapted to be used directly With a detector of the plate recti- 1936, Serial N0. 88,076

(Cil. Z50-20) cation type and which detector possesses a linear detection characteristic, and more especially to provide such circuits for use in high iidelity receivers, the circuits being reliable in operation and readily manufactured and assembled in radio receivers.

The novel features` which I believe to be characteristic 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 in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereb-y invention may be carried into effect.

In the drawing:

Fig. l diagrammatically shows a signalling system embodying one form of the invention,

Fig. 2 illustrates a modified arrangement.

Considering the circuit arrangement shown in Fig. l, it is pointed out that there is schematically illustrated only those networks of a radio receiving system which are essential to a proper understanding of this invention. Hence, the numeral l denotes a tunable radio frequency amplifier whose input circuit 2 may be coupled to any desired source of signals. The circuit 2 includes the usual variable tuning condenser 3, and the output circuit thereof may be coupled to any desired type of signal transmission network, the latter being generally designated by the reference numeral 4. The network 4 may have its output energy impressed on the tunable input circuit 5 of the demodulator, or detector, tube 6. The detected, or audio, output energy of detector t may be impressed through an audio coupling condenser l, upon an audio frequency utilization network which may comprise one or more stages of audio ampliiication, the output of the latter being terminated by any desired type of reproducer.

The receiving system may be of the high delity type, and pass to the demodulator a Wide radio frequency transmission band, such receivers being well known to those skilled in the art. Ii the receiver is of the tunable radio frequency type, then it is to be understood that the variable tuning condensers 3 and 3', as well as any of the variable condensers employed in the network d, may have their rotors arranged for mechanical uni-control. In such case the input circuits of the various radio frequency ampliers, and the input circuit of the detector, will be tunable over the desired signal frequency range,

such as the broadcast band which covers a range of 500 to 1500 kc. The receiver may be of the multi-range type and be adapted for receiving stations in the short wave ranges as well.

When the receiver is of the superheterodyne type, then it is to be understood that the tube I denotes the tunable radio frequency ampliiier preceding the usual rst detector, or it may even be the IF amplier which follows the output circuit of the first detector. If the ampliiier I is a tunable amplifier then its circuit 2 will be coupled to the usual signal collector, and if it is anIF amplifier the circuit 2 will be coupled to the plate circuit of the first detector. In any event it is to be understood that the circuits 2 and 5 may be xedly tuned to an operating IF, and this will also be true of the tuned circuits of the network 4.

The detector 6 has its circuits arranged in such a manner that it functions as a degenerative plate rectification type detector, This is accomplished by shunting the usual grid bias resistor 8 with a condenser 9 having a low impedance to radio frequency currents, but a relatively high impedance to audio frequency currents. In this way the audio frequency currents of the detected signal energy flow through the bias resistor 8, and develop thereacross the useful audio output voltage. Because of this fact the audio coupling condenser I may be connected directly to the cathode side of resistor 8, and hence the bias resistor 8 functions as the audio output load of the detector. The resistor 8 may have a magnitude of one hundred thousand ohms, and the condenser 9 may have a magnitude of one hundred mmf. The latter magnitudes are to be understood as purely illustrative in nature, and in no way restrictive upon the design of the detector circuit.

The detector input circuit 5 is connected between the control grid and one side of resistor 8, whereby the direct current voltage developed across resistor 8 acts to bias the control grid of tube 5. It will therefore be seen that detector tube 6 is of the biased type, and detects by plate rectification. The negative side of resistor 8 is connected to ground through a path which includes lead'I and the bleeder resistor II. The lead I0 is connected to the B side of the bleeder II, and it will Ybe noted that this side of the resistor is substantially -60 volts with respect to ground. Since the -I-B potential is approximately 250 volts with respect to ground, it will be seen that there is a potential difference of 310 volts provided for the energizing potentials of the detector plate and cathode.

The plate and cathode leads of the detector tube are by-passed to ground through appropriate radio frequency by-pass condensers I2, and it is to be understood that the bias resistor 8 has a magnitude such that the control grid of the detector tube is biased -4 volts with respect to the cathode in the absence of signal energy. The amplier I is of the usual construction, and its cathode is connected to ground through the customary bias network I3. The signal grid of amplifier I is connected to the negative side of bias network I3 through a path which includes resistors I4 and I5 arranged in series. In the absence'of signals, or in the absence of signals having a predetermined amplitude, the signal grid of amplifier I is maintained at a negative bias which is equal tothe direct current voltage developed across the resistor of the biasing network I3.

In addition to functioning as the normal amplifier biasing path, the resistors I4 and I5 function as a part of the automatic Volume control circuit (AVC) of the receiver. The latter control circuit will now be described. It has been previously pointed out that the detector 6 not only develops the useful audio voltage across resistor 3, but also provides a direct current voltage across the resistor which could be used for AVC bias were it not for the fact that the polarity at the cathode side of resistor 8 is not correct for the purpose desired. If the signal grid of amplifier I were connected directly to the cathode side of resistor 3, it can be readily seen that as the signal amplitude increases the signal grid of amplifier I will become more positive and increase the gain of the amplifier, instead of operating in the reverse sense as is necessary for AVC action.

Accordingly, there is employed a means for reversing the polarity of the direct current voltage developed across resistor`8, and this is accom- Vplished by connecting the cathode side of resistor vI3, through direct current connection I6, to the control grid of the polarity inversion tube I1. Condenser I8 and resistor I9 provide a lter network for suppressing the radio and audio components-in the voltage transmitted over lead I6. The cathodevof tube Il is connected to an intermediate point 25 on bleeder resistor II, and this intermediate point may have a voltage of approximately 40 volts. Since the control grid of tube I'I is connected to the cathode side of resistor 8, it will be seen that in the no-signal condition the grid of tube I'I is biased at approximately 16 volts with respect to the cathode thereof.

.The tube I'I is of the 56 type, and permits space current to flow through the plate circuit Ywhen the bias on the control-grid has been reduced to a voltage of the order of -6 volts. Hence, it will be seen that there is a delay action with regard to the iiow of plate current through resistor I5, of approximately 10 volts. The resistor I5 may have a magnitude of the order of 10,000 ohms, and it is connected between the plate of tube I1 and ground. The automatic volume control Vbias is developed across resistor I5, and -the direct current voltage developed across the latter depends upon the plate current flow of tube Ii. vBy way of illustration it is pointed out that the resistor I5 has a magnitude such that -40 Avolts of AVC bias may be developed thereacross.

The resistor I4 connects the signal grid circuit `of Yampliiier I to the plate side of resistor I5, and in this way the AVC bias developed across resistor I5 is applied tothe signal grid of amplier I. Resistor I4 and condenser I4 provide a lter network to suppress pulsating components in the voltage employed for AVC bias. It will be seen that as the plate current flow of tube I1 increases, the voltage drop across resistor I5 increases; and since the anode side of resistor I5 is negative with respect to ground, the signal grid of amplifier I accordingly becomes increasingly negative with respect to the cathode thereof.

It will now be seen that by means of the arrangement shown in Fig. 1 it is possible to employ a degenerative plate rectification detector, and yet have it act to provide a source of AVC bias for one or more signal transmission tubes. It is again pointed out that tube I1, and its associated circuit elements, function as a polarity reversal means, and for this reason the lead I 6 is denoted by the symbol AVC. It is not believed necessary for a clear understanding of this invention to point out the advantages secured with a degenerative detector. It has previously been explained that such a detector possesses a substantially linear detection characteristic upto 100% modulation on the carrier; and it has also been pointed out that this action is secured by operating the detector with a no-signal bias such that the detector functions in the manner of a diode peak rectifier circuit. The AVC action will not cornmence until the received signal amplitude is of an intensity sufficient to remove the normal cutoff bias on tube l1, and permits plate 'current to flow through resistor l5.

'I'he degenerative detector may be employed as an AVC source in a receiving system without the use of a polarity reversing circuit. Such an arrangement is shown in Fig. 2, and in the latter it will be seen that the bias resistor 8 of the detector has its cathode side at ground potential, the AVC lead 30 being connected between the opposite side of resistor 8 and the signal grid circuits of the signal transmission tubes to be controlled in gain. The reference numeral 30 denotes the filter network for suppressing the pulsating components in the AVC bias. In this arrangement the cathode of amplifier l may be grounded, the4 radio frequency by-pass condenser 3l being connected between the grounded cathode and the low alternating potential side of tuned input circuit 2. v

By proper design of the network 8-9 the useful audio voltage may be taken off from the ungrounded side of resistor 8, and the no-signal direct voltage drop across resistor can be used to provide the maximum amplification bias for amplifier l. By way of illustration it is pointed out that the resistor 8 may have a magnitude of approximately 300,000 ohms, and the condenser 9 will have a low impedance to radio frequency currents, but a sufficiently high impedance to cause the audio voltage to be developed across resistor 8. The energizing voltage for the detector tube is secured from an auxiliary rectifier 40 which functions to rectify the 60 cycle alternating current.

The numeral 4l denotes the usual power transformer of the receiver, the primary winding of which is connected to the 60 cycle source, and the transformer having the usual secondary windings adapted to feed the alternating current rectifiers for providing the energizing voltages for the different amplifiers of the receiver. The special secondary winding 42 is provided on the transformer, and one side thereof is connected to the anode of rectifier 40, while the other side thereof is connected to the ungrounded side of resistor 8 through a path which includes the resistor 43. The cathode of rectifier 40 is connected to the plate of detector tube 5 through a path which includes resistor 65 and lead 46.

The network comprising resistor i5 and condensers 4l', provides a filter arrangement for suppressing all pulsating components in the direct current voltage employed to energize the detector tube The resistor 43 functions as a part of the lter network, and also acts to keep the audio frequency impedance of the detector cathode circuit high. By way of example it is pointed out that the voltages applied to detector tube 0 may be such as to cause a 'current of l0 micro-amperes to flow through resistor 8 in the absence of received signals, and provide a direct current voltage across the resistor of approximately -3 volts.

Hence, it will be seen that in the absence of received signals the voltage drop across resistor 8 provides the normal bias for amplifier l, as well as the no-signal bias for the degenerative de tector 6. As signals are received there is developed across resistor 8 audio voltage and a direct current voltage. Since the cathode side of resistor 3 is grounded, the opposite side becomes increasingly negative in potential, and hence the negative bias on the signal grid of amplifier I is increased with a consequent reduction in gain of the amplifier. In this way the AVC is able to maintain the signal amplitude at the detector input circuit substantially uniform regardless of a wide variation in signal amplitude at the receiver collectin-g means.

It is to be understood that the arrangement of Fig. 2 is not limited to the utilization of bias resistor 8 for the provision of the normal operating bias for the amplifier' i. A bias network of the type denoted by the numeral I3 in Fig. 1 may be employed to provide the normal amplification bias. In such case the signal grid of amplier I would still be connected through lead 30 to the ungrounded side of resistor 8. It is pointed out that the arrangement of Fig. 2 may be of the superheterodyne type.

While I have indicated and described several systems 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 organizations 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. In a radio receiving system, a radio frequency amplifier, a degenerative plate rectification detector provided with a cathode load resistor across which an audio voltage and a direct current voltage are developed from detected signals, a condenser being connected across said resistor which has a high impedance to audio frequency current but a low impedance to radio frequency current, means for establishing a point of said resistor at a relatively fixed audio and direct current voltage with respect to another point on the resistor, means impressing the audio voltage developed across the resistor between the detector input electrodes and in degenerative phase, means connecting the amplifier cathode to said fixed voltage point of the resistor, and a direct current voltage connection, functioning as an automatic gain control circuit, between the second point on said resistor and a gain control electrode of said amplifier.

2. In a radio receiving system as def-ined in claim 1, additional means connected to said second point of the resistor for taking off the audio voltage.

3. In combination with a modulated signal carrier transmission tube, a detector tube including at least a cathode, a control grid and a plate, a resistor in the space current path of said detector tube, said resistor being connected in the cathode lead of the latter, a condenser, of high impedance to audio frequency current and low impedance to carrier frequency current, connected across said resistor for causing audio and direct current voltages to be developed across the resistor from detected signals, means for impressing the output of said signal transmission tube upon the input circuit of the detector tube, means establishing one side of the resistor at a relatively fixedpotential with respect to the other side, means impressing said audio voltage upon the detector cathode and grid in degenerative phase, means connecting the transmission tube cathode to said fixed potential side of the resistor, a modulation component path connected'to said other side of the resistor, and an automatic gain control circuit connected between a gain control electrode of the signal transmission tube and said other side of the resistor.

4. In a receiving system as dened in claim 1, the second point on said resistor being at a positive direct current voltage with respect to the rst point, said direct current connection including means for reversing the polarity of said positive potential.

5. In a receiving system as defined in claim l, said first point on the resistor being at a positive direct current voltage with respect to the second point, and the cathode of said amplifier being at said positive direct current voltage whereby the direct current voltage developed across said cathode resistor provides the normal operating bias for the amplifier in the absence of received signals.

6. In combination, a signal transmission tube and a detector tube of the type including a resistor in the cathode circuit thereof which develops an audio voltage and a direct current voltage from detected signals, means impressing the audio voltage between the detector input electrodes in degenerative phase, a voltage supply source for the plate and cathode of said detector, said supply source comprising a source of low frequency alternating current, a rectifier therefor and a filter network connected to the output of the rectifier, said resistor being connected in the negative side of said lter network and having the cathode side thereof grounded, a direct current voltage connection, functioning as an automatic gain control circuit, between the ungrounded side of said resistor and the signal grid of the transmission tube and the transmission tube cathode being at ground potential.

7. In combination with a signal transmission tube having input and output electrodes, a signal rectier of the type including a tube having input and output electrodes, an impedance in the space current path of the rectifier tube, said impedance developing audio and direct current voltages, means impressing developed audio Voltage between the rectier input electrodes, and direct current voltage connections between the transmission tube input electrodes and spaced points on said impedance such that the direct current Voltage developedbetween the points provides a gain control bias for said transmission tube.

8. In combination with a signal transmission tube having input and output electrodes, a signal rectifier of the type including a tube having input and Voutput electrodes, an impedance in the space current path of the rectier tube, said impedance developing audio and direct current voltages means impressing developed audio voltage between the rectifier input electrodes, and direct current voltage connections between the transmission tube input electrodes and spaced points on said impedance such that the direct current voltage developed between the points provides a gain control bias for said transmission tube, the transmission and rectier tube cathodes being at a common fixed potential which is that of one of said spaced points.

9. In combination with a signal transmission tube having input and output electrodes, a signal rectifier of the type including a tube having input and output electrodes, an impedance in the space current path of the rectifier tube, said impedance developing audio and direct current voltages, means impressing developed Yaudio Voltage between the rectiiier input electrodes, and direct current Voltage connections between the transmission tube input electrodes and spaced points on said impedance such that the direct current voltage developed between the points provides a gain control bias for said transmission tube, one of the spaced points being at ground potential, the other point becoming increasingly negative with signal amplitude increase.

DUDLEIY E. FOSTER.