US2135560A - Delayed automatic volume control circuit - Google Patents

Description

Patented Nov. 8, 1938 PATENT OFFICE DELAYED AUTOMATIC VOLUME CONTROL CIRCUIT Wendell L. Carlson, Haddoneld, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 29, 1935, Serial No. 29,003 Renewed October 1, 1937 8 Claims.

My present invention relates to automatic volume control systems for radio re'ceivers, and more particularly to such systems which utilize a delay action.

One of the main objects of this invention is to provide an automatic volume control circuit for a kradio receiver which not only has a delayed action, but also permits the cathodes of the controlled signal transmission tubes to be maintained at ground potential, while the control tube anode is normally positive with respect to ground.

' Another important object of the invention is to provide in a radio receiver provided with a demodulator diode whose anode is normally positive Vwith respect to the cathodes of the controlled ampliiiers, an automatic volume control system which is adapted to function after a predetermined signaly amplitude is exceeded; the normal grid bias for the controlled amplifiers being provided by an auxiliary diode device whose functioning is interrupted upon the impression on the receiver of the signal waves above the aforesaid predetermined amplitude.

Anotherfobject of the invention is to provide a g5 delayed automatic volume control ciruit for a receiver, which 'circuit essentially comprises a diode rectier whose vdirect current potential output is employed for automatically varying the gain of one or more ampliers of the receiver; an

,30 auxiliary rectier being utilized to provide the normal operating grid bias for the controlled amplifiers, and the auxiliary rectifier functioning to rectifyan alternating current derived from the usual alternating current supply source of the ,3 5 receiver; the auxiliary rectifier being rendered inoperative to supply the normal bias when desired signal waves are received.

Still other objects of the invention are to improve generally the simplicity and eiciency of .40 automatic volume control circuits, and more especially to provide delayed automatic volume control circuits for radio receivers which are not onlyy reliable in operation, but economically manufactured and assembled in receivers. *5 While the novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the present invention, however, both as to 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 one circuit arrangement whereby my invention may be carried into u BBC.

(Cl. Z50-20) Referring now to the accompanying drawing, there is shown in schematic fashion those portions of a superheterodyne receiver circuit which pertain to the present invention. The numeral I designates a source of I. F. energy which normally precedes the last I. F. amplifier tube 2. Those skilled in the art will clearly understand that the source I may comprise the usual signal collector followed by a stage adapted to convert the incoming signals to the I. F. energy. Such I. F. energy I0 may be derived from a pentagrid converter stage suchr as disclosed by J. C. Smith in application Serial No. 654,421, filed January 31, 1933. The numeral 3 may be considered as the I. F. output circuit of the converter tube, or it may be con- 155 sidered the output circuit of a preceding I. F. amplifier. Of course, the converter network may comprise separate local oscillator and rst detector tubes.

'I'he I. F. amplier tube 2 is shown as having its l20 cathode at ground potential; that is to say, the cathode 4 is grounded Without the interposition of the conventional grid bias resistor network. The signal input circuit 5 is connected between the signal grid 6 of tube 2 and the cathode; the g5 circuits 3 and 5 being magnetically coupled to give optimum signal transmission. The tubes preceding tube 2 are to be understood as being constructed in the same manner as tube 2; that is, the cathodes thereof are at ground potential. I The amplifier 2 is followed by tube 'I which may be of the multi-function type. The latter may be one having a diode section and a screen grid, or a pentode, amplifier section.

Tube I and its associated circuits perform four functions in the present circuit; and these will now be described. Its diode section 8-9 is connected to a network wherein signal demodulation takes place. The demodulator network comprises the tuned circuit I0, which is coupled to the tuned output ciruit II of amplifier 2, and load resistor I2. Circuits II and I0 are each tuned to the operating I. F.; the low alternating potential side of circuit I0 is connected to ground through bypass condenser I3.

The direct current component of rectified signal voltage appearing across load resistor I2 is applied to the signal grids of the controlled tubes. Thus, lead I4 is designated as the AVC connection, and is shown connected between the signal grid circuit of tube 2 and point I5 on resistor I2; the AVC path including lter resistor I6 and resistor I 'I, and the junction of the latter two being connected to ground through condenser I8. Resistor I6 and the latter provide the usual iilter 55 network for suppressing ripples in the AVIC bias applied to the controlled tubes.

The audio component of rectied signal voltage is impressed on the signal grid I9 of tube 'I v through condenser 28; an adjustable tap 2I connecting the condenser 20 to any desired point on resistor I2 thereby to provide a manual volume control device. The tap 2I is by-passed to ground by condenser 22, and the negative bias required ior grid I9 is secured from the voltage drop across resistor 23. The latter is connected in the space current path of the amplifier section of tube "I: and it Will be understood that the grounded side of resistor 23 returns to the minus terminal of the B supply (not shown). The signal grid I9 is connected to the grounded side of resistor 23 through grid leak resistor 23. Of course, the energizing direct current voltages ior the tubes in the receiver are to be understood as being derived Yfrom the usual bleeder resistor connected across the lter network of a 60 cycle source rectifier. The amplified audio output of tube 1 is transmitted to one or more audio amplifiers, and iinally to a reproducer.

Since the cathodes of the controlled tubes are at ground potential, the signal grids must be given a normal, or initial, negative bias sufficient for maximum amplification of signals of relatively Weak intensity. rIfhis is accomplished by providing an auxiliary diode 35i which has its anode 3V! connectedto lead I4 through winding 32', of transformer T, and resistor 33. The cathode 32 of the diode 32 is connected to the junction of resistors i 2 and 23; ie cathode 9 of tube l being connected to this same junction point, it should be clear that cathodes 9 and 32 may be a common cathode serving anodes 8 and 3I. In such a case a tube of the duplex diode-pentode type may be employed. The alternating current by-pass condenser 34 is connected in shunt With diode 30 and Winding 32. The primary Winding 35 is connected to a source of alternating current to be rectified; the 6l) cycle current from the receiver power supply source may be used'for this purpose.

The magnitudes of resistors 23; I2; I1; and 33 determine the voltage relations of the various circuit elements of the biasing control system. They are shown chosen, merely by Way of example, so as to normally maintain the direct current voltage relations with respect to ground as shown in the drawing. Thus, the demodulator anode 8 is at +1 volt with respect to the ground point of the receiver; Whereas the cathodes of the controlled tubes are at ground potential. Again, the drop across resistor 23 places the cathode 9 at a potential of +1.5 volts above ground; therefore, the demodulator anode 8 is at about 0.5 volt with respect to the cathode 9. This results in suicient delayed detection to suppress the reproduction of undesired background noises duringfweak signal reception. Y

By virtue of the action of the rectifier 3B, a voltage drop across resistor i'I is produced which places the junction of resistors I'I and I6 at -3 volts with respect to ground. The point I5 is maintained at |1 volt above ground by virtue'of the current oW of diode 3U during operation thereof. Since the signal grids of the controlled ,ubes are connected to this junction point, they are at the aforesaid negative bias With respect to the cathodes of the controlled tubes. It Will now be seen that as signals are received which exceed the 0.5 volt suppression bias on anode 8, a direct current voltage appears across resistor I2; this last voltage increases with increasing signal amplitude.

As it increases, it reduces the rectication action of diode 30 by biasing the anode 3l in a nega- A ciently to cause the diode 30 to be rendered nonconductive. By choosing resistor 33 low in value with respect to resistor II, a materially delayed AVC action will be obtained; as the AVC voltage developed by the diode demodulator increases,V

the normal bias voltage produced by diode 30 decreases. The sum of these voltages is substantially constant until the AVC voltageV alone isy being transmitted to the signal grids, and the diode 30 rendered inactive. Further increase of signal amplitude then results in a decrease in controlled tube gain.

It Will noW be seen that tube 'I and its associated network not only act to demodulate signals, amplify audio signals and produce AVC action; but, also, regulates the biasing action of an auxiliary diode circuit Whose function it is to provide the normal negative bias for the signal grids of the controlled tubes. Ultimately, the normal grid bias derived from diode 30 is replaced by the variable AVC bias derived from diode 8 9. By means of the present invention, it is entirely practical,

and indeed highly economical, to kdispense with the conventional self-bias grid resistors and shunt condensers normally requiredV in the controlled This vnot only reduces production costs, but minimizes regenerative unstable conditions. It is, also,to be observed that the resistor 23 not only serves to raise the anode 8 and cathode 9 above ground potential, but also functions as the grid bias resistor for the audio amplier section' of tube 'I this still further ,reduces-'the vcost of production of receivers of this type.

` While I have indicated and described a system for* carrying my invention into effect, it` Yvvill be Y. apparent to 'one skilledy in the art that my inven- 1 tion is by no means limited to the particular organization shown and described, butthat many modiiications may be made Without departing Yfrom the scope of my invention, as set forth in the appended claims.

What I claim is:

l. In combination'with an electron discharge tube Which includes at least a signal input electrode, a cathode and an output electrode, means including a device of uni-directional conductivity for providing the operating negative bias between the signal input electrode and the cathode of said electron discharge tube, and a signal vrectifier network coupled to the output electrode of said tube I forY producing a direct current potential which varies in magnitude With the amplitude of thesignals impressed on said tube, means for impressing Y tive tobias said signal input electrode When signals above a predetermined amplitude are rectied, and additional means for maintaining the rectiiier electrodes at a positive potentialy with respect to said cathode in the absence of received signals.

2. In combination with a tunedsignal amplifier of a radio receiver, a tuned 'demdulator diode circuit having its electrodes Lcoupled tov the output circuit of said amplifier, a load resistor in circuit with the anode and cathode of the diode, means for maintaining the diode anode at a positive potential with respect to the amplifier cathode during normal signal reception, additional means for maintaining the signal input grid of the amplifier at a negative potential with respect to the cathode thereof during said normal reception, and means operatively associated with the demodulator diode circuit for rendering said additional means inoperative to bias said signal grid when a predetermined signal amplitude has been exceeded, and for variably biasing said signal grid subsequent to said last named amplitude.

3. In a receiver, as defined in claim 2, said additional means including an alternating current rectifier which is coupled to a source of alternating current adapted to simultaneously energize the tubes of the receiver.

4. In a receiver, as dened in claim 2, said additional means including a device of uni-directional conductivity, and said last named device being connected to be rendered non-conductive by the direct current component of demodulated signals when said signal amplitude has been exceeded.

5. In a receiver, as defined in claim 2, said iirst named means comprising a resistor disposed in the space current path of an electron discharge tube, the signal grid of said last named tube being coupled to said demodulator network for receiving Vdemodulated audio signals therefrom.

6. In combination with a signal transmission tube provided with at least a signal grid, a cathode and an output anode, a demodulator device of the electron discharge type including at least a cathode and a cold electrode, the cathode of said tube being at ground potential, means for simultaneously maintaining the signal grid of said tube negatively biased with respect to its grounded cathode, and the cold electrode of said demodulator positive with respect to said grounded cathode, direct current connections between said last means and said demodulator device for rendering said last means inactive when signals above a predetermined amplitude are received, and a gain control connection between the aforesaid signal grid and said demodulator device for impressing a variable negative bias upon said signal grid when said means is rendered inactive.

7. In combination with a wave transmission tube having at least a cathode, a wave input electrode, and an output electrode, a wave demodulator diode having its electrodes coupled to the output electrode of said tube, a load resistor connected between the anode and cathode of said diode, means for normally maintaining the diode anode at a positive direct current potential with respect to the cathode of the wave transmission tube, additional means for maintaining the wave input electrode of said tube at a negative direct current potential with respect to its cathode, and means responsive to a predetermined signal amplitude increase for rendering said additional means inoperative, and biasing said input electrode in direct proportion to the wave amplitude increase.

8. In a radio receiver provided with a signal transmission tube, said tube including a signal grid, an output plate, and a cathode at ground potential, a demodulator diode including a load resistor between its anode and cathode, means connected between ground and the cathode side of said load resistor for maintaining the demodulator anode normally positive withv respect to said grounded cathode, means for applying a normal negative bias to the said signal grid, said last means including a diode having its anode connected to said signal grid and its cathode connected to ground through a source of direct current potential, and a gain control connection between the said signal grid and a point on said load resistor which is at a negative direct current potential with respect to the demodulator cathode when signals above a predetermined amplitude are received.

WENDELL L. CARLSON.

Images