US2073486A - Delayed automatic volume control circuit - Google Patents

Description

March 9, 1937.

Filed Jan. 25, 1935.

W. R. KOCH DELAYED AUTOMATIC VOLUME CONTROL CIRCUIT All ` HWENTOR.l

\WINFIELD R KOCH wee/M,

ATTORNEY.

Patented Mar. 9, 1937 UNITED STATES PATENT OFFICE DELAYED AUTOMATIC VOLUME CONTROL CIRCUIT of Delaware Application January 25, 1935, Serial No. 3,415

10 Claims.

My present invention relates to automatic gain control systems for radio receivers, and more particularly to improved delayed automatic volume control circuits for the signal transmission networks of receivers.

There has been disclosed by Loy E. Barton, in application Serial No. 640,946, filed November 3, 1932, a delayed automatic volume control system of the type utilizing a detector tube, particularly the second detector of a superheterodyne receiver, as a source of AVC bias for the controlled signal transmission tubes. Briefly, in the Barton system a multiple function tube is used as the detector and AVC device. The tube comprises two diode sections, and a section including at least a grid and plate; all the sections have a common cathode. One of the diode sections functions as a signal detector, and also controls the space current flow through the cathode circuit by means of the grid. The cathode circuit includes a resistor adapted to prevent the' cathode from becoming negative, with respect to the anode of the second diode section, until the signals impressed on the detector diode attain a predetermined amplitude. Beyond this amplitude the cathode becomes negative with respect to the AVC diode anode, and the AVC diode section becomes conductive.

While the system disclosed in the aforesaid Barton case is generally satisfactory, various improvements have been made by me therein to render the system more efficient. Because a low mu tube was used in the said system the AVC amplification was little. One of the objects of this invention is to use a high mu tube for the AVC function, and render it independent of the detection operation; thus securing relatively greater AVC amplification with the result that the AVC system holds the voltage at the delO tector within much narrower limits.

Another object of the present invention, and an important one, is to provide two points of delay in an AVC system of the type disclosed in the aforesaid application, the additional point of l delay being the grid of the section whose space current flow determines the conductivity of the AVC diode, such 'an improved circuit permitting the signal voltage level at the detector input to be held at a high value.

Other objects of the invention are to improve generally the eiciency and operation of delayed automatic gain control systems, and more especially to provide such systems which are reliable in actual reception, and economically manufac-V 'cured and assembled in radio receivers,

(Cl. Z50-20) The novel features which I believe to be characteristic of my invention are set forth in particularity inthe appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by 5 reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect. 10

Referring now to the accompanying drawing, there is shown a superheterodyne receiver of a conventional type. Such a receiver usually comprises a signal collector A, such as a grounded antenna circuit; one or more tunable radio fre- 15 quency amplifiers; a first detector, or converter, which may be of the composite first detectorlocal oscillator type using a 2A'7 pentagrid converter tube, or which may employ separate first detector and local oscillator tubes; one or more 20 amplifiers iiXedly tuned to the operating I. F. (which may be between 1'75 and 450 k. c.) and, finally, a second detector which feeds the demodulated currents to a reproducer through one or more audio frequency amplifiers. 25

The circuit details are too well known to require further explanation; it is sufficient for the purposes of this disclosure to point out that the signal input grids of the tubes preceding the second detector are operated with a predeter- 30 mined fixed negative bias, each tube having its bias adapted to the function of the tube.

The second detector tube is of the multi-function type, and includes a diode section and a triode, or pentode, section. 'I'he `diode anode is 35 coupled to the tuned secondary circuit I of I. F. transformer M1; the cathode of tube 2 is connected to the low alternating potential side of circuit l through a load resistor Re which is ley-passed for intermediate frequencies. The 40 grid of the triode section has the audio frequency component of rectified current impressed upon it; the grid being diode-biased, while resistor R1 suppresses the intermediate frequency component of rectified current. 'Ihe amplified audio current in the plate circuit of tube 2 is transmitted to the following audio network. The tuned primary circuit 3 of transformer M1 is disposed in the plate circuit of the last I. F. amplifier; the direct current voltage supply bleeder resistor R furnishes the proper energizing potentials to the electrodes of the tubes. For the purposes of this application it is believed sulicient to show the manner of connecting the plate 55 of the last I. F. amplifier to the +B terminal of resistor R through lead 4.

The AVC system employed, according to this invention, comprises a tube of the 2A6 type. This is a high mu duo diodetriode tube, and its construction is well known to those skilled in the art. The anode 5 is coupled to the high alternating potential side of primary circuit 3 through a condenser C1. The diode rectier circuit electrodes are connected to opposite sides of resistor R4, and condenser C1 is connected to the anode side of the latter resistor. The cathode C is connected to a point of about volts on bleeder R through resistor R2; the value of the latter depending on the delay desired for the AVC action. The anode 5 is connected to a point on bleeder R which has a voltage of +100 volts, the connection path including resistors R3, R1 and adjustable lead '1. The plate of the tube is connected to a point of +200 volts.

The grid 8 is connected to the junction of resistors R1 and R3, an I. F. by-pass condenser C'z being connected from the grid 8 to the cathode C. The by-pass condenser C2 might be an audio by-pass, as well as an I. F. by-pass. In this case thecondenser from cathode C t'o +B would not have to beso large in order to keep theV potential ofcathode Cifrom fluctuating at an audio rate. The audiocan be `by-passed by either condenser, or both. The AVC lead to the gain-controlled tubes is connected between the grids of the latter and the anode 6. The control bias voltage is developed across the resistor R5, which is connected between the anode 6 and cathode C through a path including ground, bleeder R, lead 9 and resistor R2. When the anode 6 assumes a positive potential with respect to the cathode C, thencurrent flows through resistor R5. This current ow develops -a voltage across resistor R5 which increases with signal amplitude increase. The anode side of resistor R5 becomes increasingly negative as the voltage across R5 increases. lThis results in an increase in the negativebias ofithecontrolled grids, and a reduction in the gain of the controlled tubes.

It is to beunderstood that the AVC connectionsy to the controlled grids may be such as to apply the control bias to the controlled tubes in the inverse order of the signal amplitude thereof. This is a well known mode of AVC operation, anda reference to it will be sufcient. Specific circuits therefor are disclosed by Stuart Ballantine in application Serial No. 376,163, led July 6, 1929, Patent No. 2,046,237, June 30, 1936.

To delay the AVC action of the system the anode 6 is maintained negatively biased with respect to the cathode C until the signal amplitude attains a predetermined value. This polarity control of the AVC'diode electrodes is secured by having the bias of grid 8 ydependent on the potential of the junction of resistors R1 and Ra. With aweak signal-very little-.current flows through the diode rectier circuit 5-C-R4. The current flowing throughR1-R3-R4 tends to make grid 8 positive. The grid current flow, however, keeps the grid 8 at about the potential of cathode C. As the grid voltage on a tube isA varied from negative to positive, grid current starts to flow at about 0.5 volts, and increases rapidly. This means that the D. C; resistance of the gridcathode circuit in the tube drops from many megohms to only several thousand ohms. The grid current flow depends on the voltage between grid and cathode. The voltage between grid and cathodedepends, in the circuit shown, on the impedance between grid and cathode, and therefore upon the amount of grid current. The circuit is self-regulating, and the nal grid voltage may be anything from 0.5 to several volts positive, depending on the rest of the circuit. Thus, there is a maximum flow of space current between the plate and cathode C of tube 2A6 thereby leaving the cathode C at a positive potential with respect to anode 6. The magnitude of the positive potential depends upon the value of resistor R2.

As the signal amplitude increases the rectified current through R4 increases. This progresses until the voltage across R4 is equal, and opposite in sign, to that across R3. The grid 8 becomes more negative for still stronger signals, and the space current through resistor R2 decreases. As a consequence the potential of cathode C becomes less positive with respect to anode 6. Finally, the grid 8 becomes suliciently negative to reduce the space current flow through Rz to a value such that the cathode C becomes negative to reduce the space current ow through Rz to a value suchthat the cathode C becomes negative with respect to ground. At this point the AVC diode (C-Rs-B) circuit becomes conductive and the aforedescribed AVC action commences.

A condenser is connected to the cathode side of resistor R2 to by-pass audio frequency components in the plate current of the AVC tube, and to suppress hum voltages reaching the cathode through the resistor R2. Quite often the -100 volts is secured by using the D. C. drop across part of the lter system, such as the speaker field. In this case, the cathode Will have considerable ripple voltage to ground, unless bypassed. Although connecting the condenser to +B, like the other lter condensers, should be best, it may be connected to ground, provided Rz is large, and there is a large condenser from ground to +B.

Thus, it will be seen that there are two points of delay in the AVC system. Were resistor R2 solely depended upon for delay it would require an impractically high value if it were desired to hold the signal voltage level at the detector input to a value substantially higher than one or two volts. For this reason there is introduced the delay resistor R1, having a magnitude of about I0 meghoms, and the resistor is made variable so that the delay due toit may be adjusted. The tap 'l may, also, be employed as a delay adjustment.

By connecting the diode rectier (C-5) to the primary circuit 3, an improved AVC action is secured. The signal voltage at the primary is generally a little higher than at the secondary circuit I; the selectivity to the detector will also be greater, a characteristic much desired. Further, this arrangement of using a separate AVC tube decreases the tendency to introduce hum and distortion into the detector circuit. Again, because of the high mu of the 2A6 tube, the AVC amplification is greatly increased (about ten times over the system disclosed in the aforesaid Barton case); the AVC accordingly holdsthe signal voltage at the detector withinV much narrower limits than was possiblebefore.

While I have indicatedand described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by vno means limited to the particular organization shown and described, but that many modiications may be made without departing from the scope of my invention, as set forth in the appended claims. I

What Iclaim is: y f Y l. In combination with a signal'amplifler whose gain is to be regulated, a signal rectifier adapted to produce a direct current voltage from the amplifier output whose magnitude-varies directly with the amplitude of the amplified sig-nal energy,- a diode circuit, including the cathode of said rectifier, for providing a gain control bias for the amplifier whose magnitude lvaries directly-with signal amplitude, delay means inv the diode circuit for normally maintaining the cathode positive with respect to the diode anode, means responsive to said rectifier voltage for varying the action of said delay means, and an additional delay means for delaying the action of the said responsive means.

2. In combination with a high frequency signal amplifier whose amplification is to be regulated, a signal rectifier of the diode type adapted to produce a direct current voltage from the amplifier output, the magnitude of the direct current voltage varying directly with the amplitude of the signal energy, a diode circuit comprising the cathode of said rectifier and an auxiliary anode, said last named diode circuit including an impedance for developing a control bias for the amplifier, a connection from a point of negative potential on said impedance to a gain control electrode of said amplifier, *delay means in the last named diode circuit for normally maintaining the cathode of said diode rectifier positive with respect to the auxiliary anode, means responsive to said rectifier voltage for varying the action of said delay means, and a second delay means for controlling the action of said last named responsive means.

3. In a system as dened in claim 1, a signal detector coupled to the signal amplifier output, and said signal rectifier being independent of said signal detector.

4. In a system as defined in claim 2, a diode detector coupled to the output of. said signal amplifier, and means for coupling the diode recs tifier to a point in the signal transmission network between said signal amplifier and said detector.

5. In combination with a signal amplifier whose gain is to be regulated, a signal rectifier adapted to produce a direct current voltage from the amplifier output whose magnitude lvaries directly with the amplitude of the amplified signal energy, a. diode circuit, including the cathode of said rectifier, for providing a gain control bias for the amplifier whose magnitude varies directly with signal amplitude, delay means in the diode circuit for normally maintaining the cathode positive with respect to the diode anode, means responsive to said rectifier voltage for varying the action of said delay means, and an additional delay means for delaying the action of the said responsive means, and said first delay means comprising a resistor disposed in series between the cathode of said signal rectifier and the anode of said gain control diode circuit.

6. In combination with a signal amplifier whose gain is to be regulated, a signal rectifier adapted to produce a direct current voltage from the amplifier output whose magnitude varies directly with the amplitude of the amplified signal energy, a diode circuit, including the cathode of said rectifier, for providing a gain control bias for the amplifier whose magnitude varies directly with signal amplitude, delay means in the diode circuit for normally maintaining the cathode positive with respecttothe v,diode anode,l means responsive to said rectifier voltage for varyingthe action of said delay means, and an additional delay means for delaying the action of the said responsive means comprising a source of adjustable positive potential connected yto thel latter responsive means. f 1

'7. In combination withia signaltransmis'slon tube whose gain is to be regulated, a signal rectifier of the diode ytyp-e having rthe anode there'- of connected through a signal transmission path to the output of said signal transmission tube, an. auxiliary anode disposed adjacent the cathode of said diode rectifier, and including an impedance in circuit therewith for providing a control bias adapted to be applied to said signal transmission tube for gain regulation thereof, a grid and plate associated with the cathode of said diode rectifier, and said grid being connected to a point of negative potential in said diode rectifier circuit, means for applying a predetermined potential to said grid in opposition to said negative potential, a resistor of a predetermined magnitude connected in circuit between the said auxiliary anode and the cathode of said diode rectifier for normally maintaining said cathode positive with respect to the auxiliary anode, and said last resistor additionally being arranged to have the space current between said plate and cathode flow through it.

8. In combination with a signal transmission tube whose gain is to be regulated, a signal rectifier of the diode type having the anode thereof. connected through a signal transmission path to the output of said signal transmission tube, an auxiliary anode disposed adjacent the cathode of said diode rectifier, and including an impedance in circuit therewith for providing a control bias adapted to be applied to said signal transmission tube for gain regulation thereof, a grid and plate associated with the cathode of said diode rectifier, and said grid being connected to a point of negative potential in said diode rectifier circuit, a resistor of a predetermined magnltude connected in circuit between the said auxiliary anode and the cathode of said diode rectifier for normally maintaining said cathode positive with respect to the auxiliary anode, and a resistor connected between a source `,of positive potential and the said grid for normally preventing the grid from assuming a negative bias until the signal amplitude attains a predetermined intensity level.

9. In combination with a signal transmission tube whose gain is to be regulated, a signal rectifier of the diode type having the anode thereof connected through a signal transmission path to the output of. said signal transmission tube, an auxiliary anode disposed adjacent the cathode of said diode rectifier, and including an impedance in circuit therewith for providing a control bias adapted to be applied to said signal transmission tube for gain regulation thereof, a grid and plate associated with the cathode of said diode rectifier, and said grid being connected to a point of negative potential in said diode rectifier circuit, a resistor of a predetermined magnitude connected in circuit between the said auxiliary anode and the cathode of said diode rectifier for normally maintaining said cathode positive with respect to the auxiliary anode and an adjustable resistor connected between a source of positive potential and the said grid for normally preventing the grid from assuming a negative bias until the signal amplitudexattains a predetermined intensity level.

10.-In combination with a signal transmission` tube whose gain is to be regulated, a signal rectier of the diode type having the anode thereof connectedthrough a signal transmission path to the output of said signal transmission tube, an auxiliary anode disposed adjacent the cathode of Said diode rectier, and including an impedance in circuit therewith for providing'a control bias adapted to be applied to said signal transmission tube for gain regulation thereof,ra. grid and plate associated with the cathode of said diode' rectiiierg, and; said grido being connectedA to a point of negative potential in, said diode yrectier circuit, a resistor of a predetermined magnitude 5connectedin circuit between the said auxiliary anode; and the cathode of said diode. recti- 5 er for normally maintaining said cathode positive with respect to the auxiliary anode, and a resistor connected between an adjustable source of positive'potential and the said grid for normally preventing the .grid from assuming a negative 10 bias-until the signal amplitude attains a predetermined intensity level.`

WINFIELD R. KOCH.

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