US2026357A

US2026357A - Automatic volume control

Info

Publication number: US2026357A
Application number: US599950A
Authority: US
Inventors: Laurence M Perkins
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1932-03-19
Filing date: 1932-03-19
Publication date: 1935-12-31
Anticipated expiration: 1952-12-31

Description

Dec. 31, 1935. p Ns 2,026,357

AUTOMATIC VOLUME CONTROL Filed March 19, 1932 WITHOUT A Ufa/WI 7/: I

c our/Pol w/m A 070M477:

C 0/7 TROL Jwventoo LHUHE'NCE MPEHx/N's' @w/MMM WM Patented D... 31, 1935 PATENT OFFICE AUTOMATIC VOLUME CONTROL Laurence M. Perkins, Erie, Pa., assignor, assignments, to Radio Corporation of New York, N. Y.,

13 Claims.

This invention relates to an automatic volume control for a radio receiver. Most of the commercial systems of automatic volume control use two detectors, one for normal detection of the signal, its output being audio frequency, the other for controlling the volume output of the receiver. The output of this second detector is a substantially continuous current which controls by reason of a change in the drop across the resistor in the output circuit. It is sometimes undesirable to use the two detectors because it unbalances the preceding tuned stage as this stage will have two tubes across it instead of one. posed to use one untuned circuit supplying two detectors, but this has its disadvantage in that it lowers the amplification.

An object of this invention is to use a single detector, that is, resistance coupled to the following stage. Both the audio frequency output and the direct current voltage change are obtained across the resistor in the plate circuit of the detector. In other words, the detector and the first stage of audio amplification are coupled by a resistance network so that the audio frequency is passed by a condenser to a grid of the succeeding tube. The direct current is blocked by the condenser but is passed through a filter resistance which blocks the audio frequency signal, and yet allows the voltage change to alter the grid bias supplied to one or more of the preceding amplifier tubes.

Another disadvantage of the systems heretofore in use, is that the regulation of the volume starts even on weak signals so that the maximum sensitivity of the receiver is not available when changing from a strong station to a weak station. An ideal automatic volume control should not supply a bias to the preceding stages, that is, a

bias that will operate on the upper knee of the characteristic curve of the tube. If it does, it will result in distortion and in a reduction in the sensitivity of the receiver, when weak signals are received.

' An object of this invention is to utilize the max-, imum sensitivity of the radio receiver while receiving weak signals. This has'been accomplished by the use of a rectifying device in com- I bination with a detector which gives an appreciably greater direct current voltage change than is required on the radio frequency grid for a suitable volume control.

The system is so arranged that if there were no rectifier in the volume control circuit the bias supplied to the RF grids would swing beyond the point of maximum sensitivity. In order to pre- It has been proby mesne America,

a corporation of Delaware Application March 19,

1932, Serial No. 599,950

vent the grid bias from swinging beyond the point of maximum sensitivity a rectifier has been connected so as to prevent the bias swing passing a predetermined fixed point in one direction while allowing free swing in the opposite direction. Thus, with a weak signal the rectifier allows current to pass because its anode is then more positive than its cathode. This current flowing from the anode to cathode flows through the filter resistor previously mentioned, which causes a volt- 1 age drop so that the actual voltage on the RF- grids is different from the plate voltage supplied to the detector. As the signal intensity increases the anode becomes less positive, and at the desired output, the anode is at the same potential as the cathode of the rectifier. Up to this signal I strength the bias on the RF grids is held almost constant, varying only by the change in voltage across the rectifier device-which change may be limited to one volt or less by the proper design. As soon as the rectifier ceases to pass current, due to its anode being negative with respect to its cathode, the RF grids are controlled directly by the plate voltage supplied-to the detector so that a very accurate volume control is obtained.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing wherein apreferred embodiment of one form of the present invention is clearly shown.

In the drawing:

Fig. 1 discloses a schematic wiring diagram of a portion of a radio receiver provided with an automatic ,volume control.

Fig. 2 discloses a group of curves obtained from the radio receiver.

As may be seen from Fig. 2, a radio receiver having the input plotted, as abscissa: and the output as ordinates has a curve as represented by the curve l0 where no automatic volume control is used. If an automatic volume control is used that is effective for all signals, a curve similar to the curve disclosed by the full line l2 would be obtained. From C to A the curve l2 drops and holds the output lower than that desired. The ideal condition would be to follow the curve In from A to B, then follow the horizontal curve represented by the dashed line H! from B to C. From A to B, the receiver operates at its maximum sensitivity as a change in the grid bias in either direction tends to reduce the sensitivity. The full line curve I5 is obtained ina radio receiver embodying the disclosure made in Fig. 1. That is, curve I5 approaches the ideal condition,

til

.current causing a it failing to follow the ideal condition in that a small variation in the voltage drop across the rectifier causes the curve to deviate from the ideal curve. As this automatic volume control operates equally as well regardless of the number of stages of amplification only one stage of amplification has been disclosed. This may be a radio frequency amplifier, or it may be an amplifier in a superheterodyne receiver. The output of the amplifier or amplifiers is supplied to a detector stage followed by one or more stages of audio frequency amplification. As far as this invention is concerned it makes no difference as to what circuit the output of the detector is supplied.

A resistor connected in series with a cathode 22 of an amplifier 24 has one terminal connected to a resistor 26 grounded at 28. From this it may readily be seen that the potential of the cathode 22 is considerably above ground when the amplifier 24 is functioning, in that the plate current of the amplifier 24 passes through the

resistances

20 and 26. Under normal operating conditions, with a strong signal, a grid 30 of the amplifier 24 is substantially at the same potential as the potential supplied to an anode 32 of a detector 34, in that the grid 30 is connected to the anode 32 through a resistance 36 and through an inductance 38 including a condenser 40. When the signal is weak, the rectifier takes drop through resistor 36. In the modification disclosed in Fig. 1 the plate 32 is energized from a suitable potential E supplied to a voltage

divider including resistances

42, 44, 26 and 48. The potential supplied to anode 32, however, is not equal to the terminal voltage supplied to terminal 58 as the plate current through the detector decreases by a voltage equal to the IR drop across the resistance 52. When the incoming signal is strong a large current flows through the resistance 52 which causes the potential to the plate ing in the grid 30 becoming more negative respect to the cathode 22.

A cathode 54 of a rectifier 56 is connected to a terminal 51 intermediate the

resistances

26 and 44 in the voltage divider. An anode 58 of rectifier 56 is connected to the circuit of grid 30 so that the potential supplied to the anode 58 is at all times substantially the same as the grid potential supplied to the amplifier 24. A grid or control electrode 60 of the rectifier 55 is connected to the anode 58 of that same tube. It is apparent to one skilled in the art that since the control electrode 60 is not used as such, a tube could be used which had no grid. It can be readily seen that when the potential of the cathode 54 is greater than the potential of the anode 58 no current fiows through the rectifier. This is the condition that exists when the output of the amplifier follows the horizontal portion of the curve l5, the potential of the grid 30 steadily increasing as the input decreases from C to B. When the input decreases substantially to B the potential supplied to the anode 32 is substantially the same as the potential of the cathode 54. Whenever the input falls below the abscissa of B the current flows from the anode 58 to the cathode 54 in the rectifier 56, because the voltage supplied 'to.the anode 32 of the detector 34 exceeds the potential supplied to the anode 58. As soon as current begins to flow in the rectifier 56 the potential of the grid 30 will be less than the potential of the anode 32 of the detector 34, in that the rectifier current causes a voltage drop across the resistance 36. By properly selecting the values of the rewith 32 to drop, thereby

resultsistances

25, 35, 42, 44, 48, 52 and 20, it is possible to maintain the potentials supplied to the grid 30, when the input signal is less than B, that is substantially the proper value with respect to the cathode 22. Instead of using a voltage divider 5 including the

resistances

42, 44, 26 and 48, a suit able potential can be supplied to the terminals 50, 51 by any other suitable means such as. for example, batteries or terminals on the AC source of rectified current.

While the form of embodiment of the present invention as herein disclosed, constitutes a preferred form, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow.

What is claimed is as follows:

1. In combination with a high frequency amplifier and detector, a control network for automatically regulating the gain of the amplifier, said network comprising a direct current biasing 2o path connected between an input electrode of the amplifier and the anode of said detector, and a space discharge device, connected between said path and the cathodes of said amplifier and detector, for maintaining said amplifier input elec-gg trode independent of the biasing action of said path until a predetermined amplifier energy input value is reached.

2. In combination with a high frequency amplifier and a detector, a control network for auto- 30 matically regulating the gain of the amplifier, said network comprising a direct current biasing path connected between an input electrode of the amplifier and the anode of said detector, and a space discharge device including at least a cold electrode and an electron emission electrode, connected between said path and the cathodes of said amplifier and detector, for maintaining said amplifier input electrode independent of the biasing action of said path until a predetermined amplifier energy input value is reached.

3. In combination with a high frequency amplifier and a detector, a control network for automatically regulating the gain of the amplifier,

said network comprising a direct current biasing path connected between an input electrode of the amplifier and the anode of said detector, and a space discharge diode, connected between said path and the cathodes of said' amplifier and detector, for maintaining said amplifier input electrode independent of the biasing action of said path until a predetermined amplifier energy input value is reached.

4. In combination with a high frequency amplifier and a detector, a control network for automatically regulating the gain of the amplifier, said network comprising a direct current biasing path connected between an input electrode of the amplifier and the anode of said detector, a conductive connection between the cathodes of the detector and amplifier, and a space discharge device, connected between said path and the cathodes of said amplifier and detector, for maintaining said amplifier input electrode independ- 5 ent of the biasing action of said path until a predetermined amplifier energy input value is reached.

5. An automatic volume control arrangement in combination with a radio receiver comprising a radio frequency amplifier and a detector, said arrangement including a direct current connection between the amplifier and detector cathodes,

a second direct current connection between the amplifier grid and the anode of said detector,

and a diode connected between said two connections.

6. An automatic volume control arrangement in combination with a radio receiver comprising a radio frequency amplifier and a detector, said arrangement including a direct current connection between the amplifier and detector cathodes, a second direct current connection between the amplifier grid and the anode of said detector, and a diode connected between said two connections, the diode cathode being at a less positive potential than the amplifier cathode when weak signals are impressed on the amplifier.

7. An automatic volume control arrangement in combination with a radio receiver comprising a radio frequency amplifier and a detector, said arrangement including a direct current connection between the amplifier and detector cathodes, a second direct current connection between the amplifier grid and the anode of said dectector, and a diode connected between said two connections, the anode of said diode being negative with respect to the cathode thereof to cause the diode to become non-conductive when the signals impressed on said detector increasebeyond a desired intensity.

8. In a radio receiver, a high frequency amplifier, a detector, a network for automatically controlling the gain of said amplifier so that the detector output is maintained substantially constant regardless of variations of collected signal intensity, said network including a direct current biasing path between the anode of the detector and the grid of said amplifier, and means for delaying the action of said control network until a predetermined signal intensity is attained, said delay means comprising a diode connected to said biasing path and the cathodes of said amplifier and detector in such a manner that said grid is maintained at a constant bias until said predetermined intensity is reached.

9. In an automatic volume control system for a radio receiver. which receiver comprises a radio frequency amplifier and a detector, said system including a variable biasing impedance for said radio frequency amplifier, said impedance being operatively associated with the amplifier and detector varying with the intensity of current fiow in the detector output, the value of said biasing impedance being infinite when said detector output current exceeds a predetermined value, and additional means, when said biasing impedance becomes infinite, for imparting a biasing potential to the grid of the radio frequency amplifier which varies directly with variation of thedetector anode potential. v

10. In a radio receiver, a high frequency amplifier, a detector, and a diode, a potential supply resistor, a direct current path between the detector anode and a positive potential point on said resistor, a second path including a resistor connecting the detector anode to the diode anode and amplifier grid, a connection between the diode cathode and a less positive potential point on the supply resistor, a resistor connecting the amplifier cathode to said last connection, and a connection between the detector cathode and a point on said supply resistor which is less positive than the other two points.

11. In a radio receiver, a high frequency amplifier, a detector, and a diode, a potential supply resistor, a direct current path between the detector anode and a positive potential point on said resistor, a second path including a resistor connecting the detector anode to the diode anode and amplifier grid, a connection between the diode cathode and a less positive potential point on the supply resistor, a resistor connecting the amplifier cathode to said last connection, a connection between-the detector cathode and a point on said supply resistor which is less positive than the other two points, and an audio frequency amplifier having its grid circuit coupled to the said second resistor.

12. In a radio receiver, a high frequency amplifier, a detector, and a diode, a potential supply resistor, a direct current path between the de tector anode and a positive potential point on said resistor, a second path including a resistor connecting the detector anode to the diode anode and amplifier grid, a connection between the diode cathode and a less positive potential point on the supply resistor, a resistor connecting the amplifier cathode to said last connection, a connection between the detector cathode and a point on said supply resistor which is less positive than the other two points, said first named path including a resistor.

13. In combination, in a radio receiver, a signal transmission tube, an electron discharge tube following the first tube, a signal transmission network coupling the output electrodes of the first tube to the input electrodes of the second tube, a diode having its electrodes connected to points in the space current path of said second tube such that the diode anode is negative with respect to the diode cathode in the absence of received signals, a gain control connection between the diode anode and the input grid of the first tube, and means for reversing the polarity of the diode anode with respect to the diode cathode when signal above a predetermined amplitude are impressed on said receiver.

LAURENCE M. PERKINS.