US2052613A - Automatic control system - Google Patents

Description

Sept. 1, 193$. p O FARNHAM 2,052,613

I AUTOMATIC CONTROL SYSTEM Filed Jan. 8, 1932 v gmemtok 6/ WM, w

Patented Sept. 1, 1936 STATES AUTOMATIC CONTROL SYSTEM Paul 0. Farnham, Boonton, N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application January 8, 1932, Serial No. 585,593

v14 Claims. (01. 250-20) Thisinvention relates to automatic control systems of the generaltype employed in radio receivers, and moreparticularly' to a system for automatically preventing noisy operation of a sensitive receiver. I j

The high amplification obtained when a receiver of the automatic gain control type is tuned to frequencies at which there is no signal, or only a weak signal, is accompanied by a relatively high noise output 'arising in the' tubesand circuits as well as in static disturbances. This high noise output may be quite disturbing in tuning when zones of'high noise level are passed in going from one signal frequency to another.

Circuit arrangements are known for providing a sharp cut-off for allreceived signal energy be-.

7 low a predetermined. magnitude, the cut-ofi being obtained by applying an initial bias to a diode demodulator to suppress all audio response until 1 the signal voltage impressed upon the diode rises above that value which is essential for a satisfactory signal-to-noise ratio. Unfortunately, however, the bias voltage required for best noisesuppression is ,sohigh as to produce some distortion in the audio reproduction. H

,An object of this invention is to provide aradio receiver of the type includinga noise-suppression system and an automatic gain control system, and in whichth'e automatic gain control system operate'sjto remove the noise-suppression bias from the demodulator when the radioinput of the desired signal rises above a predetermined value. More specifically, an object is to provide a radio receiver includinga detector and an automatic gain control system, the detector-being biased by the potential drop across a resistance in the output circuit of a direct current amplifier, and in which the flow of current in the resistance, and therefore the bias on the detector, is determined by therectified voltage developed by, the automatic gain'control system.

These and other objects and advantages of the inventionwill be apparent from the following specification when taken with the accompanying drawing in which the single view is a circuit diagram of one embodiment of the invention.

'3, of a. radio frequency amplifiertube-4. The amplified output from tube 4 is passed toa network, indicated generically at5, which may comprise a simple coupling impedance or may include additional amplifier stages operating at carrier and/or an intermediate frequency.

The input impedance of the diode rectifier is coupled to the output terminals of the network 5 and, as illustrated, the input impedance comprises an inductance 6 in series with a tuning con denser '1 and blocking condenser 8, the junction of the condensers being connected to ground and to the low potential output terminal of the network 5. The coupling to the network 5 may be of any appropriate type, a capacitive coupling through condenser 9 being illustrated as typical of one form.

The multiple diode rectifier l may be, and preferably is, a commercial triode of the type having a heater H, an equipotential cathode l2, grid l3 and plate I4. For convenience of description, the electrode [3 will be referred to as the grid, but it will be apparent from the following description that the grid 13 functions as the anode of a diode rectifier. The high potential side of the tuned input circuit is connected to the cathode l2 and, so far as concerns carrier irequencies, the input circuit is completed to the rectifier anodes by the radio frequency by-pass condenser l which is connected between the grounded terminal of the input circuit and the grid, and the audio frequency by-p ss condenser l6 which is shunted across the grid and plate terminals.

The tube l0 functions as a double diode having a single input circuit and individual output circuits for audio frequency and for direct cur-- rent, Referring first to the direct current output circuit, rectification of the average signal voltage takes place between the cathode I2 and plate I 4, thus developing a direct current potential across the resistor I! that is connected between plate 14 and ground, which potential is impressed on the amplifier 4 asa gain-control voltage, in the known manner, through an alternating current filter comprising series resistances l8 and shunt capacities l9. To efiect a delay in the development of direct current potentials as the received signal energy increases to that value which will provide a normal input voltage across the diode input circuit, the diode cathode I2 is connected by a lead to a source of potential which is positive with respect to the plate I 4.

0n the drawing, this direct current source is indicated by the legend +(40 volts), which negative bias of 40 volts on the anode will postpone the development of a rectified gain control voltage until the peak signal input on the diode rect current voltage for reducing the gain of the amplifier 4.

In accordance with the present invention, the direct current bias on the diode demodulator is varied automatically as a functionof the received signal energy. The audio-frequency voltage developed across resistance 2| is applied, through a coupling condenser 22, tothe voltage divider 23 which provides means, in known manner, for regulating the audio output level.

The automatic variation of the bias on the grid 13 is provided by. connecting the terminal 24 of the audio output resistor 2i to the plate P of a control tube 25 and, through a resistance 26,.to the lad 29 that is connected to the direct current source.'

The cathode K of the. control tube is grounded, and the'grid G is connected to the plate Id of the diode rectifier, preferably through one section of the alternating current filter l8, l9.' The plate circuit of the control tube 25 is shunted by a condenser 'ZTWhich has a low impedance for alternatingcurrents.

It will be apparent that, as the carrier voltage 3 E across the diode rectifier input increases to values above the bias voltage applied to the plate M, the direct current potential applied to the grid G of the control tube becomes more negative and the plate current flow through the output resistance 26 of the control tube is reduced'. 'When the direct current potential on gridG-blocks the flow of plate current through the control tube output resistance 26, the direct current potential on the grid I3 of the diode demodulator is-the same as that of its cathode lz. The demodulator therefore functions as the usual unbiased diode and the audio response is a substantially undistorted reproduction of the modulation envelope of the signal energy impressed on the diode;

When the received signal energy falls to such lowilevels that the amplified signal voltage E0 at the input circuit of tube i0 is below the value effective to causerectificationbetween plate M 'and'cathode 12, the grid G and cathode K of the-control tube or direct current amplifier 25 are at the same potential and a flow of plate current through the resistance 26 results. The potential drop due to this plate current fiow reduces the direct current potential of plate P and therefore of the terminal 24 of the audio output resistance 2|, thus making the potential of the demodulator anode, i. e.,' the grid I13, more negative than that of the cathode i2. This negative bias potential suppresses the audio output of the diode demodulator and, until the input voltage E0 rises to that substantial value which provides a rectified voltage on the grid of the control tube 25, or which exceeds the negative bias on the demodulator anode M, there will .be no development of. an-audio frequency voltage across the resistances and the: voltage divider 23.- This postponement of all audio output'until the received signal energy reaches a,

predetermined value effectively prevents noisy operation or weak signals but, since the delay'bias is automatically reduced or removed as the signal strength increases, there is no distortion in the reproduction of signals 'of substantial magnitude.

While I have described one particular embodi ment which has proved satisfactory, itwill be apparent that there is some latitude in the choice and the relative arrangement of the circuit elements which provide anautomatic variation in the delay bias voltage impressed upon the demodulator.

I claim: 7

1. The combination with a radio amplifier, and an audio transmission system including a demodulator, of directcurrent amplifier means opera-.

tive to block transmission through said audio transmission system, means including a rectifier for automatically controlling the gain of said radio amplifier in accordance with variations in the strength of a received signal, and means including said rectifier for rendering said blocking means inoperative when the input voltage on said rectifier exceeds'a predetermined value. 7 V

2. In an electrical wave transmission system, the combination with an amplifier, a demodulator, means for controlling the gain of said amplifier, said means including a rectifier developing a direct current potential which varies with the strength of a received signal, the electrodes of the demodulator and rectifier being disposed in a common tube envelope and means normally block- 1ng operation of said demodulator, of means actuate d by the direct'cu'rrent potential developed by said rectifier for reducing the efiectfof said blocking means.

I 3; In a radio receiver, the combination of a radio amplifier, a diode demodulator, direct cur-' rent amplifier biasing means impressing a bias voltage on said demodulator to block operation thereof, means including a rectifier for automatically controlling the gain of said amplifier in ac cordance with variations inthe strength of are ceived signal, and means including said rectifier for automatically varying the magnitude of the bias voltage impressed on said demodulator by said blocking means.

4. In a modulated carrier wave receiver, the

combination with an amplifier, a demodulator, and means including a rectifier for automatically varying the gain of said amplifier when the re-. ceived signal energy exceeds a predetermined low level, of direct current amplifier means biasing said demodulator to prevent the development of an audio frequency output, and means controlled by the said rectifier for neutralizing said biasing means when the received signal energy exceeds a predetermined value.- r

5. A combined demodulator and carrier volt age rectifier, comprising a double diode rectifier having a cathode and a pair of anodes, an input circuit -resonant at carrier frequency and connected between the rectifier cathode and ground, a direct current output circuit connected between one anodeand ground, an a1 ternating current output circuit connectedbetween the second anode and apoint of negative potential with respect to said cathode, and means for reducing the magnitude of the negative potential of said point; said means including a control tube having the grid thereof subjected to the potential developed by said output circuit, anda plate circuit for said control tube including areeluding a resistance in the circuit of said detector for applying thereto a delay bias voltage to render the detector inoperative for radio input voltages below a critical value, of means including said rectifier and a direct current amplifier for establishing a current flow in said resistance to reduce said applied bias voltage when the radio input voltage on said rectifier exceeds a predetermined value, and means energized by said rectifier for automatically controlling the gain of said amplifier in accordance with variations in the strength of the radio frequency voltage impressed on said receiver.

7. In a radio receiver, a radio amplifier, a demodulator diode, means including a second diode for automatically controlling the gain of said amplifier in accordance with variations in the strength of received signals, and biasing means including one of said diodes for impressing upon said demodulator diode a bias voltage that varies with the magnitude of received signals in that sense which renders said demodulator diode inoperative for weak signals and operative for signals in excess of a threshold value.

8. In the reception of radio signals by a receiver including a radio amplifier, a detector diode and an automatic gain control system including a diode; the method which comprises initially impressing a bias voltage on one of said diodes to render the same inoperative for receiver input voltages falling within a predetermined and relatively low range of values, and automatically reducing the bias voltage on said biased diode when the receiver input voltage exceeds said range of values.

9. In the operation of a radio receiver including a pair of diode detector elements, a pair of diode elements forming part of an automatic gain control system, an amplifier including a cathode and a plate, and a resistance in the platecathode circuit of said amplifier, the method of controlling the operation of one of said pairs of diode elements which comprises impressing between the same as a bias voltage the direct current potential drop produced across the resistance by plate current flow in the amplifier, and automatically varying the flow of plate current as a function of the radio input to the receiver, thereby to vary the bias on the controlled pair of diode elements.

10. In the operation of a radio receiver having a radio amplifier and a pair of diodes comprising elements in a demodulator and in an automatic gain control system, the method of control which comprises biasing one diode to render the same inoperative for receiver input voltages of less than a predetermined value, producing a direct current voltage by the rectifier action of the second diode, and employing the said direct current voltage to reduce the bias on the first diode.

11. In a radio receiver, the combination with a radio frequency amplifier and a detector, of means responsive to the magnitude of the radio voltage input to said amplifier for biasing said detector to render the same inoperative for input voltages below a predetermined level and for reducing the applied bias voltage when the input voltage exceeds said level, said means including a direct current amplifier having an output resistance comprising a biasing impedance for said detector.

12. In a radio receiver, the combination with a radio frequency amplifier, a detector diode, and a diode included in a system for automatically controlling the gain of said amplifier, of means responsive to the magnitude of the radio voltage impressed on said receiver for biasing one of said diodes to render the same inoperative for input voltages below a predetermined level and for reducing the applied bias voltage when the input voltage exceeds said level, said means including a direct current amplifier having an output resistance comprising a biasing impedance for said detector.

13. In an amplifier, means for deriving a direct current proportional to an incoming signal, a space current device including an anode and a cathode, means for normally maintaining said anode negative with respect to said cathode, and means whereby the said direct current is utilized to cause a reversal of the polarity of said electrodes, with consequent flow of space current.

14. In a modulated carrier wave receiver, the combination with a carrier wave amplifier and an audio transmission system, means including a diode rectifier for automatically varying the gain of the amplifier when the received signal energy exceeds a predetermined low level, means, including an electron discharge tube, operative to block transmission through said audio system, an input electrode of said last tube being connected to said diode rectifier for rendering theblocking tube inoperative when the signal input voltage on said rectifier exceeds a predetermined value.

PAUL O. FARNHAM.

Images