US2300876A - Radio receiving system - Google Patents

Description

1942. R. c. DICKINSON 2,300,375

RADIO RECEIVING SYSTEM Original Filed April 19, 1941 Oscillafor First Detector LFAmplifier Amplifier; I

WITN ESSES. INVENTOR 5058!? C Die/hm on.

ATTORNEY Patented Nov. 3., 1942 Q l UNITED STATES PAT-'ENT 'QFFI aszi it izzsrs'mm i 7 Q to Westinghouse Electric & Manufacturing. Company, Pittsburgh,-:Pa., a corporation l 1 of Pennsylvania Original application April 19, 1941,'Serial -No. 389,353. Divided and this application January 28, 1942, Serial No.428,48'8- 1 Claim.

This invention relates to radioreceivin'g sys temsand more particularly to circuits for eliminating interference caused by static and similar transient surges affecting. the receiver.

This application is" a" division of my"application Serial No. 389,353, filed April .19, 1941; for Radio receiving systems and assigned to the Westinghouse Electric & Manufacturing Company, which claims certain: subject matter herein disclosed.

The problem of interference due to certain high frequency discharges is an acute. one and various methodshave beenproposed for its elimination or at. least for lessening the undesirable effect produced thereby. The: nature of the electrical disturbance varies with the source of the interfering. radiation and those originating in the electrically charged atmosphere'and reaching the receiver in the form of highly damped electromagnetic impulses are especially diflicult to eliminate. Transients of this type are usually several times greater in intensity than themcoming carrier frequency energy and affect the demodulator of the receiver due to their highly damped characteristics by impact excitation.

The primary object of this invention is to cancel the effect of transient impulses reaching the demodulation circuit of the receiver byproviding means for automatically controlling the output impedance of the demodulator tube upon sudden rise of input voltage and for a duration of time necessary for the decay of transient oscillatory currents.

Another object of this invention is to attenuate automatically the audio frequency energy derived from the demodulator of the receiving system Whenever the rectified component of the carrier wave exceeds a predetermined level at a transient impulse rate.

A further object ofthis invention is to provide a control circuit connected to the demodulator portion of the receiver which produces a rectifier signal voltage whereby the effective impedance and hence the voltage output of the demodulator is decreased and'the provision of means associated with the control circuit for of a radio receiving system, together with the control circuit in accordance with this invention.

Referring to the figure, thevarious portions of the receiving system: herein shown as being of the superheterodyne type are indicated by conventional squares and only the modulator portion is shown in detail, to which the present invention is directed; The receiver may be of any conventional form inasmuch as this invention is applicable to the demodulatorqportion of any receiving system as long: as it operates as a linear rectifier of the carrier frequency'energy producing a rectified signal voltage.

The system herein shown :employs the superheterodyne principle wherein. the received carrier frequencyenergy is converted to an intermediate frequency, the latter being fedtothe demodulator tube l, which, acting as a half- ,applied to the primary winding 4 thereof. The

anode 5 ofthe demodulator tube I'is connected .to the high potential terminal of the-secondary winding 2 and the cathode 6 of the tube l -is grounded. Between ground and the other terminal of the secondary winding 2 is-Rplaced the initiating its action upon signal voltages exload resistance IL The intermediate frequency component of the signalvoltage is by-passed by a condenser efiectively'in parallel'with the resistance 1. The audio frequency component of the rectified signalling energy is taken 01f from a potentiometer which is the load resistance l of the diode l by a sliding contactthereof which connects to the input circuit of the audio frequency amplifier. In parallel with the demodulator tube l'is connected the control tube 9 in such manner that the: cathode l0 thereof connects with theanodefi ofthe tube and the anode ll of the tube -9 directly to the; grid of tube I and also to the cathode of. tube I- through aresistor 35 and the common ground connection.

The control element or grid I2 of the tube 9 is connected in series with resistor l3 to the slidingarm ll of a potentiometer 15. The grid I2 is also by-passed to ground by condenser Hi.

The potentiometer l5 forms the load resistance of another rectifier shown. here in the form of a dual purpose tube, one portion of which comprises a multi-electrode amplifier, the other being a dual anode diode rectifier. The amplifier portion of the dual purpose tube ll includes the control grid IB which is connected to the input circuit of the demodulator portion of the receiver by means of conductor l9, coupling condenser 20, and the input impedance 2| in the form of a radio frequency choke coil connected between grid [8 and ground. The screen electrode 22 is indicated to receive proper operating potentialand the anode'electrode 23 is connected to the primary winding 24 of a transformer 25,

which is tuned to the intermediate frequency, of

the signalling energy by condenser 26. The free terminal of the primary winding 24 is indicated to receive the required operating potential for the anode 23. The secondarywinding 21 ofthe transformer 25 is connected'fbetweenthe paralleled rectifier anodes 28 and 28' and one-terminal of the potentiometer 15, the other terminal thereof being grounded. Condenser 29 serves to by-pass the intermediate. frequency voltage,

to ground, the cathode 30 of the tube ll being grounded through bias resistance 3|, and bypassed by condenser 32 in shunt therewith,

In the operation of the circuit herein .described, assuming that the condition of 'tube 9 is such which will not allow thermionic conductivity, a steady signal modulated at the rate of audio frequencies will be rectified by the tube l producing a rectified signalvoltageacross: the load resistance 1 thereof; The audio frequency component of the signal is furtherv amplified by the audio frequency amplifier in order to-be reproduced inithe speaker 39. s

However; the control tube 9 in a state. of conductivity, will offer a 'low impedance path for currents in the output circuit and thereby attenuate to a marked extent the signal level due to the fact that its space current path is eifectively in parallel with the output circuit of the demodulator including the secondary winding 2 of I. F. transformer 3 and the load resistance 1. In order that the control tube 9 shall not impede normal receiving conditions, the intermediate frequency voltage is utilized asta source of steady biasing potential for the grid I2 of the tube 9. This is accomplished by the combined amplifying and rectifying .action of tube [1, theinput circuit thereof being energized through-condenser 20 by the intermediate frequency voltage which,in the output circuit of this tube, is impressed upon the secondary winding 21 of the transformer 25 in an amplified form, and then rectified by the diode portion of the tube appearing in the form-of a unidirectional voltage across the potentiometer I5. By adjusting the sliding arm I I of the potentiometer l5, a desired magnitude of biasVoltage can be obtained for the grid l2 of the tube 9 in order to counterbalance any tendency of conductivity which proper polarity of. signal voltage between cathode Ill and anode ll of the demodulator voltage would otherwise'=cause. Since-no current flows through anode l I" to resistor 35, the grid 33 of tube l is at thesame-potential as its cathode 6, and the tube l acts as a simple rectifier. Therefore, if the moving arm I4 is set so that the negative grid voltage upon the grid lz-of tube 9 is just sufficient to cause anode current cut-off, the output impedance of the demodulator circuit will be'free' from attenuation, and the signal will be heard undim'in'ished. If an impulse of stray stron ger'tha'n-the signal is now received, the resulting voltage impulse will tend to increase the voltage between cathode l and anode II of tube 9; Similarly, this impulse will increase the bias voltage produced across the control rectifier load resistance l5, tending to maintain the grid I2 sufficiently negative to counterbalance the tendency of conductivity created by the rise of anode potential in tube 9. However, due to the time delay of the resistance capacity combination inthe grid circuit of tube 9 comprising the resistor l3 and condenser l6,

- this impulse will be discharged to ground through the anode ll before the grid l2 becomes sufficiently negative to reestablish the condition of balance and non-conductivity of the tube 9.

Current flow through tube 9 causes resistor 35 to impress a negative voltage on grid 33 of tube l which will result in an attenuation of the signali the output of demodulator tube 1. Furthermore, the anode resistance of the tube 9 is of low value compared to the resistance 1 and the .voltage appearing across the output circuit of'the'demodulator is momentarily reduced to a very low value. Stable conditions are reached after the passage of the transient impulse and thesignal is again detected and passed to the audio frequency amplifier in a normal manner.

Another utilitarian feature of this circuit arrangement may be found in the elimination of inter-channel noise. The negative voltage of the, grid ,l2 .will vary with the steady signal strength with the corresponding fluctuation in the positive anode voltage of the tube 9, so that the latter will be maintained slightly above cutoff. It is thus. apparent that with. no signal,the tube 9 acts as a low impedance shunt across the rectifier I, which substantially silences the receiver when no signal is present.

I claim as my invention:

In a demodulation system, a high-frequency signal input and a signal output circuit, a demodulator tube connected to said output circuit having anode, cathode and grid electrodes, producing a rectified signal voltage,- an impedance element between said grid and said cathode, said grid being thereby biased with respect to said cathode, means for altering the condition of said grid whereby the signal output of said demodulateris attenuated, comprising a vacuum tube having anode, cathode and control electrodes connected to said output circuit and energized by said rectified signal voltage in proper polarity of thermionic conductivity, said impedance element being in series with thespace current path of said vacuum tube, and said current producing a voltage drop thereacross, and means for controlling the space current of said tube comprising a source of unidirectional voltage varying in accordance with the high-frequency signal voltage in 'said input circuit, a circuit interconnecting the control electrode of said vacuum tube and said source including a time delay network, said circuit being so adjusted that said unidirectional voltage maintains the grid electrode of said vacuum tube at a potential of space current cutoff within signal input levels of normal magnitude and frequency range, said time delay network having such characteristics as to introduce sufficient lag in the application of said unidirectional bias voltage at signal magnitude beyond modulation voltage intensities as to cause instantaneous conductivity of said vacuum tube attenuating thereby the rectified signal voltage of said demodulator tube.

' ROBERT C. DICKINSON.

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