US2300875A - Radio receiving system - Google Patents

Description

Nov. 3, 1942. R. c. DICKINSON I 2,300,875

RADIO REcmvmG SYSTEM Original Filed Aug. 12, 1938 Oscillator First Detector Audio 0 3g Amplifier;

T- INVENTOR Robert C. Dickinson.

ATTORNEY Patented Nov. 3;, 1942 Robert 0. Dickinson, Wilkinsburg, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Original application August 12, 193.8,,Serlal No.

224,566. Divided and this application April 19, 1941, Serial No. 389,353

2 Claims.

This invention relates to radio receiving systems and more particularly to circuits for eliminating interference caused by static and similar closed.

The problem of interference due to certain high frequency discharges is an acute one and various methods have been proposed 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 difiicult to eliminate. Transients of this type are usually several times greater in intensity than the incoming 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 by providing 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 of this invention is to provide a control circuit connected to the demodulator portion of the receiver which produces a rectified 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 initiating its action upon signal voltages exceeding a predetermined level and duration of time.

Additional objects and advantages will be apparent from the following description of the invention, defined in particularity by the appended claims, and taken in connection with the accompanying drawing wherein identical reference characters denote similar elements in the various figures. Referring to the drawing:

Figure l represents schematically the demodw lator portion of a radio receiving system, together with the control circuit in accordance with this invention; and

Fig. 2 refers to the same portion of the receiver as Fig. 1, showing a modification of the control cincuit.

Referring to Figure 1, the various 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 for-m inasmuch as this invention is applicable to the demodulator portion 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 frequency energy is converted to an intermediate frequency, the latter being fed to the demodulator tube I, which, acting as a half-Wave rectifier, is connected in series with the secondary winding 2 of the intermediate frequency transformer 3, the received energy being applied to the primary winding 4 thereof. The anode 5 of the demodulator tube I is connected to the high potential terminal of the secondary winding '2 and the cathode 6 of the tube I is grounded. Between round and the other terminal of the secondary winding 2 is placed the load resistance '1. The intermediate frequency component of the signal voltage is by-passed by a condenser effectively in parallel with the resistance 1. The audio fre quency component of the rectified signalling energy is taken off from a potentiometer which is the load resistance I of the diode I by a sliding contact thereof which connects to the input circuit of the audio frequency amplifier. In parallel with the demodulator tube I is connected the control tube 9 in such manner that the cathode Iii thereof connects with the anode -5 of the tube I and the anode II of the tube 9 to the cathode of tube I through the common ground connection. The control element or grid I2 of the tube 9 is connected in series with resistor I3 to the sliding arm I4 of a potentiometer I5. The grid 12 is also by-passed to ground by condenser Iii. The potentiometer I5 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 portionof the dual purpose tube ll includes the control grid I8 which is connected to the input circuit of the demodulator portion of the receiver by means of conductor I 9, coupling condenser 20, and

the input impedance 2| in the form of a radio frequency choke coil connected between grid I8 and ground. The screen electrode 22 is indicated to receive proper operating potential and 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 2 6. The free terminal of the primary winding 24 is indicated to receive the required operating potential for the anode 23. The secondary winding 21 of the transformer 25 is connected between the paralleled rectifier anodes 28 and 28' and one terminal of the potentiometer I5, the other terminal thereof being grounded. Condenser 29 serves to by-pass the intermediate frequency voltage to ground, the cathode 39 of the tube I! being grounded through bias resistance 3|, and by-passed 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 I producing a rectified signal voltage across the load resistance I thereof. The audio frequency component of the signal is further amplified by the audio frequency amplifier in order to be reproduced in the speaker 39.

However, the control tube 9 in a state of contime delay of th resistance capacity combination in the grid circuit of tube 9 comprising the resistor I3 and condenser I6, this impulse will be discharged to ground through the anode I I before the grid I2 becomes sufficiently negative to reestablish the condition of balance and non-conductivity of the tube 9. Th 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 the signal is again detected and passed to the audio frequency amplifier in a normal manner.

Another utilitarian feature of this circuit arrangernent may be found in the elimination of inter-channel noise. The negative voltage of the grid I2 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 cut-off. It is thus apparent that with no signal, the tube 9 acts ductivity, will offer a low impedance path for biasing potential for the grid I2 of the tube 9. I

Thi is accomplished by the combined amplifying and rectifying action of tube II, the input circuit thereof being energized through condenser 29 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 I4 of the potentiometer I5, a desired magnitude of bias voltage can be obtained for the grid I2 of the tube 9 in order to counterbalance any tendency of conductivity which proper polarity of signal voltage between cathode II] and anode II of the demodulator voltage would otherwise cause. Therefore, if the moving arm I4 is set so that the negative grid Voltage upon the grid I2 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 undiminished. If an impulse of stray stronger than the signal is now received, the resulting voltage impulse will tend to increase the voltage between cathode III and anode II of tube 9. Similarly, this impulse will increase the bias voltage produced across the control rectifier load resistance I5, tendin 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 as a low impedance shunt across the rectifier I, which substantially silences the receiver when no signal is present.

Referring to Fig. 2, th modification herein shown consists in the use of a triode for the rectifier tube I in such manner that it is connected in substantially the same way as tube I of Fig. 1, except that the grid 33 thereof is utilized to control the rectifying action. In this modification, the control tube 9 is connected between ground and the high potential side of the input circuit of the demodulator system, the cathode I9 being connected through coupling condenser 34 to the high potential terminal of the primary winding 4 of transformer 3, and the anode II to ground through the anode resistor 35.

The remainder of the circuit corresponds with the one previously described in connection with Fig. 1 and in order to avoid repetition further description thereof is omitted.

Considering the operation of the system-of Fig.

2, it is assumed again that the tube 9 is biased to a cut-off point. Analyzing the circuit it will be seen that the resistance 35 of tube 9 connects also the grid 33 by means of conductor 36 to ground. Under this condition, the grid 33 of tube I is at cathode potential and the tube acts as a diode rectifier. The cut-off bias potential for the grid I2 of tube 9 is obtained from the potentiometer 15 by a proper setting of the arm I4 as previously described. A transient impulse will cause anode current flow in tube 9 which must necessarily flow through the resistance 35 due to the fact that the time delay circuit of the resistor I3 and condenser I6 retards the application of negative potential to counterbalance the sudden increase of anode potential in the tube 9. In this manner the plate current flow in the resistance 35 impresses a negative bias voltage on the grid 33 of the rectifier which will result in the attenuation of the signal in the output circuit of the demodulator. It is to be noted that in the arrangement of Fig. 2, the action of the circuit is somewhat greater due to the fact that the control tube 9 receives anode current energy from a low impedance side of the input circuit and does not act merely as a shunt impedance across the demodulator tube I. v

I claim as my invention: Y

1. In a demodulator circuit, a high frequency signal input and a signal. output circuit, a demodulator tube having cathode, anode and grid electrodes connected to said output circuit producing a rectified signal voltage, a resistance between said cathode and said grid whereby said grid is biased with respect to said cathode, means for altering said condition of said grid compris ing a vacuum tube having an anode energized from signal voltage in said input circuit and connected thereto, said resistance being in the space current path of said vacuum tube, the grid of said demodulator tube and the anode of said signal input and a signal output circuit, a demodulator tube having cathode, anode and grid electrodes, connected to said output circuit producing a rectified signal voltage, a resistance between said cathode and said grid whereby said grid is biased with respect to said cathode, means for altering said condition of said grid comprising a vacuum tube having anode and grid energized .from signal voltage in said: input circuit and connected thereto, said resistance being in the space current path of said vacuum tube, the grid of said demodulator tube and the anode of said vacuum tube being interconnected whereby space current flow in said vacuum tube tends to bias the grid of said demodulator tube more negative with respect to its cathode, and biasing means for preventing space current flow in said vacuum tube during normal signal voltage transfer in said demodulator circuit comprising a source of rectified signal voltage,a connection between the grid of said vacuum tube and said source, and means for removing the effect of said biasing means upon sudden rise in signal voltage for a predetermined duration of time comprising a resistance capacity network in said connection.

ROBERT C. DICKINSON.

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