US2180355A - Means for suppressing noise in radio receivers - Google Patents

Description

Nov. 21; 1939.

P. M. HAFFCKE MEANS FOR SUPPRESSING NOISE IN RADIO RECEIVERS Filed NOV. 5, 1957 AmplQQr 9 1 Ion Jahear 4mph; Ler

Ky U A A INVENTOR PhilipMHaffcke v ATTORNEY Patented Nov. 21, 1939 UNITED STATES MEAN S FOR. SUPPRESSING NOESE 1N RIO RECEIVERS 9 Claims.

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 376) 0. G. 757) This invention relates to the suppression of noise in radio receivers, such as that due to the phenomenon commonly called static, and has among its several objects:

To provide means responsive to excessive surges of energy interceptable by a radio receiver to modify the plate or accelerating grid voltage proportionally to the excess amplitude of the energy and in a manner to offset the effects thereof;

Toprovide means for so modifying the form of the output wave of a receiver stage, in response to static surges, that the static affected portion of the wave is substantially excluded by the following stages;

To provide means for controlling the effect of static, which means is simple to apply to existing receivers and does not necessitate realigning of the set;

To provide means for effectively reducing the effect of static without causing impulse excitation in the following stages.

In the drawing:

Fig. 1 illustrates diagrammatically a circuit embodying my present invention wherein static control is achieved by reducing the plate voltage;

Fig. 2 depicts my invention applied to control the voltage on the accelerating grid of a vacuum tube;

30 Figs. 3 to 7 show various wave forms and amplitudes involved in the practice of the present invention, all of which will be explained in detail hereinafter.

It is known that noise suppression may be 35 effected by applying to a grid immersed in the electron stream, the noise component on a signal channel, whereby the signal circuit is interrupted, or the transconductance of a tube altered.

I find that the interruption for the duration of extreme surges may be accomplished by still another means and with the further advantage, that when desired this apparent interruption may be brought about without apparent shock, or rapid change of plate current, which would cause impulse excitation in the following tuned circuits in the output of the stage and in following stages.

Noise-control may-be effected by interrupting the path between the oscillator and the mixer stage for the oscillator frequency used for hetero- 50 dyning, or frequency changing, but the connections required for this will requireexperts for installation who are equipped for rebalancing of the tuning circuits, because such oscillators are usually tuned by a variable condenser which is ganged withthe condensers used for tuning the signal frequency channel, and molesting the circuits of the oscillator throws those circuits badly out of tune by the added capacity afforded by even short lengths of wire. I However, if a tube can be placed in the path- 5 of oscillator frequency and be interrupted without requiring such retuning operations, or if any one of the RF (either TRF or IRF) circuits can be broken, or the tubes made inoperative withp out a rapid change in current flow, then the 10 chances of shock excitation are reduced'to a ,desirable minimum and the chances of reducing the effects of atmospherics without secondary shock effects nearly as bad as the static itself, H become a reality. 16

The system herein proposed provides a method of accomplishing this by reducing the Ep of the stage to be affected, to either a low momentary value, or zero to even slightly negative in periods of excessive surges. By this method of plate :20 voltage control, and'by biasing the affected-tube to a point where the Eg-Ip is approximately zero during no signal periods, there can be very little or no change in the plate current in the output circuits of the tube when interrupting 1 5 the signal channel. However, if the output of the tube is sharply tuned, its response can be reduced almost to zero due to the peculiar form of the Wave delivered thereto. 7

Referring to Fig. 1, the tube 8 is of standard form and may be in either a tuned radio frequency stage, an intermediate radio frequency stage, an audio-frequency stage, or it may even be a detector. The output of the preceding stage is indicated by primary 9 of transformer i0 whereof the secondary M is connected to grid [2 and cathode it in the input circuit of tube 8. The plate I4 of tube 8 is connected to a source of positive potential through a series resistance l5 that is also in series with anode l5 and cathode ll of a diode rectifier l8. Connected to the signal channel ahead of primary 9 is a non-linear amplifier l9 that will disproportionately amplify signal components that are of greater than predetermined amplitude and the output of this amplifier is coupled through primary 20 to secondary 2! in the anode-cathode circuit of rectifier it. It will be apparent to those skilled in the art that the effect of a normal signal upon rectifier l8 will be negligible butupon the arrival of excessive surges in a static affected signal the rectifier M3 will be caused to pass a large current that will traverse resistance l5 and impress upon plate M a voltage drop proportional .to the current flowing and in opposition to the constant positive voltage impressed upon the plate, thereby reducing or even stopping entirely the flow of current in the output circuit of tube 8. It is apparent that the normal plate voltage should be adjusted to such value that the IR drop across resistance I5 will, during excessively heavy surges, completely buck out the normal plate voltage or in some instances actually change that voltage to a negative value. The amplifier I 9 may readily be so designed as to cause the rectifier I8 to secure the results mentioned even though the normal plate voltage be of the order of 250 volts or more.

The circuit depicted in Fig. 2 is in general similar to that above described and the elements therein that are common to the two have been designated by the same reference characters. However, the method of control is somewhat different. The grid 22 in this figure is normally maintained at a suitable positive voltage to accelerate the passage of electrons from cathode l3 to plate I4 and is termed an accelerating grid. It will be seen that reducing the positive voltage on grid 22 or changing that voltage from a positive to a negative value will exert a strong controlling influence on the electron stream in the tube and therefore the IR drop due to current from rectifier l8 passing through resistance 23 is applied to oppose the normal positive voltage on accelerating grid 22. Instead of obtaining the entire voltage drop across resistance 23,-an inductance 24 having a suitable impedance to cause the desired drop may be used therewith to secure certain beneficial results that follow therefrom, or the resistance 23 may be omitted and the entire drop be secured across the inductive impedance. The inductance 24 may be by-passed by a condenser 25. It is of course understood that all circuit elements are sochosen as to provide the desired time constants in the circuit. The voltage applied to grid 22 will drift from positive values of, say, 1 to 250 volts, to possibly 200 volts negative. When the constant voltage applied to grid 22 is on the order of 25 volts positive it will act as an eflicient accelerator of the electrons and allow the tube to reproduce normal signals but when it drifts to a strong negative polarity it will act as a rapid cutoff for the plate current and will create harmonics in the output circuit earlier than might be possible by plate voltage control only when that plate voltage is quite high.

Fig. 3 shows at a the wave amplitude of a normal signal and at b an assumed amplitude of an excessive surge that might be impressed upon the control grid 12, which excessive amplitude it is desired to suppress.

Fig. 4 shows at c and d the normal output waves resulting from the incoming waves shown in Fig. 3 when tube 8 is biased to operate at approximately the cutoff point on the Ey-Jp curve, or slightly therebelow so that the negative half of the incoming wave does not aifect the output circuit.

Fig. 5 illustrates the IR drop across the resistance l5 due to the output of rectifier l8. It Will be observed that the drop due to normal signal amplitude at in Fig. 3 is negligible while the drop due to excessive amplitude b of Fig. 3 is shown at e to have a large value.

Fig. 6 shows the resultant voltage on plate M (Fig. 1) or grid 22 (Fig. 2) when the IR drop developed in the rectifier circuit is properly poled and the amplitude of the static surge is very large, the negative value of that voltage being indicated at 1.

Fig. '7 illustrates the wave in the output of the tube for normal wave amplitudes at g and for excessive amplitudes at h, when the circuits are properly adjusted. It will be noted that the wave at g is unchanged on the positive half while the negative half is suppressed. When an excessive surge arrives the output builds up to normal or, due to a slight time delay in the circuit, a little above the normal value at which time the IR drop produced by the rectifier current is impressed and the amplitude of the output wave drops to zero until the incoming wave again reaches substantially normal value when the output builds up to approximately the amplitude reached before the suppressing action became effective. The fundamental wave is thus chopped into two waves of varying base line length and represents very little power at the fundamental frequency in the output circuit of the stage. If the coupling between the control stage and the one following be tuned, such harmonics will be efiiciently filtered out and prevented from passing through the subsequent stages.

The invention herein described and claimed may be used and/or manufactured by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

I claim:

1. The combination with a radio receiver stage that includes a vacuum tube having a cathode, a control grid, and an anode, and an input circuit operatively associated with said tube, of means to feed signal energy to said input circuit, a nonlinear amplifying device outside the signal channel through said receiver connected to said means, a diode rectifier, a resistance in series between the electrodes of said rectifier and also in series in the anode supply of said tube, and means coupling the output of said amplifier to the anode of said rectifier.

2. The combination with a radio receiver stage that includes a vacuum tube having a cathode, a control grid, and an anode, and an input circuit operatively associated with said tube, of a nonlinear amplifying device outside the signal channel through said receiver connected to divert a portion of the signal energy from said channel ahead of said stage, a diode rectifier, a resistance in series between the electrodes ofsaid rectifier, and also in series in the anode supply of said tube, and means coupling the output of said amplifying device to the anode of said rectifier.

3. The combination with a radio receiver stage that includes a vacuum tube having a cathode, a control grid, and an anode, and an input circuit operatively associated with said tube, of a nonlinear amplifying device outside the signal channel through said receiver connected to divert a portion of the signal energy from said channel ahead of said stage, a diode rectifier, an impedance in series between the electrodes of said rectifier, and also in series in the anode supply of said tube, and means coupling the output of said amplifying device to the anode of said rectifier.

4. The combination with a radio receiver stage that includes a vacuum tube having a cathode, a control grid, an accelerating grid and an anode, and an input circuit including a secondary inductance operatively associated with said tube, of means including a primary inductance coupled to said secondary inductance to feed signal energy to said input circuit, a non-linear amplifying device outside the signal channel through said receiver connected to said primary, a diode rectifier, a resistance in series between the electrodes of said rectifier, and also in series with said accelerating grid, and means coupling the output of said amplifying device to the anode of said rectifier.

5. The combination with a radio receiver stage that includes a vacuum tube having a cathode, a control grid, an accelerating grid and an anode, and an input circuit including a secondary inductance operatively associated with said tube,

of means including a primary inductance coupled to said secondary inductance to feed signal energy to said input circuit, a non-linear amplifying device outside the signal channel through said receiver connected to said primary, a diode rectifier, an impedance in series between the electrodes of said rectifier, and also in series with said accelerating grid, and means coupling the output of said amplifying device to the anode of said rectifier.

6. The combination with a radio receiver stage that includes a vacuum tube having a cathode, a control grid, an accelerating grid and an anode, and an input circuit operatively associated with said tube, of a non-linear amplifying device outside the signal channel through said receiver connected to divert a portion of the signal energy from said channel ahead of said stage, a diode rectifier, a resistance in series between the electrodes of said rectifier and also in series with said accelerating grid, and means coupling the output of said amplifying device to the anode of and an input circuit operatively associated with said tube, of a non-linear amplifying device outside the signal channel through said receiver connected to divert a portion of the signal energy from said channel aheadof said stage, a diode rectifier, an impedancein series between the electrodes of said rectifier and also in series with said accelerating grid, and means coupling the output of said amplifying device to the anode of said rectifier. Y

8. A method of controlling noise in a radio receiver including a signal channel having a vacuum tube that includes a plate electrode, comprising the steps of diverting a portion of the signal energy from the signal channel through the receiver, non-linearly amplifying said portion, applying the resultant amplified portion to produce a unidirectional current when said amplified portion has an amplitude in excess of a predetermined value, utilizing said unidirectional current to produce a potentiai drop, and applying said drop to reduce the positive bias voltage on the said plate.

9. A method of controlling noise in a radio receiver including a signal channel having a vacuum tube that includes a plate electrode, comprising the steps of diverting a portion of the signal energy from the signal channel through the re-.

ceiver, non-linearly amplifying said portion, applying the resultant amplified portion to produce a potential drop proportional to the amplitude of said amplified portion above a predetermined value, and applying said potential drop to the said plate to reduce the positive bias on said plate.

PHIL]? M. HAFFCKE.

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