US2135949A - Noise suppressor circuit - Google Patents

Description

Nov. 8 1938. w. VAN B. ROBERTS NOISE SUPPRESSOR CIRCUIT Filed June 17, 1957 Z'LDETECTOR NOISE LIMITER To AUDIO a A IEAMPL/F/ER cows/ewe A. /-t AMPLIFIER MPLlF/ER TO/H C TOAEAMPLlF/ER T0 1. 5 AMPLIFIER INVENTOR WALTER VA B.ROBER7'S ATTORNEY Patented Nov. s, 1938 2,135,949

UNITED STATES PATENT OFFICE NOISE SUPPRESSOR CIRCUIT Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 17, 1937, Serial No. 148,627 10 Claims. (01. 250-20) My present invention relates to noise silencing detector and the local oscillator networks will circuits, and more particularly to receivers utihave the rotors of their variable tuning conlizing noise suppressors. densers mechanically arranged for uni-control.

One of the main objects of this invention is The I. F. output of converter 2 is transmitted to provide a noise silencer circuit in which the to one, or more, stages of I. F. amplification; 5 silencing action takes place when an interfering therefore the numeral 3 is to be understood as voltage of only slightly greater amplitude than designating a network which may include one that of the desired signal is received; the disor more amplifier tubes tuned tothe operating tinguishing feature of the present silencer cir- I. F. The last I. F. output circuit 4 is coupled to cuit arrangement is the use of a multi-electrode the I. F. input circuit 5 of the second detector, 10 tube as a. diode detector which has its detecting or audio demodulator tube D. It is to be underquality controlled by the potential on a grid of stood that the coupling transformer M1 has its. the tube. primary and secondary circuits fixedly resonated The novel features which I believe to be char- 'to the operating I. F. Similarly the primary and acteristic of my invention are set forth in parsecondary circuits 4 and 5 of the coupling trans- 15 ticularity in the appended claims; the invenformer M2 are resonated to the operating I. F. tion itself, however, as to both its organization In Fig. 1 a triode D is used as a diode detector and method of operation will best be understood in combination with condenser K and resistance by reference to the following description taken in Q. The anode and cathode of tube D constitute a0 connection with the drawing in which I have inthe diode elements, while the grid 6 is used to dicated diagrammatically several circuit organicontrol the internal resistance of the diode. A zations whereby my invention may be carried diode tube S is employed for the purpose of into effect. squelching the detecting action of detector D In the drawing: whenever the voltage across the transformer sec- Fig. 1 shows a superheterodyne receiver emondary windingL exceeds a predeterminedvalue. 25 bodying one form of the invention, and A battery B2 of low voltage may be included be- Fig. 2 illustrates a modification of the silencer tween the grid and cathode of tube D to maincircuit. tain the grid normally somewhat positive, so as The invention will best be explained with refto reduce the internal resistance of the diode 0 erence to Fig. 1 which shows, for the sake of circuit to a value low enough to permit linear illustration, a conventionalized superheterodyne detection. This battery, however, is not essential receiver including a stage of intermediate freto the squelching action of the circuit. quency amplification whose gain is automatically A second battery B1 which is adjustable in controlled in accordance with signal strength. value, is used to adjust the bias on diode S so The receiver is generally of the type disclosed and that no current flows through it unless the volt- 35 claimed in my copending application Serial No. age across coil L is greater than the maximum 80,497, filed May 19, 1936. peak voltage of the desired signal. If, however,

The receiver comprises, in general, a radio frean interfering disturbance, such as for example quency amplifier I, a converter 2, an I. F. ampliis created by automobile ignition systems, sets fier 3 and a second detector D. A signal source up a transient voltage across coil L which is sub- 40 A feeds collected signals to the amplifier l, and stantially greater than the desired signal, ourthe latter will include means for tuning it rent will fiow through diode S and set up a Voltthrough a desired signal frequency range. The age across resistor R. If sufficient in intensity, signal source A may be the usual grounded anthis last voltage will make the grid 6 of detector 5 tenna circuit; radio frequency distribution line; tube D sufficiently negative with respect to its or even the collector used on mobile vehicles such cathode to prevent any flow of current through as automobiles. The amplified signals are fed the detector. Condenser C, shunted across reto the tunable input circuit of the converter 2. sistor R, is chosen as small as possible so that the It will be understood that the converter may be time required to build up the squelch voltage of the combined local oscillator-first detector across resistor R will be small. Resistor R is 50 type employing a pentagrid converter tube of preferably large, so that the interfering voltage the 2A! type, or separate tunable first detector will create a maximum voltage across R. The and local oscillator tubes may be used. In any time constant RC is chosen great enough to case, it is to be understood that the tunable cirmaintain the squelching action for the duration cults of the radio freq en y amplifier, th first of the interfering transient voltage, but, onthe 55 other hand, not so great as to keep the detector inoperative for an unnecessary length of time.

The best values of R and C for any particular installation are best determined, however, by trial. In the absence of interfering voltages the operation of the circuit is quite conventional, and both the audio output and AVC potentials may be obtained from the detector resistorQ as shown. The AVC bias is applied through lead 10 and filter resistor II to the I. F. amplifier signal grid. Of course the AVG bias can be applied to the networks and 2 as well. The AVC action is well known and need not be described.

In order to demonstrate the rapidity with which the detector is rendered inoperative as the Voltage across coil L increases beyond the predetermined value, the following calculations have Let us suppose that source B1 has been made. been so adjusted that (B1Bz), the total 'bias across diode S, is slightly greater than the maximum peak voltage of the desired signal. Let E represent an interfering voltage which is greater than this total bias. Then, assuming the internal resistance of diode S small compared to R, the rectified voltage across R will be very nearly E-(B1Bz) The total negative bias on the grid of the detector is, therefore, E-B1.

If, now, the tap T is located at a point on the coil L where the voltage is a fraction F of the total voltage across the coil, the voltage impressed upon the detector is FE. It is well known that no ourrent will flow in the detector if this voltage FE is less than the product of the grid bias and the amplification constant a of the tube. Thus, the result is obtained that the detecting action will be squelched when the interfering voltage is appreciably greater than given by the equation.

' compared to B1. This latter is the normal condition as B1 will usually be set to 10 volts, or more, in order to take care of the rather large signal voltages developed in the modern receiver.

In case it is desired to have the adjustment of the voltage B1 take place automatically, and in accordance with the received signal strength,

the circuit of Fig. 2 may be used. This circuit may be identical with that of Fig. 1 in every way, except in the method for obtaining the bias voltage for diode S. In my copending application Serial No. 108,427, filed Oct. 3Q, 1936, Patent No. 2,115,876, May 3, 1938, there is disclosed and claimed the automatic adjustment of the bias on the squelch diode. In Fig. 2, an auxiliary diode P is employed in conjunction with a high resistance M and a large capacity N to provide a positive potential on the cathode of diode S substantially equal to the modulation peaks of the desired signal. The time constant of network MN should not, however, be so large that abnormal voltage across M produced by interference voltages Will persist unduly after the cessation of the interference On the other hand, N should belarge enough so that the bias on S will increase appreciably only after squelch bias has been genconsideration is taken of the fact that the anode of, S is maintained slightly positive by the source B2. If the grid source B2 is omitted it will not be necessary to insure any particular amount ofreduction in the alternating voltage across diode S. For this reason it is preferable to use as the detector, a tube whose detecting qualities are satisfactory without requiring any positive bias on thegrid.

In both Figs. 1 and 2 a certain amount of signal voltage may reach the detector tube grid through the distributed capacity of diode S, or by actual capacity as at the point a: in Fig. 2. This however, merely assists in the detecting action of the detector, and is too small appreciably to affect the squelching action when the grid becomes strongly negative. Indeed, the aforesaid distributed capacity effects may be augmented by auxiliary capacity for the purpose of improving the detection action, provided that the alternating voltage thus introduced to the grid is not suflicient to interfere with satisfactory squelching action. It will be realized that whilea. triode detector is shown for the sake of simplicity, other tubes having suitable characteristics, such as pento-des, may be used. The connections are the same in either case, except that the screen must be connected to a suitable positive voltage if a pentode, or tetrode, is used.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my inventionis by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is 2' 1. A detector system comprising a multielectrode tube having its plate and cathode connected to a common signal input circuit as a rectifier, said tube including a control grid in the electron stream between the cathode and plate, a second rectifier connected to said input circuit to produce rectified current in proportion to the excess of input voltage above a predetermined level, and a connection from the control grid of said first tube to the said second rectifier to impress rectified voltage fromthe latter on the grid. I 2. A detecting system in accordance with claim 1, wherein a third tube is connected to said input circuit and second rectifier automatically to determine the said predetermined level.

' 3. A method of operating a signal detector of the type including a tube having at least a cathode, anode and a control electrode disposed in theelectron stream between the cathode and anode, which includes impressing signals between the cathode and anode for rectification, rectifying alternating current energy, representative of interference noise, above a predetermined amplitude, and impressing the rectified noise energy upon the said control electrode in a sense to render the first rectification inefiicient.

4. A method of operating a signal detector of the type including a tube having at least a cathode, anode and a control electrode, which includes impressing signals between the cathode and anode for rectification, rectifying alternating current energy, representative of interference noise, above a predetermined amplitude, impressing the rectified noise energy upon the said control electrode in a sense to render the first rectification inefiicient, and controlling said second rectification in response to signal amplitude variation, and in a sense to prevent the second rectification until the said predetermined amplitude is exceeded.

5. In combination with a source of signals and a load circuit, a detector tube including at least a cathode, anode and control electrode, a signal input circuit coupled to the detector anode and cathode, means electrically connected to the anode and cathode for developing a rectified voltage from signal voltage in said input circuit, a rectifier having an alternating current input circuit in which exists alternating current energy impressed on said detector anode and cathode, means in circuit with the rectifier for developing a unidirectional voltage from said alternating current energy, and means for impressing the unidirectional voltage on said control electrode in a sense to decrease the rectification efliciency of said detector cathode and anode.

6. In combination with a source of signals and a load circuit, a detector tube including at least a cathode, anode and control electrode, a signal input circuit coupled to the detector anode and cathode, resistor means electrically connected to the anode and cathode for developing a rectified voltage from signal voltage in said input circuit, a rectifier having an alternating current input circuit in which exists alternating current energy impressed on said detector anode and cathode, resistor means in circuit with the rectifier for developing a unidirectional voltage from said alternating current energy, and means for impressing the unidirectional voltage on said control electrode in a sense to decrease the rectification efiiciency of said detector cathode and anode.

7. In combination with a source of signals and a load circuit, a detector tube including at least a cathode, anode and control electrode, a signal input circuit coupled to the detector anode and cathode, means electrically connected to the anode and cathode for developing a rectified voltage from signal voltage in said input circuit, a rectifier having an alternating current input circuit in which exists alternating current energy impressed on said detector anode and cathode, means in circuit with the rectifier for developing a unidirectional voltage from said alternating current energy, means for impressing the unidirectional voltage on said control electrodein a sense to decrease the rectification efliciency of said detector cathode and anode, a

signal amplifier coupled to said signal input circuit, and means for controlling the amplifier gain with said rectified signal voltage.

8. In combination with a source of signals and a load circuit, a detector tube including at least a cathode, anode and control electrode, a signal input circuit coupled to the detector anode and cathode, means electrically connected to the anode and cathode for developing a rectified voltage from signal voltage in said input circuit, a rectifier having an alternating current input circuit in which exists alternating current energy impressed on said detector anode and cathode, means in circuit with the rectifier for developing a unidirectional voltage from said alternating current energy, means for impressing the unidirectional voltage on said control electrode in a sense to decrease the rectification eirl'ciency of said detector cathode and anode, said alternating current input circuit being the same as said signal input circuit, and means actuated by signal amplitudes below a desired level to render the said rectifier inoperative.

9. In combination with a source of signals and a load circuit, a detector tube including at least a cathode, anode and control electrode, a signal input circuit coupled to the detector anode and cathode, means electrically connected to the anode and cathode for developing a rectified voltage from signal voltage in said input circuit, a rectifier having an alternating current input circuit in which exists alternating current energy impressed on said detector anode and cathode, means in circuit with the rectifier for developing a unidirectional voltage from said alternating current energy, means for impressing the unidirectional voltage on said control electrode in a sense to decrease the rectification efficiency of said detector and anode, means for adjustably connecting the anode of the detector tube to said signal input circuit, and said alternating current input circuit being said signal input circuit.

10. In combination with a source of signals and a load circuit, a detector tube including at least a cathode, anode and control electrode, a signal input circuit coupled to the detector anode and cathode, means electrically connected to the anode and cathode for developing a rectified Voltage from signal voltage in said input circuit, a rectifier having an alternating current input circuit in which exists alternating current energy impressed on said detector anode and cathode, means in circuit with the rectifier for developing a unidirectional voltage from said alternating current energy, and means for impressing the unidirectional voltage on said control electrode in a sense to decrease the rectification efiiciency of said detector cathode and anode, and additional means for preventing rectification of alternating current energy below a predetermined amplitude by said rectifier.

WALTER VAN B. ROBERTS.

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