US2172922A - Automatic noise suppressor - Google Patents

Description

Sept. 12, 1939. CLAY 2,172,922

Filed April 4. 1936 3nventor E 2 MU/P/PA Y 6. CIA y (Ittomegs Patented Sept. 12, 1939 UNITED STATES PATENT OFFHCE AUTOMATIU NOISE SUPPRESSOR Application April 4,

1 Claim.

This invention relates to improvements in radio receiving circuits and particularly to circuits for reducing the undesirable effects of extraneous electric disturbances in a receiver.

Interference in radio receivers commonly referred to as static is one of the greatest sources of annoyance to those who listen to reception. Of the sources of noise perhaps the most common are those which result from the operation of various electrical devices. In high frequency reception the greatest offender is the automobile, each spark plug of which is a miniature transmitting station. Many other things cause undesirable interference such as light switches, violet ray machine, and innumerable electric appliances. Static, of course, such as is ordinarily caused by lightning is also periodically present.

Heretofore various attempts have been made to eliminate outside electric interference and although some of the arrangements have resulted in material improvements in noise free reception, there have been numerous drawbacks to the previous circuits. The most common means of electric interference elimination has been to cut off the high frequencies. This at the present time is undesirable since the trend is toward high fidelity reception. Other means for accomplishing the desired end have required complicated and expensive apparatus and at its best was none too effective. It has become recognized that the electric disturbances are annoying because their amplitude so greatly exceeds the amplitude of the signal being received. Electric disturbances that do not exceed the amplitude of the signal being received, while undesirable and detracting to some extent from the fidelity and rendering the reception less intelligible, are practically lost in the signal being received and ofttimes are unnoticeable.

It has been proposed to amplify the noise peaks and use the amplified signal after rectification to control the gain of the receiver. This although a decided improvement over prior methods, still has its disadvantage in that a continuous manual adjustment is necessary to obtain optimum performance on signals of various strengths, fading signals, and in tuning from a signal of one level (strength) to another. This made it necessary for the operator to be continuously on the alert to change the manual adjustment, which continual adjustment is particularly undesirable for high frequency reception where considerable variations in signal strength occur (fading) over a short space of 1936, Serial No. 72,689

strong desirable signals to distort or block them; 10

and which is effective to permit the silencer circuit to operate effectively during the reception of very weak signals. The silencer circuit at all times is so related to the strength of the desired signal that it operates effectively to silence large noise impulses even though the desired signal may vary widely in amplitude due to fading.

The noise impulse silencing is accomplished by amplifying the noise impulse and using the noise energy to block one of the I. F. amplifier tubes in such a manner that the receiver is effectively silenced for the duration of the noise impulse. This silencing is effective and is better understood by a consideration of the characteristics of the noise impulse which caused the disturbance. The noise impulses themselves are generally of extremely short duration, in the nature of one one-thousandths of a second. They are, however, transformed into impulses of longer dura tion by the various increments of the receiver itself, such as the loud speaker, telephones, etc. Therefore the problem is to silence the noise before it reaches the elements which increase the duration of the noise. This is accomplished, as stated, by using the noise impulse to control a biasing means for a cut-off tube which silences the receiver for the duration of the impulse, and which after the impulse is passed allows the receiver to resume its normal operation. The momentary cut-off during the reception of the signal is unnoticeable, even when the cut-off is caused by interference generated in the ignition systems of automobile engines, because the actual duration of the interference impulse is so short, and the cut-off period is so short that the persistence of hearing does not recognize the cut-off, the action being analogous to the action of the eye in viewing motion pictures where the pictures apparently do not flicker because the eye is unable to detect the interruption between film picture sequences.

The noise impulse circuit is made to follow the signal automatically by applying a delayed voltage to the noise impulse circuit consisting of a rectified bias which is essentially equal to cuit.

one-half of the modulation envelope amplitude in addition to a fixed additional bias, the two biases at all times exceeding the maximum, positive or negative peak amplitude of desired signal so that the noise impulse will be applied to the.

cut-off element of the circuit only when the noise impulse exceeds the amplitude of the desired signal. The additional bias may be removed from the circuit in the instances where it is found desirable when the signal strengths encountered are particularly weak.

In the accompanying drawing I have illustrated a circuit which is a portion of the con-- ventional heterodyne circuit and to which the elements permitting the operations above described have been added. Throughout the diagram, like parts have been designated by like reference characters. It will, of course, be understood that the invention is not limited in its application to a heterodyne. receiver as it may be effectively applied to other types of receivers.

Fig. 1 is a diagram showing a part of an intermediate frequency amplifier of a superheterodyne receiver and showing the noise suppressor circuit of my invention;

Fig. 2 is a modified view of the noise suppressor section; and

Fig. 3 is another modified view of the noise suppressor section showing the same adapted for operation in push-pull.

There is illustrated a part of the next to the last and the last stage of an intermediate frequency amplifier. By means already thoroughly familiar to those in the radio art. the signal is received in the preliminary stages over the usual antenna and mixed with the local oscillator signal in a mixer stage and proceeds to the intermediate frequency amplifier for amplification before rectification and further amplification for audio frequency. The signal produces a voltage across the secondary inductance L5 of the intermediate frequency amplifier transformer, the primary not being shown. L5 is tuned by condenser C and the signal is amplified in the 6D6 tube, the output of which is connected to the last intermediate frequency amplifier transformer primary L6. The primary L6 is tuned by condenser C, and is coupled to the secondary L'l tuned by C. The signal is transferred from L6 to Ll, the output of which is connected to the grid G! of the 6L7 tube, which in this instance is the amplifying grid. A constant negative potential may be applied to the grid at 20. The signal is amplified in the SL7 tube, the output of which is connected to L8 tuned by C and inductively coupled to L9 which is untuned and the output connected to tube 16, which is operated as a diode rectifier, where the signal is removed from the I. F. current and reduced to audio frequency. The remainder of the circuit for amplifying the signal at audio frequency and rendering the same audible, is not illustrated since it may be conventional. The circuit thus far described is the conventional cir- The 6L7 tube, however, (which might be any I. F. tube) is used instead of a 6D6 as in'the preceding stage since it provides the additional control electrodes and has the characteristics which effectually assist in the accomplishment of my invention.

It is desirable in some places in the intermediate frequency amplifier to introduce a control element which will, and is capable of, silencing or weakening the signal which eventually becomes audible. The grid G3 of the tube 6L7 has been found to be suitable and practical for use with such a control element, and the means for actuating the grid is connected thereto to cause the tube to control the signal in the unique manner about to be described.

Connected to the grid GI of the SL7 tube by conductor A is the grid of the amplifier tube 6K7. The desired signal together with the electric disturbances which create noise are received, amplified and conducted to the 6L7 and SK? tubes simultaneously. The output of the 6K7 is connected to the primary Ll of a special transformer M, which transformer is provided with two secondaries L2 and L3, the primary and secondaries having approximately a 1:1 ratio. The input circuit to the transformer M is conventional. The output of secondary L2 is connected to a plate of the double diode tube 6H6 which has a diode and cathode indicated as AVC for automatic volume control and another diode and cathode indicated as noise, the purpose of which will hereinafter be explained, and through the diode load resistance R2 to the corresponding cathode. The signal voltage in L2 causes a current fiow between the diode cathode plate during the periods when the plate is positive in relation to the cathode. This causes a voltage drop across the resistor R2 which is accumulated on the condenser C2 producing a negative potential or voltage at the ungrounded end of R2 and C2 closely proportional to the maximum amplitude of the desired signal. This negative potential is conducted through the lead 2| and the resistor R6 to the AVG circuit which operates in the conventional manner, being connected to the grids of the 6D6 tubes, varying the negative bias on the grids of the I. F. tubes in accordance with the amplitude of the received signal to increase the negative potential on the grids when the signal increases, then reducing the amplitude of the signal and decreasing the negative potential on the grids when the signal decreases and thus increasing the amplification. The condenser C3 in conjunction with the resistor R6 operates to filter undesired high frequency potential from the AVG control circuit, leaving only the desired AVC controlling voltage and bypassing the high frequency to the ground.-

In a similar manner, a voltage is produced across the secondary L3 of the transformer M and applied to the associated noise diode plate and cathode of the 6H6 tube. However, the plate of the noise diode is at all times impressed with a negative bias from the AVG section of the tube through the lead 23 and resistors R1 and R4. The cathode of the noise diode may be operated at a slight positive potential in relation to ground. The negative bias on the noise diode is practically equal to the maximum amplitude of the desired signal at all times, being only limited by the slight inefficiency of the 6H6 tube. It may be desirable in some instances to make the ratio of L2 to L! slightly larger to provide a step-up for increasing the bias on the plate of the noise diode.

Due to the negative voltage impressed on the noise diode plate and the fact that the corresponding cathode is positively biased, a voltage appearing across L3 will cause no current to flow between the noise diode, cathode and plate as long as the Voltage due to the desired signal is nearly the same, or the same across L2, as it is across L3. As previously stated, L2 and L3 may have equalconstants and coupling to Ll in order to produce this equality. However, they may be properly ratioed to produce any desired constants. v

Circuit elements R6 and C3 have been chosen to provide a long time constant with the result that sudden large peak noise impulses produced in the AVG section of the 6H6 tube have little or no effect on the AVG voltage appearing across C3. In the case of the noise diode section of the tube 6H6 the reverse is true since C4'has a very small value and sudden peak noise voltages produce large negative impulses across C4. A lead 24 connects the plate of the noise diode tube through a blocking condenser and filter L4 to the element G3 of the 6L7 I. F. amplifier tube. A normal operating bias is impressed on the element G3 through the coupling resistance R5. C and R5 have been chosen to have along time constant at audio frequencies.

When a signal is being received which is free from noise impulses due to static or other types of interference, it will be apparent from the foregoing description that a negative voltage substantially equal to the desired modulated carrier amplitude will be built up across C2. This voltage provides a constant biasing potential which is impressed on the noise diode plate through the resistance R! and R4. Since L2 and L3 have closely similar constants and coupling to LI, the voltages appearing across the secondaries L2 and L3 will be equal. However, it is to be noted that the cathode of the noise diode is biased to a small positive potential in a manner familiar to those versed in the art, such as a tap 01f of a voltage divider, with the result that the potential differences between the noise diode plate and the cathode exceed the peak signal voltages appear-,- ing across L3 so that no current can flow between the noise diode elements. Since no current flows between the noise diode elements, no audio or rectified voltage will appear across C4 from the action of the noise diode. Thus the bias impressed on G3 through the coupling resistor R5 is undisturbed and remains constant, permitting normal operation of the receiver.

However, should a strong interference impulse, caused by static or a wave such as is caused by the ignition systems of automobiles, be picked up by the receiver during the operation of the elements as just described, a peak voltage proportional to the magnitude of the interfering impulse will appear across both of the secondaries of the transformer M. This will produce a momentary increase in the rectified potential across C2. However, since the long time constants of C3 and R6 and R1 and C1 are present, an interfering impulse of short duration will not cause any appreciable change in the AVG or in the normal bias impressed on the noise diode plate through R4 and L3. The noise impulse is, however, amplified in the secondary L3 equal with L2 and will exceed the normal operating bias of the noise diode causing the noise diode section of the tube to function to provide a rectified impulse of negative potential across C4 for the duration of the interfering impulse. Due to the long time constants of the blocking condenser C5 and coupling resistor R5, the negative potential corresponding in amplitude and operation to the noise impulse is impressed on the control element G3 of the 6L7 tube through the lead 24. As previously stated, the control element G3 is so designed that a relatively small increase in negative potential effectively blocks electron flow from the cathode to the plate, stopping the amplifying action of the tube and blocking all signals through the amplifier as long as thenegative potential is maintained,- which is for the duration of the noise impulse. The noise impulse having passed the negative potential furnished by the noise diode tube being removed'from the control grid G3, the tube and amplifier immediately resumes normal operation. As previously stated, the'operation of the noise impulse is so small (approximately of a second) that the persistence of hearing refuses to recognize the interruption.

' Irrespective of the strength of the desired signal, the plate of the noise diode will always be biased with sufficient negative potential so that it will not generate any current which will block the operation of the receiver unless it is a sudden impulse of predetermined higher magnitude than the received signal. These conditions appertain even though the receiver may be tuned in succession to stations differing widely in received strength or in the presence of a signal which is fading. However, if during normal operation of the receiver one or a series of noise impulses of short duration exceeding the strength of the received signal at that particular instance, is received, the noise diode will immediately function to bias the control element G3 of the 6L7 tube and block the amplifying operation of the tube and silencing the receiver for the duration of the undesired impulse or impulses.

' In Fig. 2 I have shown a modified form of the noise suppressor section although its principles of operation are substantially the same. It will be clearly seen by inspection that the circuit is the same as that of Fig. 1 except that the secondaries L2 and L3 are wound as a single, tapped secondary in which L2 embraces the entire secondary and is connected to the cathode of the 6H6 tube for the AVG section; and L3 includes only a portion of the complete secondary and is connected to the cathode of the noise section of the 6H6 tube. This provides the increase in potential for the AVG tube which supplies the biasing voltage to the noise section of the tube as previously described. I have found that this has the advantage that the secondary may be operated at ground potential for audio frequencies; and that it also permits the noise diode plate to be free from all AC poten-1 tials excepting the potential applied by a noise pulse, when it becomes conducting as previously explained. These circuit refinements render the circuit more free from leakage and strays and are found to provide a very satisfactory performance.

Fig. 3 shows exactly the same thing as Fig. 2 except that two 6H6 tubes are operated in pushpull and the secondary of the transformer M therefor is provided with five taps, the center tap D of which connects to the plates of the tubes, the ends F and F of the inductances connecting to the cathodes of the AVG section of the 61-16 tubes and the intermediate taps E and E connecting to the cathodes of the noise section of said tubes.

This circuit performs in the same manner as described for Fig. 2 except that the push-pull action permits the two 6H6 tubes to act on both sides of the carrier envelope efficiently to suppress noise regardless of whether the undesired disturbance is greater on one side of the envelope or the other.

Although the transformer Ll is shown as provided with inductive coupling, it is obvious that other types of coupling might be used and that the drawings have illustrated invention for the purpose of explanation and are not to be inter-,-

preted as limiting invention otherwise than is expressed in the accompanying claim.

Having thus described my invention, I am aware that numerous and various departures may be made without departing from the spirit and scope thereof.

I claim:

In a circuit of the class described, an intermediate frequency amplifier at least one amplifying tube of said amplifier having a control grid for blocking the amplifying action thereof, means operable by a signal impulse to block the said si nal in said receiver, comprising a second amplifier connected to the circuit ahead of said first amplifier in parallel therewith, the output of said second amplifier being connected to the input of a double diode rectifier, one section of said double diode rectifier being included in an automatic volume control circuit connected to an intermediate frequency amplifier and operable to automatically control the volume of the receiver, the other section of said double diode rectifier being in a noise suppressor circuit connected to and operable to supply a negative potential to said control grid, said automatic volume control circuit having a long time constant and connected to supply a negative bias to said noise circuit, said bias being proportionate to a received signal and adapted to prevent rectification in the noise circuit during the reception of a signal free from noise impulses, said noise circuit being operable upon reception of a noise impulse of greater amplitude than the desired signal to supply nega tive potential for the duration of the impulse to said first mentioned control grid to block the signals in the receiver.

MURRAY G. CLAY.

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