US2298083A - Control circuits - Google Patents

Description

G. w. FYLER I CONTROL cincuus Filed Nov. '4, 1940 Oct. 6, 1942.

Inventor: George W. F91

ttor'ney.

His

from any suitable tector I l Patented Qct. 6, 1942 con-moi. cmourrs George W. Fyler,

New York Stratford, Conn, General Electric Company,

assignor to acorporatlon of Application'Novcmber 4,1946, Serial No. 354,160

, 14 Claims. (CL 250-20) My invention relates to ticularly adapted for use receiving apparatus.

One of the main objects of my invention is to provide improved noise suppression circuits which effectively discriminate between-undesired noise impulses and desired signals and prevent such impulses from appearing in'the output of radio or television receiving apparatus.

Another object of my invention is to provide improved control circuits for preventing noise transients from appearing in the receiver output whenever these transients level which is automatically determined by the carrier level of the received signal as well as its modulation level.

A further object of my invention is to provide improved control circuits which may perform the dualfunction of noise suppression circuits and voltage doubling automatic gain control circuits.

.The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. l diagrammatically represents a portion of the'circults of a radio receiving apparatus embodying my invention; Figs. 2 and 3' represent modifications of the circuits of Fig. 1; Fig. 4 diagrammatically represents a portion of the circuits of a television receiving apparatus embodying my control circuits parin radio or television invention; and Fig. 5 represents a further modification of my invention.

In the several figures, corresponding reference numerals have been placed on corresponding elements to facilitate their comparison.

The radio receiver .circuits of Fig. 1 include a detector II and an amplifier I6. These may be, for example, the second detector of a superheterodyne radio receiver and the first audio frequency amplifier. High frequency .carrier waves,

modulated by desired audio signals, are supplied signal receiving apparatus (not shown) to the detector input through the tuned input transformer ID. The signal deis of the diode type, having an anode l2 and cathode l3. The signal detector circuit extends from the anode I2 through the secondary of the input transformer l0, adiode load resistor l4 and high frequency by-pass capacitor in parallel, to the cathode l3. v

The operation of this portion of the circuit is well known to those skilled in the art and will not be detailed here. The detector is effective to demodulate the carrier'waves impressed'upon the transformer Hi. In a superheterodynereceiver these carrier waves will generally be of an intermediate high frequency lower than the carrier frequency of the received signals. The demodulaexceed a threshold voltages appearing stage of audio amplification or a tion products developed across the diode load resistor i4 include audio frequency components and a direct current component. The audio fre-' quency components are coupled to the grid of the audio frequency amplifier I6 through the coupling capacitor H. .Theamplified signal in the output of the amplifier it are supplied to any suitable utilization circuit (not shown). This may comprise a further sound translating device.

As is well known, interfering electrical disturbances of short duration and considerable magnitude are often received on the signal channel along with-the desired signal. These may seriously distort thedesired signal, or even mask it completely, whenever they approach or exceed the magnitude of the useful signal. Such disturbances are generally grouped under the term impulse noise, as distinguished from hiss or thermal agitation noise. These disturbances may arise from various causes. For. example, natural atmospheric static surges, man-made electrical disturbances, as from high frequency apparatus, ignition systems and the like, and other shock excitation effects in the receiving system.

' it is desired to In order to limit the effect of such noise impulses, a noise suppression network is provided in accordance with my invention. As shown in Fig. 1, this network includes a second diode de tector I8 reversely connected in parallel to the signal diode ll through a resistor l9 shunted by a capacitor 20. The time constant of the network I9, 20 is selected so as as compared to the frequency within the band of frequencies which amplifier 16. For example, itmay be of the order of about one-tenth of a second. Further, the resistor I9 has a very high value as compared to diode load resistor H. For example, it maybe of the order of about ten megohms with a 50,000 ohm diode load resistor. This network functionsto be relatively long period of the] lowest signal transmit to the audio frequency .former l and the 2 to reduce the interfering effects of noise pulses in a manner now to be described.

It will be observed that the diode I8 is in circuit with the secondary winding of the transoutput network It, I 5 and that it is poled to pass current through these elements, when conductive, in the opposite direction from current flowing in the signal diode circuit. Thus, this circuit extends from the anode 22 of the diode I8, through the network I9, 20, the network l4, l5 and the secondary winding of transformer It, to the cathode 2|.

The signal diode I l passes current on the peaks of those half cycles of the applied carrier wave which make its anode positive, and the. modulation voltages appearing, on the network [4, l5 reproduce the modulation envelope. On the peaks of alternate half cycles, the maximum voltage across the diode ll equal ,to the sum of the maximum diode load voltage on the network l4, l5, and the peak voltage of the. applied carrier waves. These two voltages are approximately equal and hence the peak voltages applied across the diode l8 and network I9, 20 are approximately equal to twice the peak amplitude of the carrier waves.

Due to the relatively long time constant of the network I9, 20 the diode l8 functions as a rectifier on the outward peaks of audio modulation and charges the capacitor 20 up according to the peak values of the signal and modulation levels. The diode l8 therefore draws'only the small current necessary to replenish the charge on the capacitor 20 which leaks off through the resistor IS. The potential on capacitor 20 varies with the modulation envelope so that noise peaks can be limited substantially at the audio peak level. At zero modulation, or very low levels of modulation, limiting takes place approximately at the carrier level. The bias potential on diode l8 thus may vary between approximately two and four times the unidirectional component of the carrier voltage developed on load resistor l4.

Assume now that an undesired noise impulse of greater magnitude than the audio modulation and of short duration is impressed on the secondary of the transformer I 0. A low impedance path for voltages of either vided through one or the other of the two diodes, since the diodes are reversely connected to pass current through the output load resistor H in both directions and capacitor 20 is of low impedance for impulses which are conducted by. diode l8. .Therefore, since no demodulation of the noise transient takes place, it is effectively suppressed, producing substantially no effect upon the grid of the amplifier IS.

The internal resistance of the signal diode II should be'approximately equal to the internal resistance of the diode iii. In a particular radio receiver embodying my invention, values of about 10 megohms for resistor l9 and about .01 microfarad for capacitor were found to be suitable.

It is common practice to utilize the direct current component of the demodulated signals appearing on resistor I! for automatic gain control. dotted outlines in Fig. 'l. Unidirectional potentials, negative with respect to ground, are developed at the left-hand'end of resistor l4 and applied throughw. low-pass filter, comprising a resistor 23 and capacitor 24, to the gain control connections of preceding stages of the receiver, not shown. The arrangement and operation of the gain control connections will readily be unis approximately derstood by those skilled in the art without elaboration. .The potentials are applied to control electrodes of one or more of the tubes preceding the detector to reduce the receiver gain in accordance with the strength of the received signals, as is well understood in the art.

Since the bias voltages developed across the resistor l9 and capacitor 20 are approximately twice those appearing on resistor H, the noise suppression circuit can be utilized to perform the additional function of a voltage-doubling automatic gain control circuit. Thus a more effective gain control action may optionally be secured by connecting the upper end of resistor I9 to the ain control connections over conductor 25. Further, the resistor l9 and capacitor 20 serve as a low-passfllter, eliminating the need for additional filter means.

A small amount of signal distortion must inevitably be caused by the limiter circuit due to the intermittent charging current drawn by network l.9, 20.- High fidelity reproduction requires a very low order of harmonic distortion. A circuit which reduces signal distortion to a negligible value although with some reduction in the noise suppression action is illustrated in Fig. 2.

- It will be observed that it differs from the noise polarity is now pro- Conventional connections are shown in suppression circuit of Fig.

1 only in that the lower end of resistor I8 is connected to the lefthand end of resistor I4 rather than directly to ground. The potential on resistor l9, and therefore the discharge current, is now reduced by approximately one-half. This is apparent from the fact that the maximum voltage impressed on diode l8 and resistor I 9 is now only the peak value of the carrier waves supplied from transformer l0 and does not include the approximately equal voltage developed on resistor H. The discharge circuit for capacitor 20 now includes resistors l4 and IS in series, but since the and .01 microfarad for capacitor 20.

provides improved noise suppression, especially trated as a battery 26. The source is poled to produce a small current flow through the diode l8. Generally, the potential value is not critical. Five or ten volts have been found satisfactory in a typical receiver circuit.

The addition of-the battery 26 ,increases the age on the primary winding of transformer l0,

In Fig. 4 my invention is shown in connection with the final picture signal circuits of a television receiver. The signal detection and noise suppression circuits, which are shown in detail, are essentially the same as the circuits of Fig. 1 and corresponding elements have been supplied with corresponding reference numerals. The demodulated video signals appearingupon the load resistor M are supplied to a video amplifier, indicated schematically by the block 21, and thence to a cathode ray device 28 of the usual type employed in such circuits for reproducing the picture image. The synchronizing and sweep circuits of the receiver have .been omitted to simplify the drawing, as they form no part of my invention.

The functions of the signal detection and noise suppression circuits of the television receiver of 'Fig. 4 are fundamentally the same as previously described in connection with the circuits of Fig. 1. The principal difference resides in the choice of circuit constants for proper reproduction of the much wider band of signal frequencies, which may extend from about 60 cycles to 4 megacycles or higher. A series peaking inductance 35 is connected .in circuit with the output load resistor ll for improving the high frequency response of the detector circuit in a manner familiar to those skilled in the art.

The positive modulation peaks of the carrier correspond to the peaks of the synchronizing pulses in one form of television transmission system in current use. In the television receiving apparatus of Fig. 4 noise impulses will be limited substantially to this level. In a particular television receiver embodying my invention it was found that line and frame synchronization was improved on weak signals in the presence of noise. thereby demonstrating the practical utility of my improved circuit.

The noise suppression circuit of Fig. 4 may also be utilized to provide a voltage-doubling automatic gain control action. The gain control connections of the preceding stages of the television receiver, not shown, may be supplied with control potentials over the conductor 25. This is particularly effective in video circuits since the peak amplitude of the television signals is constant in the aforesaid system of transmission.

Rectification in diode l8 therefore takes place only on the fixed level of the synchronizing pulses and the gain control potentials will be proportional to this level.

A further modification of my invention embodied in a radio receiving apparatus is illustrated in Fig. 5. The signal detection circuit is the same as before, but the noise detection circuit is supplied with high frequency potentials from the primary winding of the input transformer l0, rather than from the secondary wind ng, as

in the circuits described heretofore. An alternative connection of the noise diode l8, resistor l9, and capacitor 20 is also shown, the resistor l9 being connected in shunt to the diode l8 rather than the capacitor 20. The detection action of this form of noise suppression circuit is not substantially different from the circuits previously described, as will be understood by those skilled in the art without detailedanalysis.

The diodes H and ill in Fig. 5- are poled to pass current through the diode load resistor and capacitor M in opposite directions, when conductive, as before. The maximum voltage on diode I8 is the sum of the maximum diode load voltage on the network 4, I5 and the peak voltin this case. If desired, noise impulses may be impressed simultaneously on the signal diode H and on the noise diode 18 in the conductive direction, depending upon the direction of winding the primary of transformer III with respect to the secondary. I

,It may be found desirable, in some cases, to supply the signal and noise detector circuits with high frequency potentials from two separate secondary windings on the transformer I0, both i being coupled to a common primary winding. As

in the case of Fig. 5 this permits a choice of polarities for the individual secondary windings to secure most effective noise suppression action.

Furthermore, the tuning characteristics for the input circuit of the signal diode may be made different from those of the input circuit for the noise diode so as to provide more-effective suppression of noise impulses in the signal diode load circuit I4, l5.

It will thus be apparent that I have provided improved noise suppression and automatic gain control circuits which require a minimum of elements and which are readily adapted to existing apparatus. embodiments of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination with a source of signalmodulated high frequency potentials subject to ed to have demodulated signal potentials developed thereon, a second detector circuit serially comprising a second unilaterally conducting discharge device having an anode anda cathode, said output impedance and a second impedance,

said devices being poled to pass current through said output impedance in opposite directions when conductive, means coupled to said source for impressing said high frequency potentials on both of said circuits in parallel, and peak rectifying means including said second device and second impedance for biasing said anode negatively with respect to said cathode, said last means being operative at all times to effect peak rectification of both said high frequency potentials and said demodulated potentials, said second device and second impedance having a low .impedance for impulses which exceed said bias and which are of a polarity to drive said cathode negative, whereby impulses of either polarity exceeding said bias find a conducting path through said output impedance and through one or the other of said devices.

2. In combination with a source of signalmodulated high frequency potentials subject to interfering noise impulses, a signal detector circuit coupled to said source and serially comprising a unilaterally conducting discharge device and an output impedance adapted to have demodulated signal potentials developed thereon, a second detector circuit coupled to said source in parallel to said signal detector circuit and serially comprising said output impedance, a capacitor, and a second, reversely connected unilaterally conducting discharge device having an anode and a cathode, a resistor connected from the While I have shown particular either polarity exceeding said bias find a conducting path through said output impedance and through one or the other of said devices.

3. In combination with a source of signalmodulated'high frequency potentials subject to interfering noise impulses, a signal detector circuit serially comprising said source, a unilaterally conducting discharge device, and an output impedance across which demodulated signal potentials are developed, a second detector circuit serially comprising said source, said output impedance and a second, reversely connected, unilaterally conducting discharge device having an anode and a cathode, said circuits being energized in parallel from said source, and rectifying means including said second device for biasing said anode negatively with respect to said cathode, said means being operative at all times to effect peak rectification of both said high frequency potentials and said demodulated potentials, said second device having a low impedance for impulses which exceed said bias and which are of a polarity to drive said cathode negative, whereby impulses of either polarity exceeding said bias find a conducting path through said output impedance and hrough one or the other of said devices. V

4. In combination with a source of, carrier waves modulated in amplitude in accordance with a band of signal frequencies and subject to undesired noise transients, a signal detection circuit 'serially including a rectifying device, a signal output impedance and said source, a second rectifying device reversely connected across said first device through connections including a network having a resistance element and a capacitance element, said network having a relatively long time constant as compared to the period of the lowest signal frequency in said band, and means comprising said second device for developing at all times a negative self bias through peak rectiiication of the waves impressed thereon, said second device being conductive to pass current through said output impedance only in response to transients of one polarity which exceed said bias.

5. In combination with a source of signalmodulated high frequency potentials subject to interfering noise impulses, a signal detector circuit serially comprising an input impedance coupled tosaid source, a rectifying device and an output impedance adapted to have demodulated signal potentials developed thereon, a second detector circuit serially comprising a second input impedance coupled to said source, a second rectifying device having an anode and a cathode, a capacitor and said output impedance, said circuits being energized in parallel from said source and said devices being poled to pass current through said input impedance in opposite directions. when conductive, and peak rectifying means including said d'evice and said capacitor for biasing said anode negatively with respect to said cathode, said means being operative at all times to eflect peak rectification of both said high frequency potentials and said demodulated potentials, said second device and said capacitor having a low impedance for impulses which exceed said bias and which are of a polarity in drive said cathode negative, whereby impulses of either polarity exceeding said bias find a conducting path through said output impedance.

6. In a modulated wave receiver for operation on can'ier waves modulated over a band of signal frequencies and subject to interfering noise transients, the combination'comprising a signal detection circuit serially including an input impedance, a signal rectifier and an output resistor by-passed for carrier frequencies, a noise suppression diode having an anode and cathode reversely connected across said signal diode through a capacitanceelement, and a resistance element of high resistance as compared to said output resistor connected in shunt to said capacitance element through a source of low unidirectional potential, said elements having a relatively long time constant, said source being poled to bias said anode positively.

7. In a modulated wave receiver for operation on carrier waves modulated over a band of signal frequencies, said receiver having connections for varying the receiver gain in accordance with gain control potentials'applied thereto, a signal demodulating circuit serially including aninput impedance, a signal diode and an output load impedance by-passed for carrier frequencies, a second diode reversely connected across said signal diode through connections serially including a resistance element shunted by a capacitance element, said elements having a relatively long time constant as compared to the period of the lowest signal frequency in said band, peak rectifying means comprising said second diode and said elements for developing potentials across said elements-substantially corresponding to the peak values of the waves appearing across said detector diode, and means for applying said potentials to said gain controLconnectionsto reduce the receiver gain.

8. In a. modulated wave receiver for operation on carrier waves modulated over a band of signal frequencies and subject to interfering noise transients, said receiver having connections for varying the receiver gain in accordance with gain control potentials applied thereto, a signal demodulating circuit serially including an input impedance, a unilaterally conducting discharge device and an output impedance across which demodulated signals are developed, a second unilaterally conduoting discharge device having an anode and a cathode and being -reversely connected across said first device through connections serially including a resistance element shunted by a capacitance element, said elements having a relatively long time constant, as compared to the period of the lowest signal frequency in said band and peak rectifying means compris ing said second device and said elements for developing potentials on said elements, through peak detection of the waves appearing on said first device, which bias said anode negatively with respect to said cathode, said second device having a low impedance for-noise transients which exceed said bias and which are of a polarity to drive said anode positive, and means for applying said potentials to said gain control connections to reduce the receiver gain.

signal detection circuit 9. In combination with means for supplying aves modulated in amplitude by desired signal iltages and subject to undesired noise inter- :rence, a signal detection circuit coupled to said leans comprising a unilaterally conducting deice and an impedance adapted to have the deiodulated signal voltages developed thereon, a .oise suppression circuit coupled to said means omprising said impedance, a second unilaterally onducting device and a time constant network,

aid circuits being energized in parallelfrom said ource and said devices being poled to pass curent through said output impedance in opposite lirections when conductive, and means including :aid second device and said network for developng a negative bias for said second device sub- ;tantially corresponding to the peak modulation level of said waves for all values thereof.

10. In combination with means for supplying high frequency carrier waves modulated in amplitude by desired signal frequency waves and subject to undesired noise transients, a signal detection circuit coupled to said means comprising a rectifying device and a load resistor bypassed only for carrier frequencies, a noise suppression circuit coupled to said means comprising said load resistor, a second rectifying device and a resistance-capacitance network having a time constant longer than the period of the lowest desired signal frequency, said circuits being energlzed'in parallel from said source and said second device being poled to pass current through said load resistor in the opposite direction from said first device when conductive, and means comprising said second device and said network for impressing a negative bias on said second device substantially corresponding to the peak modulation level of said carrier waves for all values thereof, whereby transients of one polarity exceeding said bias are conducted by said noise suppression circuit.

11. In radio receiving apparatus for translating carrier waves modulated over a band of desired signal frequencies and subject to undesired transint interference, means for supplying said waves to an input impedance, a signal demodulating circuit serially including said input 1mpedanc, a signal detector and an output load resistor by-passed only for carrier frequencies, a noise suppression circuit serially including a noise detector reversely connected across detector through a resistance-capacitance network having a time co taut longer than the period of the lowest desired signal frequency in said band, means comprising said noise detector and said network for impressing a negative bias on said noise detector substantially corresponding to the peak modulation level of said carrier waves for all values thereof, whereby transients of one polarity exceeding said bias are conducted by said noise suppression circuit.

12. In combination with means for supplying modulated by desired signal waves to undesired noise interference, a

and subject serially comprising said means, a signal detector and an output impedance adapted to have the demodulated signals developed thereon, a noise suppression circuit serially comprising said means, said impedance, a noise detector and a being energized in parallel from said means and said detectorsbeing poled to pass current through said output impedance in opposite directions when conductive, a resistance and a source of unidirectional potential connected in series across said capacitance, and means comprising said noise detector, said capacitance and said resistance for impressing a negative bias on said noise detector substantially corresponding to the peak modulation level of said carrier waves, said source being poled to oppose said bias.

13. In radio receiving apparatus for translating carrier waves modulated over a band of desired signal frequencies and subject to undesired transient interference, means forsupplying said waves to an input impedance, a signal demodusaid signal resistance-capacitance network lating-circuit serially including said input impe'dance, a signal detector and an output, load resistor by-passed only for carrier frequencies, a

noise suppression circuit serially including a.

noise detector reversely connected across signal detector through and a resistance element compared to said load to said capacitance element tional bias source, said elements having a time constant long as compared to the period of the lowest desired signal frequency in said band, and means comprising said noise detector and said elements for impressing a negative self-bias on said nois'e diode substantially corresponding said of high resistance as to the peak modulation level of said carrier waves,

and said fixed bias source being poled to oppose said self-bias.

14. In radio receiving apparatus for translating high frequency waves modulated over a band of desired signal frequencies and subject to uninterference, said apparatus having connections for varying the gain in response to gain control potentials impressed thereon, means for supplying said waves to an input impedance, a signal demodulating circuit serially including said input impedance, a signal rectifier and an output load resistor by-passed only for said high frequency waves; a noise suppressing and voltage-doubling gain control circuit serially including a second rectifier reversely connected across said signal rectifier through a having a time constant longer than the period of the lowest desired signal frequency in said band, said noise suppressing and gain control circuit being eflective to develop potentials On said network which bias said rectifier negatively substantially in accordance with the peak modulation level of said high frequency waves for all values thereof, and means for im said potentials upon said gain control connections.

GEORGE W. FILER.

capacitance, said circuitsa capacitance element, 1

resistor connected in shunt through a-unidirec-

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