US2265883A - Signal amplitude limiting circuit - Google Patents

Description

Dec. 9, 394i. A. R. APPLEGARTH, JR

SIGNAL AMPLITUDE LIMITING CIRCUIT Filed July 16, 1938 Patented Dec. 9, i941 SIGNAL ADHLITUDE LDWITING CIRCUIT Alexander R. Applegarth, Jr., Philadelphia, Pa., assignor, by mcsne assignments, to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application Iuly 16, 1938, Serial No. 219,644 19 claims. (ol. 17a-'1.5)

This invention relates to signal1 amplitude limiting circuits for use in electrical systems, and has for its general object the elimination or reduction of undesirable effects produced in such systems by interfering signals whose amplitudes exceed some predetermined level. The limiting circuit of this invention is applicable to communication circuits in general, but inasmuch as several of its most novel and interesting aspects are more readily appreciated in those cases wherein the requirements are most exacting, the invention will be described as applied to a television receiving circuit.

Another object of the invention is to provide means for compensating for the disadvantageous effects introduced in amplitude limiters by the inherent or fortuitous interelectrode capacities of non-linear impedances available for use therein.

A further object of the invention is to provide signal amplitude-limiting means adapted to reverse the polarity of signals exceeding a predetermined amplitude and to reinsert them in the system with reversed polarity, to thus avoid the deleterious eiects of the said fortuitous interelectrode capacities.

Still another object of the invention is to provide automatic control for such a system, whereby the limiting means are continuously maintained at maximum efficiency.

The invention may be clearly understood by reference to the accompanying drawing, the sin gle figure of which is a diagrammatic illustration of the video signal channel of a television system embodying the invention.

A detailed description of the invention as applied to a television receiving circuit will depend to some extent upon the polarity of modulation. of the received signals, and, therefore, it may be well to differentiate briefly between two general systems of modulation now well known in the art, to wit, positive modulation and negative modulation. Negative modulation is that form of modulation wherein increased light at the camera tube results in decreased transmitter output, while decreased light at the camera tube results in increased transmitter output. In short, there is an inverse or negative relation between light values at the camera tube and the transmitter antenna current or carrier level. Positive modulation, on the other hand, is that form of modulation wherein increased light at the camera tube results in a correspondingly increased transmitter output, and conversely. Here there is a direct or positive" relation between light and antenna current or carrier level.

, In the employment of either of these systems, the final video signal is, of course, supplied to the picture tube control grid in such polarity as to result in a positive picture onthe viewing screen. Yet, the two systems respond quite differently to the interfering noise pulses. It will be appreciated that, in general, the eect of a noise pulse on the video signal (as obtained from the second detector diode of Ia superheterodyne receiver) will be either to decrease or increase momentarily the signal amplitude. However, the diode current cannot be decreased below zero, although it can be increased, depending on the energy of the noise pulse, to a considerable extent, and hence the worst disturbances occasioned .by noise pulses are those which act in the direction of increased carrier, while the lesser disturbances act in the direction of reduced carrier.

In the reception of negative modulation signals, these worst disturbances occur in the direction (electrically speaking) of less light, or, let us say, in the black direction. Such disturbances appear on the viewing screen as black spots or, more accurately, as black sections of a scanning line, which may be followed by a White spot while the lesser disturbances may appear as white sections of a scanning line.

In the reception of positive modulation signals, on the other hand, a reverse condition obtains, which, as will be shown, is considerably more deleterious to the reproduced picture. Here the worst noise pulse disturbances cause the picture tube to record more light, and for high energy noise pulses the control grid of the picture tube may be driven so positive as not only to result in an abnormally high degree of illumination, but also to defocus the cathode ray beam so completely as to produce a high intensity light spot considerably more than one normal scanning line in width. This white spot may sometimes Ibev followed by a black spot. If theinterference signal is at all persistent, the effect is to ruin the entertainment value of the picture for all practical purposes.

Referring now to the figure, there is illustrated an amplitude limiter constructed according to the invention and disposed in the video frequency channel of a television receiver designed to receive positive modulation signals. In the gure, l is a full wave second detector which serves to rectify the I. F. television signal impressed on the detector input coil 2. The rectified video frequency signal appears across thev resistor 3. The D. C. component of the output isv isolated from this resistor by means of the blocking condenser 4. Normally this condenser would be placed in the lead 5, as is common in the usual Rf@ coupling systems, but in this case the distributed capacity to ground of a condenser large enough to prevent undue low frequency discrimination would seriously limit the high frequency response of the system, and, therefore, the con denser l has been placed in the low tension or ground side oi the circuit, as shown. Dfue to the very low impedances of the usual video frequency circuits, the condenser must be con: siderebiy larger than those employed in the usual audio frequency ampliers, and may he as high as microiarads, or more. rihe polarity oi the detector output is such that mcreased light 'at the camera tube, as well as the more troublesome noise pulses, causes the voltage of the upper or cathode end oi the resistor 3 to become more positive. This fact is oi importance in the ac tion of the limiting diode G, as will be pointed 1i r se Tl?. mniunptle iligzrests of simplicity, the invention will @rst Toe described without reference to the automatic control :features of the circuit which are nrovided by the diode l and its associated circuits. it may, therefore, be assumed rorn the .moment that the points il and il are bondeordirectly together. With this siinplication, the ngure discloses, in general, a video frequency' source 3,. a video frequency amplier lil, a non-hnear impedance G and coupling condenser il serially connected therebetween, and a picture "tube l2 which utilizes the amplified V. F. output of the amplier it. Associated with the non-linear impedance (diode t) is a triode i3 whose function will he explained hereinafter. Diode lli, it will -be observed, is included in a loop circuit comprising the resistor 3, a variable polarizmg voltage source (the potentiometer M), and the resistor l5. By means oi the potentiometer it, an ad justable positive polarizing voltage or bias may he impressed on the anode of the diode o, and hence for input signals of such a magnitude that the cathode is at no time driven positive with respect to the anode, the diode will function generally as an ordinary linear series arm in a 1r network whose shunt arms are the resistors 3 and l 5, and except for an unimportant loss occasioned by the insertion of this network, virtually the entire V. F. output of thedetector l will be impressed on the input of the V. F. amplifier ill. In order to give the desired limiting action, the potentiometer lli should be adjusted so that, within range of useful picture signals, the cathode of the diode d will never be driven positive with respect to the diode plate, While for voltages in excess of the maximum picture signal, the cathode will be driven positive with respect to the plate. The effect of this action ideally is to open-circuit completely the series arm of the fr network for high amplitude noise signals, and hence to remove the signal source from the V. F. amplifier input. Under these conditions the voltage existing on the control grid of the picture tube would then include the xed bias, the background control, manual or automatic as the case may be, plus Whatever transient might be occasioned by the abrupt cutting oi of the amplier input.

The diode 6, it will be noted, is employed in this circuit as a series type of limiter rather than as a shunt type. n the latter connection where the diode is used to shunt noise pulses to ground (by becoming conductive for voltages above a xed level) it has been found that the minimumimpedance of such diode is too high to eiect any Sill appreciable noise pulse energy loss and this due largely to the very low impedance clrcui4 with which it must be associated in televisie circuits. Thus, the diode load may be of tk order of only a few thousand ohms, and this roughly comparable to the lower limit of a dl ode impedance during the time it is conductim In the series limiter connection of the drawim the diode is made to operate conductively for no1 mal picture signals, while becoming non-conclue tive for noise signals which exceed a predeter mined ievel. This arrangement has proven ver satisfactory in low impedance circuits for it dis criminates very sharply between signals lyin Von either side of any predetermined level.

it very high frequencies, however, the simpl series limiter thus fardescribed falls short oi th ideal in one important particular. Due to a for tuitous or inherent shunt capacity between th electrodes of the diode t, there will still be pro vided a comparatively low impedance path fo some or the higher video and noise pulse irequen cies, even when there is no conductive pat] through the diode. it is, therefore, desirable t provide a means for compensating for this fail ure oi' the diode limiter to discriminate agains the higher frequencies, and also, especially i1 television receivers, for impressing a reverse o black signal on the V. F. amplifier. This i one function of the signal inverting triode il which, constitutes one of the most important fea tures of this invention--namely, a means for com pensating for certain disadvantageous effects re sulting from the inherent interelectrode capacit: of the diode The triode lll receives its iixed plate and gric voltages from a voltage divider lG-ll-li con nected across the B supply which is grounded af the juncture of divider elements I4 and il. The relative magnitudes of the grid and plate voltage: are controlled by the setting ot the potentiometel i@ which is adjusted to a point where the tube operates under the conditions of plate current cuton for signals not exceeding the peak level o1 the picture signals, and for such signals the triode has no effect on the amplifier l0. However, when a noise pulse having an amplitude in excess of thev peak picture signal arrives, the triode becomes operative, and there appears in its plate circuit a pulse which is reversed in polarity with respect to the initiating pulse. This pulse appears across the V. F. amplifier input and because of its polarity, may drive the picture'tube grid negative and thereby completely blank the beam for the duration of the noise pulse, or it may simply oppose the noise pulse.

I'he combination of the diode 6 and the triode I3 is desirable for another reason. It will be apparent that the t-riode in supplying the inverted noise pulse (negative) to the diode plate will tend to increase the inverse voltage across the diode, and hence sharpen its normal cut-off characteristic. This obviate's the necessity of operating the diode polarizing potential uncomfortably close to the peak picture signal. The triode is referred to hereinafter as the noise signal inverter, since it does, in fact, invert or reverse the polarity of the noise impulses. It is noteworthy that when the diode 6 is conducting, the low impedance which it introduces between the input and output circuits of the triode results in self-degeneration therein, thus largely preventing the presence of undesired or inverted signals in the plate circuit of the triode which might obtain due to inadequate bias on the triode grid. On the other hand, this same diode, when non-conducting, lntroduces a very high impedance between the triode grid and plate thus preventing self-degeneration in the signal inverter.

The noise signal inverter hereinbefore d scribed may be operated to give a wide variety of results. For example, by a suitable choice of therefore, an important object of this invention values for the condenser I8 and resistor I 9, the

frequency characteristic of the inverter may be made similar to that of the diode 6 in its nonconducting state, i. e., when only the small interelectrode capacity of the diode is considered.

Under these conditions, it is possible to fix the/ gain of the inverter so that it just balances out the higher frequency components which would otherwise reach the V. F. ampliiier by way of the diode capacity. Care must be exercised, of course, to prevent differences in phase shift in the two paths from precluding complete cancellation.

On the other hand, the RC input to the inverter may be so chosen as to make the inverter, in general, equally responsive to all the component frequencies present across the vsource 3, while the gain may be increased to a value such that a high amplitude noise pulse (therefore, of positive polarity and representing white) across the resistor 3 will appear inverted (of negative polarity and representing black) at the V. F. amplifier, and will consequently register as a completely black section of a. scanning line on the screen of the picture tube, regardless of the fixed bias orbackground control voltage applied to the picture tube control grid. In practicing the invention, it has been found desirable to maintain the gain of the inverter I3 as low as possible consistent with the results desired, in order to avoid the generation of transients in peaked V. F. ampliers'. Where the inverter gain has been set unduly high, there has been,

found to result a transient White spot immediately following the blacked out scanning line segment. This transient is a result of shock excitation ofthe peaking circuits often associated with high fidelity, wide band, video amplifiers. Thus, in the amplifier IIJ there is shown an inductance 20 which is designed to resonate with the input capacity of the tube as'some high frequency, as is well understood in the design of very wide band amplifiers. If the band width required is s narrow that peaking circuits are unnecessary, no trouble is, in general, experienced from these transients.

It will be understood that the invention is by no means limited either to one or the other of the two loperating conditions hereinbefore described, for there are many operating characteristics lying within the limits of these two general modes of operation and which are obtainable by means of simple inverter circuit and gain adjustments.

In the circuit as described, if the strength of the received signal were to change appreciably, the polarizing voltage applied to the anode of diode 6 would no longer be the optimum value, and there would be necessitated an adjustment of the potentiometer III which, as has been hereinbefore described, determines the level at which limiting occurs. Such changes in signal Ilevel might be occasioned by tuning to a different signal, by a fading signal, by failure of an A. V. C. system to maintain a fixed input to the detector I, or, in television systems, by a change in general illumination at the camera tube. It is.'

to provide a means whereby this polarizing voltage is automatically maintained at a level which varies in accordance with the level appearing at the picture tube. To this end, the dioderectier 'I is coupled to the picture tube grid by means of the condenser 2l and some of the video signal is thus rectified, causing a direct current potential to appear across the potentiometer 22. A portion of this voltage is filtered by means of the R-C combination 23-24 and is applied between the points 8 and 9, as shown in the gure. polarity of the rectified output is such that the point 8 is more positive than the point 9. It will be observed that the diode 'I is conductive only for the negative swings of the picture tube grid voltage, which represent the blanking portions of the received sginal. The picture signals themselves represent the positive swing of the grid,

and, therefore, no energy is taken from these signals to drive the diode 7. The inclusion of this filtered and rectified voltage in theloop con-` taining the diode 6, the resistors 3, I and I5 adds to the constant polarizing potential provided by the source B, a voltage which is pro'- portional to the signal amplitude at the picture 'tube and which serves automatically to insure the correct polarizing potential on the plate of the diode S.

It has already been stated that the diode 6 is coupled to the detector I for A. C. only, D. C. blocking being provided by the condenser 4. If the center tap of the coil 25 were grounded directly, the D. C. component of the rectified signal would be impressed across the resistance 3, and it will be noted that the polarity of this component would be such that for increasing signal level, the cathode of diode 6 would becomeincreasingly positive, and this is equivalent to a decrease in the polarizing potential applied to the diode anode. This, of course, is diametrically opposed to the desired conditions, and it is for this reason that the diode 6 has been isolated from the D. C. component of the detector output.

It is possible, of course, to use the limiter of the invention in the reception of negative modulation signals. The system illustrated may be adapted for such signals merely by reversing the plate and cathode leads of the detector I. In this adaptation, the limiter will operate to limit those lesser disturbances which occur in the direction of more light, which have been described hereinbefore. In another adaptation, the detector I may be left connected as shown in the drawing whence the limiter will again act to limit the more serious disturbances which, for negative modulation, act in the direction of less light, or black. In this case, the triode I3 and this can be accomplished readily by connecting two of the limiters in cascade, one to limit pulses of one polarity, the other to limit pulses of the opposite polarity. By connecting a plurality of l limiters of one polarity in cascade, it is, of course, possible to very considerably increase the sharpness of the amplitude discrimination. I

The,

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f In one experimental model from'which satis f factory results were obtained, the circuit elements had the following values: 15-5,000 Ohms .merely for purposes of disclosure.

tion'in any electrical lsystem wherein the normal y.limiting capabilitiesof diodes are found to bef unsatisfactory. f

I claim: I

shunt with saidl amplitude-limiting means fory ltransmitting said undeslredsignal components to ysaid utilization meansr inreversed polarity.' while rejecting said'desired'signal lwhere said desired signal is unaccompanied by said undesired signal. f 5. In an electrical system, a source of signals, f

signal utilization means, la diode .space discharge y device serially connected between said source and i 1osaid utilization means for transferring signalsy therebetweem'meanslfor biasing said diode to establish a transmission level and thus discrimi inate betweensignals of different amplitudes,

there being a lfortuitous capacity between the f electrodes of said space discharge device tending n to ytransfer an undesiredy signal component, an-

; other l.space l discharge device rcomprising triode elementsk connected inishunt relation with said f diode, and means for biasing said triode in render ritl non-responsive .to signals to which said diode f 1. In an electrical system, aL source of signals,

f signalutilization means, a space discharge def vice lserially connected `between rsaid source and said utilization rmeans for transferring signals therebetween, means for biasing said space dis-f charge deviceto establishA a transmissionk level amplitudes, there being a fortuitous capacitybetween the electrodesof said space discharge def vice tending to transferan undesired signal com;- f

ponent, and means in shunt with said capacity y for counteractingthe effect of said capacity to f ythereby minimize the transferiof said undesiredr component, said last-mentioned means being operativel only forsignals whose amplitude ex-r ceeds said transmission level. i f e f 2. In an electrical system, a source of signals,

signal utilization means, signal amplitude-limit-'- ing means including a space discharge device serially connected between said source and said utilization 'Ameans for controllably transferring energy therebetween, there being a fortuitous capacity between the'electrodes of said space discharge device tending to transfer an undesired signal component from said source to said utilization means, and compensating means comprising a second space discharge device in shunt with said amplitude-limiting means for minimizing the transfer of said undesired signal component, said compensating means being operative for signals exceeding a predetermined amplitude level, but substantially inoperative for signals not exceed lng said amplitude level. Y

3. In an electrical system, a source of signals, signal utilization means, signal amplitude-limiting means connected between said source and said utilization means, for conductively transferring only those signals whose amplitude does not exceed a predetermined level, and amplituderesponsive means in shunt with said limiting means for transmitting to said utilization means with reversed polarity only those signals whose amplitude exceeds said level.

4. In an electrical system, a source of signals Y including a desired signal component having a maximum amplitude not greater than a predetermined level, and an undesired signal component of magnitude greater than said vpredetermined level, signal utilization means, signal amplitudelimiting means connected between said source and said utilization means for transmitting said desired component to said utilization means and for rejecting said undesired component, and amplitude-responsive signal-inverting lmeans in f and thus discriminate between signals of different; f

is responsive and to render the triode responsivey to signals transferred by said fortuitous capacity.

f' 6, lIn an electrical;system,fa source of` signals, f

signal utilization means, a polarizablenon-linear l impedance serially connected' between said source and said utilization means for controllably trans' f ferring energy therebetween, means for polarizying said impedance to establish -a transmission llevel and .thuszdiscrimin'ate between signals of different amplitude, said impedance being char' acterized by a tendency to transferfan undesired signal component,y compensating means in shunt with said impedance for minimizing the 'transfer' f yel. said undesired signal component, means for lrectifying ar portion of .thesignal supplied to said utilization means, to thereby obtain.` a unidireiv` tional voltage whose amplitude varies with the `transmitted'signal, and meansy for applying said voltage tosaid impedance yso as to vary the'y polarizationy thereof and thus vary said transmission level.

7. In a system for the reception of television signals, a source of video frequency signals having a low impedance, a 1r-type coupling network, a signal utilization means including a 'cathode ray tube, the aforesaid elements being connected in cascade, the series element of said coupling network comprising a non-linear impedance for rendering said coupling network conducting for signal amplitudes not greater than a threshold level but substantially non-conducting for signal amplitudes greater than said threshold level, signal amplitude responsive means`associated with the signal channel for deriving therefrom a polarizing voltage and for supplying said derived voltage to said non-linear impedance, whereby the said threshold level is caused to vary in accordance with the average amplitude of the said video frequency signal, and signal transfer means connected in shunt with said non-linear impedance for transferring signals from said source to said utilization means in reversed phase with respect to signals transferred by said coupling network, said signal transfer means being operative only while said coupling network is rendered non conducting.

8. In a system for the reception of television signals, a source of video frequency signals having a low impedance, a r-type coupling network. a signal utilization means including a cathode ray tube having a control grid, the aforesaid elements being connected in cascade, the series element of said coupling network comprising a than said threshold level, signal amplitude responsive -means coupled( to the control grid of said cathode ray tube for deriving therefrom a polarizing voltage and for supplying said derived voltage to said non-linear impedance through the shunt elements of said nnetwork, whereby the said threshold level is caused to vary in accordance with the average amplitude of the said video frequency signal, said signal amplitude responsive means comprising a diode rectifier and lter, and signal transfer means connected in shunt with said non-linear impedance for transferring signals from said source to said utilization means in reversed phase with respect to signals transferred by said coupling network, said signal transfer means being operative only while said coupling network is rendered non-conducting.

9. In a system as defined in claim 8, wherein the diode rectifier elements are so coupled to the control element of the cathode ray tube that energy is absorbed therefrom substantially only during the'blanking portions of the signal.

10. In an electrical system, a. source of signals, a 1r-type coupling network, and a signal utilization means, the aforesaid elements being connected in cascade, the series element of said coupling network comprising a non-linear impedance comprising a space discharge device, a polarizing source connected to the shunt elements of said network and forming with the network a loop circuit through which a polarizing potential is applied to said device, whereby the said. non-linear impedance transmits only those signals not exceeding a predetermined level, and signal-inverting means in shunt with said nonlinear impedance for transmitting signals exceeding said predetermined level to said utilization means in reversed polarity with respect to the signals transmitted by the said network, said signal inverting means being normally inoperative and rendered operative only when said signals exceed said predetermined level.

11. In an electrical system, a ource of signals, a vr-type coupling network, and a signal utilization means, the aforesaid elements being connected in cascade, thel series element of said coupling network comprising a non-linear impedance comprising a space discharge device, a polarizing source connected to the shunt elements of said network and forming with the network a loop circuit through which a polarizing potential is applied to said device, whereby the said non-linear impedance transmits only those signals not exceeding a predetermined level, and signal-inverting means in shunt with said nonlinear impedance for transmitting signals exceeding said predetermined level to said utilization means iny reversed polarity with respect to the signals transmitted by the said network, said signal-inverting means comprising an electron discharge tube having a plate circuit including one of the shunt elements of the said 1r-network, said signal inverting means being normally inoperative and rendered operative only when said signals exceed said predetermined level.

12. In an electrical system, a source of signals, a 1r-type coupling network, and a signal utilization means, the aforesaid elements being connected in cascade, the series element of said coupling network including a non-linear impedance comprising a space discharge device, a polarizing source connected to the shunt elements of said network and forming with the network a loop circuit through which a polarizing potential is applied to said device, whereby the said non-linear impedance transmits only those signals not exceeding a predetermined level, signal-inverting means in shunt with said non-linear impedance for transmitting signals exceedingsaid predetermined level to said utilization means in reversed polarity with respect tothe signals transmitted by the said network, said signal inverting means being normally inoperative but rendered operative when said signals exceed said predetermined amplitude, means for deriving from said system a polarizing voltage Whose amplitude varies with the amplitude of signals supplied to said utilization means, and means for applying the derived voltage to said space discharge device to thereby vary said level in accordance with the amplitude of transmitted signals.

13. In an electrical system, a source of signals, a 1r-typecoupling network, and a signal utilization means, the aforesaid elements being connected in cascade, the series element of said coupling network comprising a diode space discharge device, means for seriallyl applying a polarizing voltage to the said diode through the shunt elements of said l11F-network, tothereby establish a transmission level and thus render said diode non-conducting to signals which exceed a predetermined amplitude, there being a fortuitous capacity between the electrodes of said diode tending to transfer an undesired signal component during the non-conducting state of said diode, another space discharge device comprising triode elements associated with said diode, the' triode input being connected in shunt with the shunt input element of said 11F-network, and the triode output being connected in shunt with the shunt output element of said 1r-network, and means for biasing said triode to render its grid-plate transconductance substantially zero to signals to which said diode is conductive and to re-establish said triodes normal transconductance to signals transferred only by said fortuitous capacity.

14. In a television receiver a source of signals of video frequency, a coupling means, a signal utilization means comprising a picture reconstituting device having a viewing screen and a control electrode, the aforesaid circuit elements being connected in cascade, a diode space discharge device serially connected in said coupling means between said source and said utilization means, means for applying a polarizing voltage to said diode to thus render said coupling nonconductive to noise signals exceeding a predetermined amplitude, said noise signals tending to give rise to the generation of large high intensity bright spots on said viewing screen, a second coupling means connected between 4said signal sourceV and said signal utilization means, said second coupling means including a second space discharge device having at least a control grid, a cathode, and an anode, the signals supplied by said second coupling means to said utilization means being of opposite phase to those supplied by way of said first-mentioned coupling means, and means for so biasing said second space discharge device that said second coupling means is operative only when said noise signals exceed said predetermined amplitude, whereby only a small black spot appears on said viewing screen instead of said large bright spot.

15. In an electrical system, a source of signals, a signal utilization means, a first and normally operative signal transfer path connected between said sourceand said utilization means, a second and normally inoperative signal transfer path connected between said source and said utilization means, one of said signal transfer paths in cluding means for reversing the polarity of signals transferred by said one path with respect to those transferred by the other path, means for rendering said first path inoperative to transfer conductively signals exceeding a predetermined amplitude level, and means for substantially simultaneously rendering said second path operative when said first path is rendered inoperative.

16. In an electrical system, a source of signals, a signal utilization means, an electron discharge device having a cathode and an anode, there being a fortuitous capacity between said cathode and said anode, means for connecting said device in series in a first signal path between said source and said utilization means, biasing means for normally maintaining said device conductive, a phase-reversing means, means connecting said phase-reversing means in a second signal path between said source and said utilization means, and means operatively associated with said second path' for normally rendering said second path inoperative, said last-mentioned means being adjusted to render said second path operative in the presence of signals exceeding a predetermined amplitude level, said biasing means being adjusted to render said first path non-conductive at substantially the instant that said second path is rendered operative.

17. An electrical system according to claim 16, characterized in that the gain of said second signal path is so adjusted that the signal transferred by said second signal path, while operative, nullifies the signal transferred through said fortuitcus capacity while said first signal path is non-conductive.

18. In a wide-band signal transmission system,

escasas a source of signals, a signal utilization means, wide-band signal transfer means coupling said source and-said utilization means. said transfer means includingI an electron discharge device having a cathode and an anode, the internal anode-cathode path thereof being serially connected between said source and said utilization means for controllably transferring signals therebetween, said device having a fortuitous capacity between said cathode and said anode tending to transfer an undesired signal from said source to said utilization means, a resistancecoupled amplifier connected between said source and said utilization means and in shunt with said electron discharge device, said amplifier being connected and adjusted/to transfer signals from said source to said utilization means in such phase and magnitude as substantially to nullify the effect of signals transferred icy way of the said fortuitous capacity, and means for biasing said amplifier so as to render it normally inoperative but operative in response to signals exceeding a predetermined amplitude.

19. In an electrical system, a source of signals, a signal utilization means, means to transfer signals from said source to said utilization means including a bidirectional non-linear impedance comprising a diode space discharge device serially connected between said source and said utilization means, there being a fortuitous capacity between the electrodes of said space discharge device tending to transfer an undesirable signal from said source to said utilization means, and means for nullifying 'the effect of said undesired signal on said utilization means, said nullifying means including a unidirectional highly nonlinear transfer path connected between said source and the output of said diode.

ALEXANDER R. APPLEGARTH, JR.

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