US2395615A - Radio-frequency carrier-signal limiting system - Google Patents

Description

Feb. 26, 1946. l.. FLcUR-rls 12,395,615

RADIO-FREQUENCY CARRIER-SIGNAL LIMITING SYSTEM I Filed oct. 2a, 1941 2 sheets-sheet 1 L. F. CURTIS Feb. 26,1946.

vRADIOLFREQUENCY CARRIER-SIGNAL LIMITING SYSTEM Eiied oct'. `2e [1941 2 sheen-sheet 2 93m w r N o s Se: Siam, QwNm 9.@ o w- INVENTOR L E Lis F. GuRTls` ATTORNEY 2,395,615 RADIO-FREQUENCY CARRIER-SIGNAL tric-s G SYSTEM Leslie F. Curtis, Great Neck, N. Y.,

Hazeltigie Corporation, a corporation Ware assigner to of Dela- Application OctoberA 28, 1941, Serial No. 416,851

' 14 claims. (o1. 17a- 44) The present invention relates to an improved carrier-signal limiting system and, particularly, to an improved limiting system of the type which utilizes multi-electrode vacuum tubes and accomplishes limiting action by operation of the tube beyond'anode-current cutoff during a portion of each carrier-signal cycle.

Of the numerous types of limiting systems heretofore proposed, the self-biased vacuum tube type has perhaps the most promise by virtue of such outstanding advantages as its rapidity and uniformity of limiting action, 4its simplicity and ease of adjustment, itsself-biasing feature which eliminates the necessity of xed biases with their Y atten t disadvantages, and its freedom from the requirement of any form The prior art limiting systems of this nature have employed a single vacuum tube having a control electrode connected to aninput circuit including a grid condenser and grid leak to develop its own control-electrode bias by peak rectification of carrier signals translated through the system.

of neutralization.

Limiting occurs on all carrier signals of intensity greater than that required to bias the vacuumtube to cutoff.

Such single-tube limiting systems of the prior art have the disadvantage that limiting is delayed in time more than is desirable due in large measure-to the fact that the grid provides only half-wave rectication, Aa limitation which reduces the speed with which the bias potential can be developed. This delayed action is also due in partto the fact that the values of the grid leak and grid condenser required for adequate sensitivity are necessarily so large that the time constant of the system is appreciable. Transient disturbances cannot, therefore, be limited as completely as are the translated carrier signals 'and the output of the limiter system is materially reduced over an appreciable interval upon the application thereto of a large positive pulse of tran-` sient disturbance. Any attempt to decrease the time constant ofthe grid condenser and grid leak impairs the sensitivity of the system over itsl entire` rangeA of operation and increases the value of signal input at which limiting begins. There' is the additional disadvantage that single-v tube limiters of this nature-do not have balanced input and.. output circuits, whereby even-order harmonics which occur by virtue of the'limiting actlonappear inthe output circuitof the limiter and cannot be balanced. out in the coupling between the system and the succeeding amplifier or detector stage. e rv YIt is an object of the invention, therefore,- to provide a new and improved carrier-signal' limiting system which avoids one or moreof the of the priorart devices.

It vis-a further object of the invention to pro.

above-mentioned disadvantages and limitationsy cuits connecting the tubes in vide a carrier-signal limiting system having a self-bias circuit of relatively short time constant in which is developed the required self-bias'potential and one which, therefore, provides exl tremely rapid limiting action.

It is an additional object of the invention io provide a carrier-signal limiting system having improved sensitivity over its entire' operating range and particularly the portion thereof just before limiting begins.

Inaccordance with one embodiment of the invention, a radio-frequency carrier-signal limiting system comprises a pair of vacuum-tube repeaters having predetermined operating characteristics, andindividual input and output cirpush-pull relation, the input circuits being adapted to have radiofrequency carrier signals applied thereto and the output circuits being highly selective against all harmonics of the carrier4 signals applied to` the input circuits. The system also includes means for limiting to a predetermined value the maximum amplitude of carrier-signal output of the system comprising capacitance means composed primarily of the input capacitances of the repeaters and the distributed capacitances of the input circuits and an impedance commonto the input circuits for the magnitude of which variesin accordance with the amplitudeof the applied carrier signals, the impedance being so proportioned with relation to the operating characteristics of the vacuumtube repeaters that the carrier-signal translating cess of that necessary to produce a self-.bias po tential of sufficient magnitude to bias the tubes e y to cutoff.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and lits scope will be pointed out in the appended claims.

Referring now cuit diagram, Apartly schematic, of 'a complete frequency-modulated carrier-signal receiver embodying the invention; Fig. 2 is a graph illustrating anode-current and control-electrode potential relationships for each of the tubes and is used in explaining` the operation of the invention; Fig.. 3 represents steady-state input-output characteristics of the limiting system over its useful range of operation; Fig.'v 4 is a graph illustrating a method of graphical solution of certain equations derived in a mathematical lanalysis of the operation of the inventio'n; and Figs. 5 and the limiting system ofA the invention.

developing a self-bias potential to the drawings; Fig. 1 is a cire oi the Fig;

6 are graphs represent- Referring now more Particularly to Fig. 1,l there is represented 'schematically a complete frequency-modulated carrier-signal receiver .of a

conventional design embodying the present in-` vention in a preferred form. 'Ihe receiver includes a radio-frequency amplifier ill having its input circuit connected to an antenna system II, I2, and having its output circuit connected to an oscillator-modulator I3. 'Connected in cascade with the oscillator-modulator I3, inthe order named, arean intermediate-frequency ampliner Il of one or more stages, a carrier-signal limiting system I5, more fulLv described hereinafter, a

frequency detector and automatic amplication control systeml I6, an audio-frequency amplifier Il of one or more stages, and a sound reproducer `the various units thereof unnecessary. Considering briefly the 'l operation of the receiver as a whole, and neglect ing for the moment the operation of the limiting system I5, presently to be described, a desired frequency-modulated carrier signal is selected and amplified by the radio-frequency ampliiier I0, converted to a frequency-modulated intermediate-frequency carrier signal in the oscil-v lator-modulator I3, amplified in the intermediate-frequency ampliier I4,l limited to a predetermined substantially constant amplitudeA by the v limiting system ligand detected by the frequency detector |'5,thereby to derive the audiofrequency modulation components. The audio- Y `frequency components are, in turn, amplified in the audio-frequency ampller I'l land are `reproduced by the sound reproducer i8 vin a conventional manner. The automatic amplincation control bias developedby the automatic amplification control system ofunit i6 is effective to control the ampliilcation of one or more of the units I0, I 3 and 'I4 to maintain the signal input i. lto the limiter I5 within a relatively `narrow range for a wide range of received signal intensities.

y Referring now more particularly to the portion of the system embodying the present invention, the limiting system I5 comprises a pair of pentode-type vacuum-tube repeaters `I.9,'2|l having predetermined grid-cathode rectincation characteristics and predetermined input voltage-output current operating characteristics providing 'relatively sharp anode-current cutoi. The penjtode type ofvacuum tube is, as is well known, one which includes a screen electrode between `its control electrode and its anode electrode for reducing the capacitance between the last-mendenser 2l to the mld-frequency ot the range of frequency deviation of the carrier signal applied to the transformer 22. The frequency response of the tuned circuit 2|, 22 is broadened by resistors 24, 24 winch arel connected across individual halves oi' the transformer secondary winding 2i. There is provided means for limiting to a predetermined value the maximum amplitude ofv carrier-signal output of the system comprising capacitance means composed primarily of the ingrid rectlcation of the applied carrier signal.

The input electrode capacitances of tubes I9 and 2l are represented by dotted-line condensers 26 and 21, respectively, and the capacitance to ground of the center-tap of the secondary winding 2I by the condenser 28', although thesecapacitances may be comprised in part by the distributed capacitances of the individual input cirf cuits.

The output electrodes of vacuum-tube repeaters I9 and 2|! are connected in individual output circuits comprising individual halves of the primary winding 28 of an output transformer 29 coupled to the frequency detector I6 -ina` conventional manner. The primary winding 28 is tuned by a condenser 30 to the mid-frequency of the range ,of frequency deviation of the carrier signal applied to the limiting system. and the frequency-response characteristic of the tuned circuit thus formed is broadened by the resistors 3|, 32 winch are connected across individual halves of the transformer primary winding l2l. Vacuum tubes I8 and v2l) are suitably energized circuits. Assume now that a lrelatively weak carrier signal is applied to the input circuits of the tubes. The control grid oi each tube rectifies individual half-cycles of the `carrier signal to develop across the resistor 25 a unidirectional negative-bias potential. This self-bias potential,

-' lwhich charges the inherent input-circuit capactionedelectrodes. Each of tubes ,I9 and 20 thus includes/means for reducing the capacitive coupling between lthe input and output electrodes thereof, the screen electrode comprising means for efiectivelydecreasing' the translation of carrier signals directly from the input to the output circuits of the repeater tubes by interelectrode :capacitive coupling through the vacuumy tube repeaters. The input electrodes of vacuum tubes I9 and. 20 are connected in individual input circuits comprising individual halves of the secondary winding 2I- of an input transformer 22. The secondary winding 2l is tuned by a conitances represented by the condensers 26, 2B' and 21, increases in magnitude until the control grids are biased suiilciently negatively that peak rectification o'f the applied carrier signal occurs, which usually takes plac in a few carrier-frequency cycles. i A

Referring now to Fig. 2,-which represents the 'operating characteristic of either of thetubes lil or ,20, the axis` ,-Il is the zero grid-bias axis of the characteristic while the axis a-a represents -th'e axis of the carrier signal'applied to the control grids of the tubes, this axis being displaced from the axis Il-h by an amount equal to the bias Ve1 developed across the resistor 25 by peak rectiilcationof the applied carrier. signal reprecharacteristic is not exceeded, the carrier signal is amplified in the hunting system. I5 and appears v in the output ythereof with substantially undis-r from a source of space current 33. The individsion of the resistor 25 in torted wave form as represented by curve c. The time constant of the input circuit comprising the inherent capacitances 2B, 26', 2l and the resistor 25 is of the order of two or less microseconds,`.for example; or in terms of frequency is of the order of several cycles, for example, 3 to 5 cyclesfor an applied carrier signal having a frequency of 3 megacycles, so that for all practical purposes the bias developed across the resistor 25 may be considered a constant bias over any given carrierfrequency cycle, as represented by the linear axis steady-state translation characteristic of the limiting system of Fig. 1 with the resistor 25 propor tioned in the manner presently to be described. It will be observed from this gure that the output of lthe limiting system increases rapidly to the point m, at which limiting begins, and that v the output is thereafter substantially constant Assume now thatv the carrier wave applied to the limiting system increases in amplitude. The control grids of tubes. I9 and 20 again peakrectify the carrier wave to develop acrossresistor 25a larger self-bias potential. Thus a carrier signal having an amplitude represented by curve d produces a self-bias potential of magnitude e2 to bias both tubes beyond cutoff during a portion of each carrier-signal cycle, whereby only a portion of each positive half-cycle ofA the applied carriersignal is ampliied and appears in the output circuits of tubes I9 or 20, asrepresented by curve e. Since tubes I9 and 20 are connected in push-pull relation, the individual outputs of' the tubes have a phase difference of 180 degrees proper choice for `the value of resstor` is,v o f for increasing intensities of carrier signal applied -tol the limiting. system. The' steep slope of the portion o, m of this curve indicates that the limiting system has a high ampliiication for carrier signals of suiiiciently low intensity that limiting does not occur, and that limiting begins.\at a relatively low amplitude of the carrier-signal input, both of which are highly desirable characteristics of a limiting system. lf the value of the resistor 25 is chosen too small, the translation characteristic of the limiting system continues to rise for all values of applied carrier-signal amplitude, as represented by curve y. Irthe resistor 25 is too large, the output of thelimiting system increases until begins and thereafter rapidly decreases,`"as" represented by curve z. The

course, that which renders the output of the limiting system substantially constant over the major portion of its translation characteristic.

and there is applied to the output tuned circuit,

l 28, carrier signals having the wave form represented by the combined output signal curves e, e' for the two tubes `in push-pull. The tuned circuit 28, 30 selects'only the fundamental-'frequency component, represented by broken-line curve f, and applies this component through the output transformer 29 to the detector of unit I6. Should the'carrier signal applied to the input circuits of tubes .I9 and 2li increase still 'further in amplitude, as represented by curve g, a larger self-bias potential es is developed across resistor 25 and tubes I9 and 2Ii` are biased even further Actually, the steady-state translation characteristic of the system depends not only upon the value of the resistor25, but additionally upon the control grid-cathode conductance characterv istic of the tubes I9 -and 20 and on the control- 35 mathematical analysis of the operation ofthe l beyond cutoff. There appears in the output circuit ofeach tube correspondingly smaller portions of positive half-cycles of the carrier signal as represented by curves h, h'. The fundamental component Ic of the output wave h, h' is here again selected by the tuned circuit 28, 30 and applied to the detector of unit I6.

The value of resistor 25 is so proportioned, in a manner presently to be considered in greater detail, in relation to certain operating characteristics of the vacuum tubes I9 and 20 and the capacitances of condensers 2,6, 26' and 21, that the fundamental component of the resultant'carrier signalsdeveloped in the output circuits of tubes I9 and 20 is substantially constant in amplitude for all values of applied carrierl signals in excess of that necessary to produce a'self-bias potential of sufiicient magnitude to bias the tubes to cutoi during at least a portion of eachcarri'ersignal cycle. By proportioning resistor 25 in this manner, the steady-state carrier-signal translathe'developed grid bias and the point of clipping the common portion of grid bias voltage -necessary for anode-current cuto. This will 'beeclear from'the following limiting system.

The control-grid current of each of the tubes- I9 and 20 is expressed by the following relation:

. ig=Aeg3/2' (1) where:

V`1A=a constant derived from the actual grid voltage-grid current characteristic of the particular tube used and is equal to the grid current for a grid potential of one volt; and

eg=the instantaneous positive control-grid cath-- ode voltage.

termined by the following relation:

' pagaran-en where Um=the transconductance of eitherfof the I' 'I9 and 20;.and Ec=the` control-grid bias rent cutoi.

since tue-controlada unscathed lements or tubes I9 and 20 act asa diode re constant circuits, n Y 26', .21 and the resistor; 25, develop a steady bias I r, the time between the-grid and cathode elements as previ- I ously pointed out, whereby the voltage eg which.

produces grid current has a wave shape corresponding to the clipped peak of a cosine wave.

being determined by the. vfollowing relation:

E=the peak value ofthe applied signal voltage; 6=wt and a =the angle, having plus andminus values, dur--V ing which4 grid current flows. A

The anode current lof each of these tubes is denecessary for plate cur-- prislng the condensers 26,

a=sral=ieee a-eee i5/1 ai The man grid current dui-ing eeen e'yeie ofthe applied signal voltage. isdetermined by integration between the angles -a and +o of the gridrelation:-

where: A

R=twice the value of the resistor 25 since the latter resistor is effectively two resistors in parallel. one of which is included in the input circuitof vacuum tube I8 and'the'other in the ,input circuit of vacuum 'tube 2l insofar as the present mathematical analysis is concerned.

combining Equations '5 Asind o gives the relation:

Y 1=' IAE 3i/2(1-eoe ai Theplate current flows over a'larger angle b than tire arid current as determined by the ex ri: is convenient to utilize a parameter P, where: `ieri/iii, (9)

"ijhenEquationSmaybewritten: -h

The instantaneous plate current the angles-b and -i-b has the value:

' =aiPse eoe o-eeeti (1 1) Since cne'umitersysiemhasenmea oimiuceircuit, only the fundamental component of this plate currentis useful in developing a steady-state voltage acro the output circuit. The peak value' of thiscomponent, which is determined by integration of Equation il between the angles '-b' and +I. hasavalue by the relation: 'I,.=g.;E.:(beee bein 6)' The steady' Leiste tram ung i:iiartiste-istiev of eachhalfoftbelimitingsystem,comprisingone u co of under the assumption that the amplitude of the of the tubesil or' Il. may be derivedby simultaneous solution of vEquations 'i'. 10, and l2. A

general mathematical! solution of these equations' .is not feasible but a graphical solution may readily'. be had. Thus', referringto Fig.' i, ciirvea A, fB andcare obtained byplotting coso, calculated from Equation 7, against P (plotted conveniently as relative Vinputin decibels) for assumed values of'RAEelf. CurvesD, E and F are obtained by plotting cos. b, calculated from Equation ifor thesame-limitsofeos a, against?. CurveGis denvedbyniottinstnenmeuon "-'missin asoaois From'Equatlonsland3.thevalueoftheinstantaneousgrldcurientmaylieexpreiisedbythe lcosa (z).

in rie.; by meramente: am veroeei construetion lines, the arrow heads indicating the order of procedure from one set' of curves to another.

In this graphical' solution, it is convenient to plot bothP and f l w-eoe bein bi to allogarithmic or, decibel scale. Root-meanl0 square values of the fundamental component of theplatecurrentare. l y Ecgg.

multiplied by the ordinates of relative output.

The root-mean-square'values of the input voltage Imultiplied by the ordinates of relative output.

It has been found that values of RAEeU? between :i5- and 40 give a substantially constant or uniform steady-state input-output translation characteristic of the limiting system for values of P from 0.6 to 10 whichis equivalent to a decibel rangeof-4iioi- 20 db.v i The actual noise disturbance appearing in the audio, output of the frequency-modulated carrierm wave receiver depends upon the nature of the interfereneethe band width andymmetry of the amplifier stages including the audio stages and any compensation forpre-eniphasis in' the latter, as well as lupon the operatingeharacteristic of the limiting system. If no amplitude transients are translatedby the limiting system, the limitirm Y' action can be said to be perfect. No audio output is produced by ampli de variations in a perfectly balanced frequency etector in the absence of 40 frequency deviation, but during the deviations accompanying desired frequency modulation sudden changes in amplitude produce noise output which depends on the product of the instantaneous detector. The function of the limiting system is to maintain the carrier-signal output amplitude asA constant as possible, and the amount of amplitulle variation of the carrier -signalas applied to the frequenydetector is a measure of the carriersignalappliedtothelimiting systemis -or decreased by a' -transient noise pulse. for example, a pulse yof rectangular wave ,.Inthisanalysisitwillbe assumed thatthe changes of control-gridbias of the tubesl l! and 20 due tothe transient nolse'puise are suiiiciently slow relative to the period ofthe carrier signal applied to the limitingsystem that the grid cur- 'rent during the Vtransient; may be expressed by Equation'. Thisequation is not exact when cos o is negative, but this is'a condition not normally encountered in practice so the error is not there- -viation and the relative change in amplitude of the carrier signal applied to;the frequency de- *l enceintes from minnen :messina eos o. 'me 'zo 'fore sisnincens. .As will be seen later, the signin- I' v'method of graphically solving Equations '1, Vlil-and 'l2 by theuse of curves A to G. inclusive, to defXtermine any givenpoint on the atealLv-'state t'aslation-characterisuc, 'represented by curve 3, foranyvalue'ofltAEel/Js indicated 15 trol-grid current as a fimctlon of the angle al cant intervals are those `when the control-grid Equation 5 expresses the magnitude of the conaseaeis during which grid current `hows. The magnitude of the grid current may 'also be expressed by the following relation:

Where I ef-the instantaneous magnitude of grid bias dur- `ing the' period of the transient amplitude change of the carrier signal applied to the limiting system; R=the value of the resistor 25; and C=the total capacitance in sh t to"the resistor l 2'5, this capacitance equaling the sum oi the f capacitance of condensers 25,526' and 21.

Combining Equations 5 and`13 givesthe ex- K1=a constant depending onthe initial \condi tions when time t=0. Y

Equation may be simplied into the forma:

vlimiting system may be vas follows:

1. Grid bias greater' than Erl-Ec During this phase, grid current and platecurrent of vacuum tubes i9 and'20 are both cut ofi.

There is no audio output whatsoever except for'- the decay transients in the circuits following the limiting' system, which transients are not further considered.

2. Grid bias greater than Ea but less than Eri-Ea During this phase there is no grid curent,-but

the plate current increases with time as the grid bias decays. The decay of grid bias during phases 1 and 2 is determined only by the time constant RC of the control-grid circuit t FE# Re 3. Grid bias less than Ea During this phase, the grid bias decays according to Equation 20 and the plate current continues to increase until the steady-state condition is reached.

When the amplitude of the carrier signal applied to the limiting'system l5 is suddenly in- During the Y growth of the control-grid bias from lzero at time t=0, the specific value oi Kx is;

During the decay onthe control-grid bias from.

creased, as by a positive noise transient, the control-grid bias increases according to Equation 18 until the steady-state condition is reached. The plate current of the tubes i9 and 2t initially assumes a large value and decays to the 'steadystate value.

The plate current during any of these'phases may be determined by calculating the angle of plate-current'ow corresponding to cos a' (or cos a") vfrom Equation 10, remembering that vcos a' and cos b' now vary withtime. Thus y or the calculated lvalue of eos a is greater than transient unity. When the calculated. value of cos b' is greater lthan unity, plate current is cut off.

rent is e@ -from an adaptation of Equation 12 and where P is the value of En/Ea following the application of the transient noise pulse to the applied carrier `signal lIlle broken-line curve 'J of Fig. 5 'represents' the change o f control-grid bias when the 'carrier signal applied to the limiting system I5 has vthe plitude-modulation characteristic represented by thesolid-line curve K. there beim! a negative nents.- one plotted from data calculated from .Equation 18 for the condition of a positive increase oi carrier-signal amplitude, as during the intervals te to ta using one set of values for the divided into three parts The fundamental component of the plate curl pulse appearing during the interval t1 -to ti and va' positive noise impulse. during the interval from t: to t4. CurveJ has three compoconstants oi Equation 18 and is to t4 using another lset of values for the constants oi' this equation. one component from Equation 22 during the period oi' decreased amplitude of the applied carrier signal, as during the period ti to t: and to ts using the proper values for the constants of Equation 22 for these periods, and a third portion plotted from data calculated from Equation as during the period of time from ts to iniinity. The circuit constants of the limiting system I B, in deriving data for curve J, were assumed to have the following values:

. R=50,000 ohms.

Capacitance in shunt to the resistor -=total capacitance. of condensers 26, 26' and 21=40 micro-microiarads.

Time constant=2 microseconds. s

The broken-line curve L of Fig. 6 represents the variation of plate current oi.' either of vacuum tubes' I9 or `2li when the amplitude of the car- Y very small time constant of itsinput circuit. This.

rier signal applied to the limiting system i5 l changes in the manner represented by curve K oi Fig. 5. Curve L is plotted from data calculated from Equation 24 when the circuit' constants oi" the limiting system I5 have the values just stated. In order to compare the transient limiting characteristic of the limiting system of the present invention with a prior art type of limiting system similar to that of the limiting system il except that only a single vacuum tube is used therein, the broken-line curve M of Fig.'5 and the broken-line curve N of-Fig. 6 represent the change ot control-grid bias andthe change of plate current' of this prior art type of limiting system when the amplitude of the carrier signal applied to the limiting system varies in accordance with curve K oi Fig. 5, the circui-J constants oi the limiting system being assumed to have'the following values: v

Self-bias grid resistor=l00,000 ohms. 4 1

Capacitance in shunt to. self-bias grid resistor= 250micro-microfarads.

Time constant=25 microseconds.

The curves L and N of Fig. 6 have perhaps. the greatest .y comparative sign'iiicance since the change of platey current of the 'limiter tube is a.- direct measure of the output of the limiting systenr and thus is a measure of the transient-noise disturbance applied tothe -irequeiicydetecto whichiollows the limiting system.` From curve L, it will be evident -that the output of the limiting system I! of the present invention is reduced to zero during the time tito tz when the applied carrier signal has zero amplitude, has a sharp peak of relatively low amplitude and extremely.

short duration .immediately subsequent to the time is, has a much largerpealr amplitude of :isomers at the time aand does not resume its steadystate output until approximately 65 microseconds after the' amplitude of the carrier signal is again normal at the time t4. The transient amplitudelimiting characteristic, represented by curveN, of such prior art limiting system produces distinctly audible components during the occurrence oi, andthe recovery from, transient noise disturbances which may -be more serious than audible components produced by the original transient noise pulses without a limiting system. The audible components of the limiting system I5 ot the present invention, on the other hand, are very small. v'I'he improved transient amplitude-limiting characteristic oi the limiting system of the present invention is effected in large part vby the ltime constant is determined by the resistor 2l and theminherent capacltances 20,' 26 and 21. Consequently, the transient amplitude-limiting characteristic of the llimiting system I5 is greatly improved by any reduction of the inherent ca-` pacitances of its input circuit, there` being no need for physical capacitance in shunt to the resistor 25 as is necessary in the prior art limiting system discussed, the characteristic of which is represented by curves M and N of Figs. 5 and 6 respectively. l

The limiting action of the limiting system il is' substantially independent ol the anode potential-'of tubes I8 and Il as long as the anode potential is above that required for distortionless vtranslation of relatively weak carrier signals. By

virtue of the use of a pentode type of tube which operates essentially as a constant current or curv` rent regulated device the limiting action is also substantially independent of the anodeload, which aiiects only the gain of the limiting system for carrier signals of intensity below that atwhich limiting occurs.

As illustrative oi.' a speciiic' embodiment of the invention, the following circuit constants are given for an embodiment oi the invention shown' short duration immediately after` the carriersignal amplitude has increased at the time ts. and has zero output for a short time interval of V approximately 3 microseconds immediately after thecarrier-signal amplitude is reduced to normal value at the time t4. In comparison, the prior art limiting system has a small peak amplitude over an appreclably larger interval immediately f .after the time t2, has another which persists over substantially the entire interval ta to t4 after the amplitudeof the carrier signal is increased/jas byl a positive transient noise pulse, .is completely cut ci! for an interval of 35 microseconds after the carrier-signal amplitude is reduced to normal 3 megacycles It will be evident from the above description of .the invention that a carrier-signal limiting sys- 'tem embodying the invention has the advantages,

ilrst, that the system has improved gain over its entire translation characteristic and especially over that portion of its characteristic prior to the occurrence of limiting, and, second, that the magnitude of the selfbias potential of the system increases rapidly-with increasing values oi carrier-signal intensity, whereby limiting begins atV low values of carrier-signal input amplitude.

The limiting system has balanced input and output circuits so that even-order harmonics of the translated carrier signal are balanced out in the coupling to the succeeding amplifier or detector stage. 'I'he system is extremely eilective in limiting transient noise disturbances, since the time constant ofthe circuit across which the selfbias potential is developed is relatively'short, be-

ing only of the order of a few microseconds or,

in other words, a few cycles of the carrier signal..

Transient noise disturbances of either polarity are eectively limited by the system. The limiting system of the invention has the additional advantages that it is of simple. circuit arrangement, is easily adjusted for proper limiting action, and its operation is stableand substantially independent of changes of anode energization andv anode loading. y

While there has been described what is at presof this invention, it will be obvious to those skilled in the-art that various changesand modiiications may be made therein withoutdeparting from the invention,. and it is, therefore, aimed 3. A radio-frequency carrier-signal .limiting system comprising, a pair of vacuum-tube repeaters having predetermined operating and relativiy sharp anode-current cutoff characteristics. individual input and output circuits connesting said tubes in push-pull relation, said 'inis put circuits beingadapted Ato have radioffrequency carrier signals applied thereto and said output circuits being', highly selective. against all harmonics of the carrier signals applied to said input circuits, and means for limitingto a preent considered to `be the preferred embodiment l5 in the appended claims to cover all such changes and modications as fall within the vtrue spirit and scope of the invention.

What is claimed is: l. A radio-frequency carrier-signal limiting system comprising, a pair of vacuum-tuberepeaters having predetermined operating1 characteristics, individual input and output circuits connecting said tubes in push-pull relation, said input circuits being adapted to have radio-frequency determined value the maximum amplitude of v carrier-signal output of said system comprising capacitance means composed primarily 'of the input capacitances of said repeaters and the distributed capacitances of said input circuits and an impedance common to said input circuits for developing a self-bias potential the magnitude oi which varies in accordance withthe amplitude -of the applied carrier signals, said impedance being so' proportioned with relation to said operating and anode-current -cutoii characteristics that the carrier-signal translation characteris tic of `said-system is substantially constant ior values of applied carrier signals in excess oi that .necessary to produce a self-bias potential oi' sumcient magnitude to bias said tubesto plate-cur,

carrier signals applied' thereto and said output` 30 termined value the maximum amplitude'of ca rrier-signal output of said system comprising cacapacitances of said repeaters and the distributed capacitances of. said input circuits andan impedance common to said input circuits tor-developing a self-bias Apotential the magnitude of which -varies in accordance with the amplitude of the applied carrier signals, said impedance being so proportioned with relation to said operatrent cutoff. l 4. A radio-frequency carrier-signal limiting system comprising. a' pair of vacuum-tube' repeaters having'predetermined operating and grid-l cathode recticatio'n characteristics, individual l input and output circuits connecting said tubes vin pushpull relation, said input circuits being adapted to have radioefrequency carrier signals applied thereto Vand said output circuits; being highly selective against all harmonics of the car-- rier signals applied to said input circuits,and

means f orlimiting to a predeterl'nined.value'the maximumamplitude of carrier-signal output'of said system comprising capacitance means composed primarily of the input capacitances oi said repeaters and the distributed capacitances oi' vsaid ing characteristics that the carrier-signal translation ^characteristic of said system is substantially constantl for values of applied carrier signals in excess of that necessary to produce a selfbias potential of sufcient magnitude 'to biassaid 2. A radiirequency carrier-signal limiting system comprising, a pair of vacuumatube repeaters having predetermined operating characteristics, individual input and output circuits conput circuits being adapted to have radio-fre;

output circuits being highly selective against all input circuits and aw impedance common to said input circuits for deriving by grid rectiiication of said applied carrier signals a self-bias potenv tial the magnitude of which varies in accordance v with the amplitude of theapplied carrier signals, o said impedance being so proportioned with relation to said operating and grid-cathode rectiiication characteristics that the carrier-signal translation charsacristiel or saidsysiem is substanvvtially constant Ivor values ofapplied carrier sig-l necting said tubes in push-pull relation, Vsaid in, miv

input circuits, and means for limiting to a pre- 'in determined value the maximum amplitude o! carrier-signaloutpuof said system comprising capacitance means composed primarily of the input capacitances oi said repeaters and the distributed/ self-bias potential the magnitude of which varies in accordance with the amplitude of applied carrier signalsgsaid resistor 'being so proportioned with relation to said operating -characteristlcs of said system is substantially constant for values of applied carrier signals in excess o! that neces-1v sary to produce a self-bias potential of sufiicient `magnitude to bias said tubes-to cutoff.

' capacitances of said-input circuits and a resistor, F5-

nals in excessof that necessary'to produce a selfbias potential of sutlcient` magnitude to bias said quency carrier signals applied thereto and said tubes to wma' l 5. A 'radiofrequency carriersignal limiting I. ste com risin', aair of vacuum-tube re- ,harmonics of the carrier signals applied to said`v sy m p g p 'being `adapted to-have radio-frequency carrier signals applied thereto andsaid output circuits being highly selective against all harmonics of the'carrier. signals applied to said input circuits.

and means for limiting to a predetermined value the maximum amplitude of 'carrier-signal output "of saidsystem comprising capacitance means y composed primarily lof the input capacitances of that the carriersignal( translation characteristic said repeaters andthe distributed capacitances of said input circuits and an impedance common .15 bias potential the magnitude ,of which varies in carrier signals, said impedance being so propor- 'tioned with relation to saidcharacteristic that the carrier-signal translation characteristic of said system vis substantially constant for values oi applied carrier signals in excess of that necessary to produce a self-bias potential .of sumcient f magnitude to bias said tubes to cutoff,

teristics and predetermined input circuit capacitanccs, individual input and` output circuits connecting said tubes in push-pull relation, said input circuits being adapted tohave radio-freoutput `circuits being highly selective against all harmonics of the carrier signals applied to said input circuits, and means for limiting to a pre-` determined value the maximum amplitude of can-ier-signal output of said system comprising capacitance means composed primarily of said input`capacitanoes oi' said repeaters and the disltributed capacitances of s aid input circuits and an impedance common-to said input circuits for developing a self-bias potential Atheuiagnitude of which varias in accordance with the amplitude oi the applied carrier signals, said impedance being proportioned with relation to said capaci- 8 4 A l asesora accordance with thc amplitude ci the implicav c cnc vcit, inuividutiinnutnndcutput circuits ccnnecting said tubes in push-pull relation, said input circuits being adapted tov have radio-frequency carrier signals applied thereto and said s output circuits being nigniy 'selective against nu harmonics ofthe carrier signals applied to' said input circuits, and means for limiting to a predetermined value the maximum amplitude of' a-.i'esistor R common td'said input circuits for developing a 'self-bias potential the magnitude oi which varies in accordance with the amplitude of applied carrier Signals, Said resistor being so proportioned with relationV to said cutoii' bias Ec tand said constant'A that -the product RAEcll is quency carrier signals applied thereto and said substantially between 35 and 40, whereby-the carrier-signa1 translation characteristic of said system is substantially constant for values of f applied carrier signals from 0.6 to Iltimes saidv bias Ec.

' currentv to cutoiI-and the control-electrode current is A e331 where es is the instantaneous positive voltage applied to the control electrode and A is a constant equal to the value o1' control-electrode current for-a control-electrode potential of tances of said limiting means to cause said lim' 3o one volt, individual input and output circuits iting means to have a time constant of the order of only ai'ew microseconds, whereby the carrier` connecting said `tubes in push-pull relation, said input circuits being adapted to have radici-irequency carrier signals applied thereto and having predetermined input capacitances and said outat put circuits being highly selective against an nar- 1 monies o! the carrier Signals applied to said input circuits, and means for limiting. to a predetermined value the maximum amplitude of carriersignal output of said system comprising a resistor -peaters having predetermined operating charac- 40 R. commento said input circuits for developing a teristics and predetermined input circuit capacitances, individual input and output circuits connecting. said tubes in push-pull relation, said linput circuits beingadapted to have radio-frequency carrier signals applied thereto and said 4 5 stant A that the. product RAEcl/2 is substantially self-bias potential the magnitude of which varies .with reiaticnto said cutcfr biss Ec and scudccnoutput circuits being highly selective against all harmonics of the carrier signals applied tosaid 5 1 input circuits, and means for limiting tti a. p determined value the maximum amplitude of capacitance 'means composed primarily ofvsaid input capacitances of said repeaters and the distributed capacitances .of said input circuits and an impedance common to said input circuits for developing a self-bias potential the magnitude of which varies in accordance with the amplitude of the applied carrier signals, said impedance being proportioned with relation to said capacitances of said limiting means to cause said limmicroconds, whereby thevcarrierasignal translation characteristic of saidl system isl substantially 'c'onstant for values of applied carrier signals in excess of that necessary to produces self- `is a constant equal to the value of control-electrode current i'or a control-electrode potential ot between and 40, whereby the steady-state car. riet-signal translation characteristic of said systemis substantiallyI constant for values of applied e. carrier signals from 0.6 to 10 times said bias Ec, carrier-signal output of said system comprising said input circuit vcapacitances being sufficiently small with relation to the value of said resistor that-said capacitances and said resistor have a `time constant of the ordero! only a few microseconds, thereby to minimize the eiect in said- 55 output circuits of undesired transient amplitude vvariations`of the applied carrier' signals.

10. A radio-frequency carrier-signal limiting v system comprising, a pair .of vacuumg-tube l peaters having predetermined operating characiting means to have a time constant less than two 6 terlstics, individual input and output circuits connecting said tubes in push-pull relation, said input and output circuits being' tuned to the mean frequency ct radio-frequency carrier signals applied to said input circuits and said output cir-i- '4 bias potential of suiiicient magnitude to bias said 'cuits' being. highly selective against all harmonics of the carrier signals applied to said input circuits, and means for limiting t0 a predetermined value the maximum amplitude of carrier-signal output of said system' comprising capacitance 7 means composed primarily `of the input capacitances of said repeaters andthe distributed ca-v pacitancespf said vinput circuits and an impedance common to said input circuits for developing a self-bias potential the magnitude of which varies in accordance with thc amplitude ci the applied carrier signals, said impedance being so proportioned with relation to said operating characteristics that the carrier-signal translation characteristic of said system is substantially consignal translationcnaracterisucor said'sysm is substantially constant for values of applied carpeaters having predetermined operating characteristics, individual input and output circuits con necting said tubes in push-pull relation, said input circuits being adapted to have radio-frequency carrier signals applied thereto and said output circuits being highly selective against all harmonics of the carrier signals applied to said Y input circuits,lmeans for effectively decreasing the translation of carrier signals directly from Asaid input to said output circuits by inter-electrode capacitive coupling through said vacuumtube repeaters, and means for limiting to 'a predetermined value the maximum amplitude ofcarrier-signal output of said system comprising capacitive means composed primarily of the input capacitances of said repeaters and the distributed capacitances of said input circuits and an impedance common to said input circuits for developing a self-bias potential the magnitude of which varies in accordance with the amplitude of the applied carrier signals, said vimpedance being so proportioned with relation to said operating characteristics that .the carrier-signaltranslation characteristic Aoi said system is substantially constant for values of applied carrier signals in excess of that necessary to produce a self-bias potential of suiiicient magnitude to bias said tubes to cutoff.

12. A radio-frequency carrier-signal limiting system comprising, a pair of vacuum-tube repeaters having predetermined operating characteristics and including means for reducing the capacitive coupling between the input and output electrodes thereof, individual input and output circuits connecting said tubes in push-pull relation, said input circuits being adapted to have radio-frequency carrier signals applied thereto` and'said output circuits being highly selective against all harmonics of the carrier signals applied to said input circuits, and means for limiting to a predetermined value the maximum am- `plitude of carrier-signal output of said system comprising capacitance means composed primarily of the input capacitances of said repeaters and the distributed capacitances of said input circuits and an impedance common to said input circuits for developing a self-bias potential the magnitude of which varies in accordance with the amplitude of the applied carrier signals, said impedance being so proportioned -with relation to said operating characteristics that the carrierrier signals in excess of that necessary to produce a self-bias potential of sufficient magnitude to bias said tubes to cuto?.

13. A radio-frequency carrier-signal limiting system comprising, a pair of vacuum-tube lrepeaters having predetermined operating characteristics and each including a screen electrode between the control electrode and anode electrode thereof for reducingthe capacitance between said last-mentioned electrodes, individual input and output circuits connecting said tubes in push-pull relation, said input circuits being .adapted to have radio-frequency carrier signals applied thereto and said output circuits being highly selective against all harmonics of the carrier signals applied to said input circuits, and means-for limiting to a predetermined value the lmaximum amplitude of carrier-signal output of said system comprising capacitance means composed primarily of the input capacitances of said repeaters and the distributed capacitances of said input circuits and an impedance common to said input circuits for developing a self-bias potential the magnitude oi' which varies in accordance with the amplitude of the applied carrier Signals, said impedance being so proportioned with relation to said operating characteristics that the carriersignal translation characteristic of saidsystem is substantially constant for values of applied carrier signals in excess of that necessary to produce a self-bias potential of sufficient magnitude to bias said tubes to cuto.

14. A radio-frequency carrier-signal limiting system comprising, a pair of pentode vacuumtube repeaters having predetermined operating characteristics, individual input and output cir,- cuits connecting said tubes in push-pull relation, said input circuits being adapted to have radiofrequency carrier signals applied thereto and said output circuits being highly selective against all harmonics of the carrier signals applied to said input circuits, and means for limiting to a predetermined value the maximum amplitude' of carrier-signal output of said system comprising capacitance means composed primarily of the input capacitances of said repeaters and the distributed capacitances of said input circuits and an impedance common to said input circuits for developing a self-bias potential the magnitude of which varies in accordance with the amplitude of the applied carrier signals, said impedance being so proportioned with relation to said operating characteristics that the carrier-signal translation characteristic of said system is substantially constant for values of applied carrier signals in excess of that necessary to produce a self-bias potential oi sufficient magnitude toebias said tubes to cutoff.

LESLIE F. CURTIS.

Images