US2420248A - Amplitude limiter circuit - Google Patents

Description

May 6, 1947. W, R, KOCH 2,420,248

AMPLITUDE LIMITEE CIRCUIT I Filed July 19, 1944 4 Sheets-Sheet 2 l p A?, fia, gaar'- ,e 353050 -ff- INVENTOR. W//vF/z-'La P. Koch' I BY May 6, 1947. w. R. KOCH 2,420.248

AMLITUDE LIMITER CIRCUIT Filed July 19, 1944 4 sheets-shea a ,FEE 4 V04 rags /X May s, 1947. W,R KOH' 2,420,248

I AMPLITUDE LIMITEE CIRCUIT Filed July 19, 1944 4 Sheetst-Sheet 4 ull! AAAAAAA 0i' E E? l 4f ZO INVENTOR.

WWF/5L@ E. Moc/7 rmAQ/vy Patented May 6, 1947 2,420,248 AMPLITUDE LIMITEE `CIRCUIT Winiield R. Koch, Haddonfield,

Radio Corporation of America,

Delaware N. J., assignor to a corporation of Application July 19, 1944, Serial No. 545,664 6 Claims. (Cl. Z50-20) My present invention generally relates to amplitude limiting systems, and more amplitude limiter circuits adapted for use in receivers of angle modulated carrier waves.

In the reception of angle modulated carrier waves it is necessary that the demodulator of ,the receiver be presented with modulated carrier wave energy Whose amplitude variations are an accurate representation of the angle modulations of the Vreceived waves. This is insured by employing an amplitude limiting device -prior to the discriminator section of the demodulator to prevent undesired amplitude variations in the received'angle modulated Waves from causing la response in the output of the demodulator. The generic term angle modulated is to be understood as including frequency modulation (FM), or phase modulation (PM), or hybrid modulations having characteristics of both FM and PM. By way of specific illustration the present description will be explained in connection with a system for receiving FM carrier waves, although my invention is not restricted to such reception.

One of the most widely known andV used forms of discriminator-rectifier circuits is that disclosed and claimed by S. W.4 Seeley in his U. S. Patent No. 2,121,103, granted June 21, 1938. As commonly used in accordance with the Seeley patent, the discriminator section comprises a pair of coupled resonant circuits each tuned to a predetermined operating frequency (usually lthe intermediate frequency of a receiver of the Widely-used superheterodyne type). The amplitude limiter tube usually includes in the plate circuit thereof vthe primary of the two coupled circuits, while a pair of opposed rectifier devices are connected to opposed ends of the secondary circuit coil which is center-tapped. The rectified volttages of the rectier devices are combined in polarity opposition, and the resultant voltage is accurately representative of the frequency variations of the received FM waves.

The well known frequency deviation vs. voltage output characteristic of .the Seeley demodulator system is generally an inclined, approximately linear characteristic whose upper and lower ends are bent in opposite directions. The linearity of the characteristic depends upon the degree of coupling between the primary and secondary circuits of the discriminator section, and the effective selectivity of these circuits. Usually the coupling is approximately critical coupling, under which condition the primary circuit voltage response has a pronounced double hump.

Unlike the normal I. F. transformers preceding the limiter tube, the response curve of the priparticularly to 'l mary of the discriminator transformer is of large i importance. The pronounced, double-hump response of theV primary provides a correction for inherent central curvature in the normal frequency deviation vs. voltage output characteristic oi the tdemodulator.

The amplitude limiters employed in the past with the Seeley discriminator, or more generally any discriminator Whereinthe primary one of coupled tuned circuits possesses a doublehumped response curve, have generally been so constructed as inherently to alter under varying .conditions of operation the optimum response curve of the discriminator primary circuit. For example, in one form of prior amplitude limiter circuit the object has been to maintain a constant signal voltage at the plate of the limiter tube. The effect of such aV constant voltage source across the discriminator primary circuit is effectively to shunt the latter by a substantially zero impedance. Y As a consequence the response curve of the primary circuit is flattened out under conditions of operation which will hereinafter be further described, and the voltage response characteristic ci the secondary may approach that of a single tuned circuit. Because of this the linearity and length of the discriminator characteristic are reduced. Hence, the advantages of a pair of coupled tuned circuits are largely lost. The frequency deviation vs. voltage output characteristic will depend on the strength of the received FM waves, and for strong signal reception the characteristic `will have a shorter length, and be of increased slope, than for Weak signals.

It may, therefore, be stated that it .is an important object of my present invention to provide an improved amplitude limiter, and, more particularly, an amplitude limiter which affords an output of uniform amplitude and which does not deleteriously `affect the operation of the succeeding discriminator. More speciiically, one of my objectives is to preserve under varying conditions of operation the desirable double-humped response curve of the primary circuit of the discriminator transformer by feeding FM Waves thereto through an amplitude limiter tube circuit which possesses a substantially constant plate current characteristic constant plate voltage characteristic.

Another important object of my invention is to provide an amplitude limiter tube of the screen grid type, wherein the control grid and screen grid voltages are varied at a relatively fast rate in response to applied signals so as to maintain the plate current flow substantially constant over a range of signal voltage amplitudes above a predetermined value.

Another object of my invention is to provide an amplitude limiter whose tuloe has the plate thereof operated at a positive direct current voltage of relatively high value, While the tube is provided with a screen grid whose positive direct urrlll voltage is lower than the plate voltage in contradistinction to aV Another object of my invention is to improve the efficiency and construction of FM receivers by improving the amplitude limiter operation and Y preventing the limiter from adversely affecting the desired response characteristic of the discriminator.

A more specific object of my invention is toY provide an amplitude limiter tube whose signal input grid circuit and screen grid circuit each include resistorcondenser networks of relatively short time constants, and whose plate circuit contains only very small direct current impedance.

A further object of my invention is to provide an amplitude limiter tube of the type including a screen grid having means in circuit therewith to permit it to vary over a wide range oi positive voltages thereby to co-operate in maintaining a substantially constant plate current, and other means being operative to prevent the screen Voltagefrom becomingtoo low and causing the gain of the tube to fall for weak signals.

Still other objects of my invention are to improve amplitude limiter circuits for FM reception, and more particularly to provide a limiter-discriminator network of desirable characteristics.

Still other features of my invention will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows one embodiment of the invention;

Fig. 2 shows idealized response curves of the discriminator transformer;

Fig. 3 illustrates typical discriminator charac'- teristics, both with and without the invention;

Fig. 4. s hows limiter characteristics both cal- 'culated and experimental;

Figs. 5a, 5b,Y 5c show of the limiter tube for signals respectively;

Fig. 6 illustrates graphically the effect of varygraphically the Yoperation week, medium and strong 'ing the value of R in the screen grid circuit of the limiter tube;

Fig. '7 shows a modification of the system of Fig. 1, and

Fig. 8 illustrates a further modification of Fig. 1.

Referring now to the various figures ofthe accompanying drawings, wherein like reference letters and numerals in the different figures designate similar elements, there is shown in Fig. 1 so much of an FM receiver as is essential to a proper understandingA of my invention. It is assumed that the limiter and demodulator of Fig. 1 are embodied in a superheterodyne receiver, since that form of receiver is most widely employed at present. The customary selective radio frequency amplifier, converter and I. F. amplifier precede the input transformer I which feeds the I. F. signals to the signal grid 2 of limiter tube 3. Those skilled in the art of radio communication. and more specifically FM communication, are fully aware of the details of circuit design prior to limiter tube 3. The received FM waves may havea carrier or center frequency in any of the known frequency bands allocated to FM or vPM reception. The present FM Abroadcast range extends from 42 to 50 megacycles (mc.)

Assuming operation in the 42-50 mc. range, the

selector circuits of the receiver between the antenna and discriminator network will each be designed satisfactorily to pass a band of frequencies of the order of 200 kilocycles (kc.) This relatively wide band pass selector characteristic is required, Ibecause in compliance with present standards of FM broadcasting the maximum fre- `swing of a selected modulated carrier.

Y quency deviation at each FM transmitter is up to 75 kc. on either side of the normal carrier frequency. As pass band width of 200 kc. insures the acceptance of the overall kc. frequency The frequency variations of the signal energy are, of course, representative of the modulation applied to the carrier wave at the transmitter, the extent of the frequency variation or deviation being proportional to the amplitude of the modulating signals while the rate of deviation is dependent on the modulation frequencies per se. Since a PM wave essentially differs from an FM wave in that the extent of frequency deviation is proportionately higher for the higher modulating frequencies, it will be clear that the FM receiver may be employed for detection of PM waves with de-emphasis correction subsequent to the demodulator.

Assuming, now, that there has been applied to the primary circuit 4 of I. F. transformer l the FM energy whose center or carrier frequency has been reduced to the I. F. value of 4.3 mc., each of circuits 4 and 5 is tuned to the I. F. value. The circuits `4 and 5 are coupled to provide a substantially band pass response characteristic about 200 kc. wide. As previously stated, this is also true of the selector circuits prior to transformer l. Limiting is accomplished by means of tube 3, shown as a pentode tube by way of specific illustration. 'Ihe cathode 6, either indirectly or directly heated, is preferably connected directly to ground, while the low potential side of input circuit y5 is connected to ground by a resistor R1 shunted by condenser C1. The magnitudes of these components R1 and C1 are so chosen as to provide a relatively short; time constant. For

' example, and in no way restrictive, R1 may be 150,000 ohms while C1 mat7 be 22 micro-microfarads. The control grid 2 is connected to the high alternating potential side of circuit 5.

The plate or anode 1 of limiter tube 3 is connected to operate at a relatively fixed direct current potential. Thus, the plate 1 is connected through the inductance 8 of the primary circuit P of the discriminator network to a point on a direct current source (not shown). having a potential of, for example, +250 volts. It will be observed that between the +250 volts point and the plate 1 there is no material direct current resistance, the direct current resistance of coil 8 .being very small. This is the reverse of the construction of limiters well known in the prior art, wherein substantial resistance is deliberately introduced into the plate circuit to reduce the normal plate voltage to such a low positive value that the tube Will readily saturate. The screen grid 9 of tube 3is connected to the +250 volts point through a. resistor R whose upper end is connected to ground by condenser C. The values of R and C are so chosen that network R, C has a relatively short time constant. By way of specific example, and in no way restrictive, R may have a value chosen from a range of 33,000 ohms to 120,000 ohms. The condenser C may have a value of the order of 68 micro-microfarads. The normal no-signal voltage of the screen grid 9 will be relatively small compared to the. plate voltage. E'or example, the no-signal screen voltage may be as lo,w as -L-'IDf volts. However, during s'gn'al reception' the volt.- age of. the screen will vary atv a relatively rapid rate, by virtue of the short time constant of network RI., C'. Here, again, the presentY limiter construction is. the reverse of" that employed in the prior art, since in the.V past' it has usually been proposedto maintain the screen voltage" invariable throughout signall reception while permitting. the plate voltage to vary. The suppressor grid' I U. isconnectedl to the cathode 6 within the' tube envelope, and .performs its usual functionY oil suppressing, secondary electron emission from plate 1.

. The coil's and I I are of the respective primary and. secondary` windings of the di'scriminat'orA transformer. Coil 8 and condenser 3 provide the resonantprimary circuit P, while the secondary coil 'II and' condenser II in parallelv with it provide the resonant 'secondary circuit S. Each of circuits P and' S is tunedlto the operating I. F. value of' 4.3 mc.. These circuits are preferablyv coupled so as to provide a. substantially doublehumped response curve for the' primary circuit P, while providing arband pass curve for the second'ary circuit. The high alternating potential side oiprimary circuit Pis connected to the'mid'- point of coil II" through a direct current blocking. condenser I2 which functions as a direct connection so far asthe I. F. currents are concerned. In other Words condenser I2 impartsv no phase shift to the I. F. currents applied to themidpoint of coil I'I`.

The circuitsP and S constitute a discriminator network. oi the type described inthe aforesaid Seeley patent, and referred to herein asa Seeley discriminator. It is relatively widely used'in FM receivers, and its functions are well-known to those skilled in tion. It is sufiicient for rthe purposes of the present application to explain that at the instant` when the appli'edll.. F. energy hasa fre-V quency equal to the resonant frequencies of circuits P and S, then the signal. voltage across circuitn S will vbe 90 out of phase with the Voltage across primary circuit P. This is due to the magnetic coupling between resonant circuits P and VS'. However, the connection including condenser I`2 will also inject into the circuit S- primary signal; voltage which has not' been subjectedtoanyphase shift. l y

Due to the fact that the primany voltage is applied to the mid-point of coil II, it will' be seen that from each end( of coil I`I to ground there will exist primary signal voltage in phase quadrature .with the induced phase-shifted signal voltage, the induced voltagesin each half ofthe secondary coil II being of opposite polarity. Hence, there will exist between each end of coil It and ground a resultant voltage which isthe vector sum of the phase quadrature-related voltages across each. half of secondary winding II and the primary voltage. These resultant voltages will be of equal magnitude at the instant when the I. F. energy applied to circuit P i's equal. to the resonant frequencies of the discriminator circuits. However, should the instantaneous signal frequency deviate or shift with respect to the, predetermined reference ire quencies of' circuits P and S, then the resultant voltages at the opposite ends of coilr II will' become unequal because of phase changes away4 from the quadrature phase relation. The inequality wil'l be dependent upon the magnitude the art of. radio communicaoff frequencyy deviation f from the,I vrefe-rence free quencyt; while the 'directionfoff the inequality-i will depend upon the direction of'frequency' d"e4 viati'on. In this way' the frequency-variable waves areV translatedJ orjtransformed into -a-*pair of vvoltages whichy are equal in' magnitude at the instant when the signalfrequency is equal to thev discriminatorreference frequency; but vvl'iicli;`

vary with respectr to each otherfor-"frequencyk deviations fromv the: center-frequency.'n The "func-` tion` of the tube I31i's-toprovidea pair offrec tier devices for recti'lfying the; aforesaid-"pail-f of'variablevoltages; i f

Tubel I3, while shown-io`y'wa1y-of exampleas a- (5l-I6- type tube embodying Va pair'of separate dit. odes, may be'repla-ced' by a pair of independent diode tubes or other'suitable rectiersf.V The. anode HI'. and cathode I5}o f the-upper" diode device 'aref connected' between the upper sideof circuit S-and the upper end of loadresistor I6. The lower diode device II, I8 is connected between the -grounded end of' load resistor IS'and the lower side of 'cire' cuit Si Thefmidpoint of secondary coil I Is i'scon#l nected by I. F. choke coil 2Q to thevjunction of load: resistors I-Sand ISP.y Hence, theresultant I. Fl voltage' applied to-anodevI4iwil1ibe rectified', and'- the rectified voltage will appear'4 across resistor I6. In the same way theresultantv I. F. voltage applied to diode anode I'I will be rectied, and' the rectified voltagel will appear across resistor I9. Since the rectified voltages across resistors"|6f and I9 are combined in polarityA opposition rela-1 tive to ground,`the resultant potentialvv at'l the endA of resistor' I6 connected to cathode I5?V will be-Zeroat thel instant when the frequency'of the lpplied FM- Waves is eqrialv to the yreference frequency of the discriminator' circuits P and S, andv will vary* in magnitudeV and polarity depending upon the extent and'direction of frequencydeviation.` 'I-he resistors lila" and I9-are each bypassed` by suitable- I. F; lbypass condensers, and the modulation frequency component of the resultantfrectiedvolt'- age is applied through coupling -condenservZIf to anysuitable audio frequency Vmodulation. ampliner-network.

In Fig. 2` there are shown the primary-'and sec. ondary response curves'Pi-'andjS'i-of circuitsland. S- respectively. 'Ihese curves are securedv by plot-1 ting frequency" as abscissa against response asi ordinate. Asexplained previously, itrislhighly desirable to have the response curve Pi markedly double-bumped. The secondary response curve Sri`s relativelyv flat-topped with somewhat ofl a depression-atits center. In practice ythe trans'-A former windings 8 and II= are coupled approximately to the critical coupling value, .which .provides the curve Pt. Itshould be noted that the primaryis. coupled' to the` diodes. I4, I5- and II,4 I8 both through, the transformer windings andi throughcondenser I2'. Because of the connectionincluding condenser IZ, the response curve of the primary circuit P.(as well as the secondary circuit Si) is important tothe operation of the discriminator. Thefbest overall` discriminator characteristic, bothJ as to` linearity andA length, is ob tai-ned when the response o'f'primary P is substantially-in accordance with the double-humped curveP'i in Fig. 2. v Y Y Fig, 3 the-curve D relates frequency as abscissaagainst'- voltage output vas ordinate.'- Theportion of curve'Dloetween the peaks thereof is substantially linearL by virtue of the correction introduced by the double-humped primary response curve Pi. If, now,V there occurs 'any flattoning ofthe primary' response curve, say to Pi (Fig. 2), `the eiect will beto change the overall discriminator characteristic from D toward or beyond the curve D (Fig. 3). Such a change would be highly undesirable, since it shortens and otherwise deleteriously affects the linear portion of the discriminator characteristic.

Returning now to the limiter tube, the known arrangements of the prior art wherein a constant signal voltage is maintained at the limiter plate 1 may cause upon reception of signal energy of varying intensities approximately the change in primary response curve from P1 to P1'. 1 This may be explained as follows. If the screen grid voltage is maintained invariable, Vand the direct current plate voltage is permitted to vary in order to provide a constant signal voltage output from the primary circuit, there is provided the equivalent of a zero impedance across the primary winding 8. -That is, no matter what impedance is put in the plate circuit of tube 3 thevoltage across it will lbe the same. Since this means in effect that the primary 8 is being shunted by a very low resistance, the eiect is greatlyto dampen the primary response curve. Hence, the desirable doublefhumped curve P1 may in the reception ofv strong signals be changed to the relatively attened response curve P1.V At the same time the selectivity due to circuit P will largely be cancelled, and the tendency will be for the secondary circuit S alone to supply selectivity whereby the discriminator transformer will act like a single tuned circuit with a rounded top, instead of the relatively flat top S1 of Fig. 2. If the voltage across the primary of the discriminator transformer were held constant, the overall discriminator characteristic would depend on the strength of the received signal, would be narrow for signals strong enough to cause limiting, and might actually change from curve D to one even Worse than D'. If, on the other hand, as in the use of my invention, the limiter plate voltage is kept high so that it never swings over the knee of the plate-current vs. plate-voltage characteristic, the discriminator characteristic will remain like D.

What is desired, however, is a constant signal frequency current Vin the plate circuit of the limiter tube. This corresponds to a very high plate resistance of the limiter tube. If this is many times as high as the impedance of the primary circuit P, as is 4usually the case, then the overall discriminator characteristic D will be the grid at the preceding tube. My present view is same for weak signals as for strong signals. VFurther, the linearity of the characteristic will be preserved, since the desirable double-humped re..

sponse curve P1 will be maintained and will not be degenerated to response curve P1. l

I secure the desired limiter action by the relatively simple arrangement shown inFig. 1. The resistor R in the screen voltage lead land associated condenser C cause the screen voltage to vary at a fast rate so that carrier amplitude variations due to amplitude modulation will vary the electron Ilow to the plate 'l in a manner to maintain a substantially constant signal frequency plate current. In Fig. 4 I have shown four curves secured by plotting voltage on grid of tube pre-V ceding limiter as abscissa against voltage applied to diodes as ordinate. Curve I is a calculated curve using an idealized tube characteristic with an invariable screen voltage, but `with R1=10 megohms. In other words, R is not in circuit with the screen grid 9, and the condenser C is removed. Also Yno plate limiting impedances are employed. The calculated curve shows that the output Vof the limiter tube dropsy appreciably as the signal voltage rises above the threshold value of 0.02 volt on the,"preceding grid. Curvek tial droop in the horizontal part of the charac-" teristic. The eiiectof introducing` suitable resistance, shunted by suitable capacity, into the screen grid circuit is shown by curve 4, Here the resistor R is given a value of 33,000 ohms, and C is chosen of the order of 68 micro-microfarads; R1 is 150,000 ohms and C1 is 22 micro-microfarads. The limiter characteristic is now ilat above the signal input voltage of 0.04 volt on the thatwhat has been done is thatthe screen grid has been given a rapid time constant so that it functions in the manner of a fast-acting auto-` matic gain control element and provides self,- regulation of the limiter tube, augmenting the fast-acting gain control action of the grid 2 caused by network R1 and C1 inthe grid circuit. A point involved in my invention is that grid circuit limiting alone does not give a Vuniform output from the limiter-discriminator circuits for diiferent intensities of applied signal voltages. Plate circuit limiting when used with grid circuit limiting changes the discriminator'characteristic. By including a fast time constant resistor-condenser combination in the screen grid circuit, the limiter can be made to give a uniform output, and the discriminator characteristic can be maintained without change at varying signal intensities.

The magnitude of screen resistor R determines the threshold or limiting point of the limiter tube. In order to provide limiting action at as low an input vvoltage as possible, a low normal screen voltageis desired. This necessitates a relatively high value of R. Hence, in Fig. 6 I have shown a family ci?v applied signal voltage vs. effective limiter output current curves to show the eiTect on the limiting point of the magnitude of R. At lower initial or normal operating voltages of the screen grid, say +70 volts for example, thev limiting will commence at lower signal input voltages. It is desirable, therefore, to use higher values (say of the order of 120,000 ohms) for R when receiving weak signals than when receiving strong signals. There could be provided an adjustment'for the value of R so as to permit the set operator to adjust the limiting point depending upon the reception area he finds himself in. It will be noted from, Fig. 6 that the value of R may be chosen in order to provide the flattest limiting characteristic, or a second value may be chosen which will start limiting sooner for weaker signals, but which will not be quite so flat.

Since it is highly desirable to provide the networks R, C and R1, Crwith as short time constants as possible, it may be found under some conditions that condensers C andC1 donot have suiciently low impedances forroptimum operation. In Fig. 7 I have shown means enabling use of a time constant'of each of R, C and-R1, C1 while using condensers C and C1 which might otherwise present more than the desired impedance. Each of condensers C and C1 is shunted by a series-resonant `circuit consistingof a coil ing weak signal inputs.

30 Aand .39 and condenserv3| and 3| respectively,

each tuned to the operating I, F. value. These respectiveseries-tuned circuits 30, 3l and '30','31 provide a very low impedance for the I. F. current. The condensers C and C1 are necessary Vto bypass harmonics,.but can -be of smaller capacvthe gain 'for weak, vor low amplitude, input signals. VBy employing diode 40, shown in Fig. 8, in association with .screen grid 9 it is possible vto use a higher value for screen resistor R than would normally be the case without the diode.

The higher the value of R the lower the value of signal input voltage for which limiting begins.

Diode l0 has its cathode connected to the screen grid end of R while its anode is connected -to a suitable point E2 on potentiometer resistor 4l connected between the -l-B voltage lterminal and ground. Hence, point 42 is normally less .positive than the +B end of resistor 4l. A condenser 43 bypasses point 42 and the diode. anode to ground for I. F. currents. The signal grid network R1, C1 is shown in an electrically equivalent arrangement relative to that shown in Fig. 1. The diode 4B is normally conductive for weak signal input to circuit 5. Hence, the voltage of screen 9 will be at the potential of point 42 dur- I-Iowever, when the in- .put voltage at signal grid 2 becomes stronger the screen current through R becomes less, with the result that the cathode of diode i0 may for rapidly recurring potentials become more positive than itsanode, inasmuch as the latter is grounded `for I. F. currents, thereby disconnecting the screen grid 9 from point 42. The full voltage across the series resistor R is, therefore, applied to the screen grid, and the limiter tube from this point on acts as it does inthe circuit of either Fig. l or Fig. 7. Diode 40, therefore, provides ya controlled minimum screen voltage 4for the .limiter tube, but withoutaffecting the plate voltage.

Regardless of whether or not the modifications of Figs. '7 and 8 are used, thelimiter `circuit of my present invention `functions to .provide a substantially uniform and constant signal frequency current in the plate circuit of the limitertube. This corresponds to a very high plate-resistance of the limiter tube. It is preferred that the Vplateresistance of the limiter .be many times as high-asthe impedance of primary circuitl P. In .this way the desirable response curve P1 shown in Fig. 2 will be maintained.

In Figs. a, 5b and -5c `I have graphically 'and 'ideally shown the manner in which the limiter .grid '2 of limiter tube 3. The signal grid is normally at ground potential. Upon the signal energy charging the grid 2 positive during the irst few positive cycles thereof, grid current ows and .10 produces a voltage across R1 thereby charging condenser C1 negative. This negative bias, indi- 'cated `by broken line X, `offgrid 2 determinesethe operating point'of the tube. 'As the signal passes Ythrough its alternate negative 4'and positive cycles, the screen and'plate rcurrents veach will follow the "curve b Ias to contour, `th'e plate current, in the -tubes which 'I have .used being, however, of the 'order of fourtimes the sCreen'grid'Current.

Upon 'the vsignal intensity increasing to a', for example as shownin Fig. 5b, the negative'bias for grid 2 'increases "tothepoint where thetube characteristic looks like a .class B "-a'mplier characteristic. 4"I'h'e positive half o'i each signal wave produces a rectied plate 'currentpu'lse b. Sincethe 'time'constantof R1, C1isfast,"thebiasjon gri'd2 'will 'quickly .follow the signal yamplitude variations, i. e.,wil1becomemore negative as "indicated in Fig. 5b by the 'line .X vbeing'movedfurther'tothe left. The :fast-acting control network R, C will functionto provide a gain control 'voltage Ydue to screen current .flow decrease which tends 'toreduce'the dropin'resstor R, causing higher screen voltage which in turntends toprevent areduction in instantaneous plate current now. This eiect is also shown in Fig. 5b which 'indicates that .such increase in the positive screen voltage 'raises 'the plate current curve from the Kfull -line A -to the broken line B. 'Ihis allows .theplate current upon 'a ,positive half-cycle of signal voltage tobe 1in accordance with the brokenline b" IinFg. 15b instead of the shaded area which shows what the-operation would be if the resistor R were not present in the screen grid circuit. -This Yfurther ymeans that the signal `frequency current inthe .plateciiu cuit is increased yfrom that Vcorresponding to vcurve 3 ofFig. '4 Vto that correspondingto curve 4 ofthat figure. In Vother words, eachfast change inscreen voltage :causes the tube to operate along a new characteristic of the family of ,possibleplate current-control grid `(grid i2) characteristics. The gain control by the screen grid isin a sense `to op- Ypose-.thesignal carrier variations due toamplitude modulationV which would .otherwise occur followingicurve '3 in Fig. .4. vAlthough theapeakvalue ofplate current Jhas `been-increased in Fig. 5b over 'Fig 5a, the average plate current tends to .re-

main uniform because .the average plate current, under the conditions `0i Fig. 5b, will 4be only `of the order oi 30% of peak value,as compared with a much larger Apercentage in Fig. 5a.' This is largely due to the breaks in the plate-current Iias the negativegrid .potential reaches ori-exceeds cutoff value.

In Fig. 5c Athere are shown the curves a. vand b".,for strong signal reception. The ltube .now

`acts in the manner-of a class C amplier, and the control by the variable screen voltage-is Aagain shown. It will vbe notedthat `the platecurrentcontrol grid characteristichasbeen raised further to the position B', and the ybias .line X has been .moved still .furthernegative The rise-in -the plate current peaks -is compensated by increased time spacing between -the plate current pulses. The vshaded area again illustrates the `decreased plate current which wouldl result if resistor fR were omitted from lthe screen grid/circuit.

Summing -up the actionofithe limiter :tube Figs. V5a,=5l7-and 5c show that as the fapplied signal amplitude increases, the average screen current'fdecreases by reason of the increased negative bias on grid 2. The voltage drop across the screen resistor R is dependent on the average screen current `flowing through the res'istorff-obnsecuent1y when the average screen-current decreases,.`the effective screen voltage increases by virtue of a decrease in voltage drop across the screen resistor. This tends to increase the average screen current. The higher screen voltage, also tends to bring up the average plate current. The overall action is such as to tend to maintain a constant average plate current. This, in turn, will tend to produce a signal frequency component of the plate cur- .rent that is uniform for all input amplitudes above the threshold value. So far as the relations between the R, C networks in the control grid circuit and screen grid circuit are concerned, it may generally be stated that the time constant in the screen grid circuit is relatively faster than that in the control grid circuit. Generally speaking both time constant networks may be chosen from a range of the order of 1 to 10 microseconds.

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

What I claim is:

1. In combination between a source of frequency modulated wave energy and a frequency Vdiscriminator circuit having a predetermined response characteristic, an amplitude limiter tube including a cathode, signal grid, plate and auxiliary cold electrode adjacent the plate, means for applying a positive potential to each of the auxiliary cold electrode and the plate, means in circuit with each of said signal grid and auxiliary cold electrode for providing a fast-acting gain control over the limiter tube for carrier amplitude variations in excess of a predetermined value, and means in circuit with the auxiliary electrode for providing a controlled minimum voltage therefor.

2. In combination between a source of frequency modulated Wave energy and a frequency discriminator circuit having a predetermined response characteristic, an amplitude limiter tube including a cathode, signal grid, plate and auxiliary coldelectrode adjacent the plate, vmeans for applying a positive potential to each of the auxiliary cold electrode and the plate, means in circuit with each of said signal grid and auxiliary cold electrode Vfor providing a fast-acting gain control over the limiter tube for carrier am.. plitude variations in excess of a predetermined value, said last means consisting kof a resistor and a condenser cooperating to provide a relatively short time constant, and a series resonant circuit, tuned to the center frequency of said wave energy, electrically connected to each resistor and associate condenser for rendering more rapid each time constant.

3. In a system for receiving frequency modulated carrier wave energy, an amplitude limiter tube provided with a cathode, a signal grid, a plate and a screen grid interposed between the signal grid and plate, a signal input circuit connected between said signal grid and cathode, a frequency discriminator comprising a primary circuit and a secondary circuit tuned to a common frequency, said primary and secondary circuits being coupled to impart a substantially double-bumped response curve to the primary circuit, means for establishing said plate at a substantially. 'constantpositive potential of relatively high magnitude, a Vfirst Qresistorand con- 'denser vnetwork inV circuit with said signal. grid and cathode providingt a. relatively short time constant, a second resistor and vcondenser network in circuit with said screen grid for providing a relatively shorter time constant than the lfirst time constant and auxiliary Ameans in circuit with said screen grid providing a controlled minimum voltage therefor. Y Y

. .4. In combination between a source of frequency modulated wave energy and a frequency discriminator circuit having apredetermined response characteristic, 'an amplitude limiter tube including a cathode, signal grid, plate and auxiliary cold electrode adjacent the plate, means for applying a positive potential to each of the auxiliary coldrelectrode and the plate, means in circuit with each of said` signal grid and auxiliary cold electrode for providing a fast-acting gain control over the limiter tube for carrier amplitude variations in excess of a predetermined value, and diode means in circuit with said auxiliary cold electrode for providing a controlled minimum voltage therefor.

5. In combination, between a source of frequency modulated wave energy and a frequency discriminator circuit having a predetermined response characteristic, an amplitude limiter tube including a cathode, signal grid, plate and aux.. iliary cold electrode adjacent the plate, means coupling said grid to said source, means for applying a positive potential to each of the auxiliary cold electrode and ,the plate, means in circuit with each of said signal grid and auxiliary cold electrode for providing a fast-acting gain control overthe limiter tube plate current, for carrier amplitude variations in excess of a predetermined value, said last means consisting of a resistor and a condenserco-operating to provide a relatively short Atime constant,Y and means connected to each resistor to render more rapid each time constant. 5

.6. In combination between a source of frequency modulated wave energy and a frequency discriminator/circuit having a predetermined response characteristic, an Aamplitude limiter tube including a cathode, signal grid, plate and auxiliary coldV electrode adjacentA the plate, means lfor applying a positive potential to each of the auxiliary cold electrode and the plate, means in circuit with each of said signal grid and auxiliary cold electrode for providing a fast-acting gain control over the tube space current for carrier amplitude variations in excess of a predetermined value, said frequency discriminator cons1sting of a primary circuit anda secondary circuit coupled to impart a double-humped char- REFERENCES CITED The following references areA of record in the file of this patent:

UNITED STATES PATENTS Name Date Dome Dec. 9, 1941 Number y

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