US2340429A - Amplitude modulation limiter circuit - Google Patents

Description

Feb. 1, 1944. J. A. RANKIN 2,340,429

AMPLITUDE MODULATION LIMITER CIRCUIT Filed July 10, 1941. 2 Sheets-Sheet 1 1 .mzlo I 2, v j I v IH B "i a I INVENTOR ATi'ORNEY Feb. 1, 1944. J. A. RANKIN AMPLITUDE MODULATION LIMITER CIRCUIT 2 Sheets-Sheet 2 Filed July 10, 1941 IINVENTOR Jbkndl? BY 'f ATTORNEY Another important object of -cult of relatively higli impedance.

"axis of the applied modulated waves.

Patented 1944 I 1 2, 94 MODULATIONLIMITER John A. Rankin, Port washingtomhl. assignor 'to Radio Corporation of America, a corporation 0! Delaware I 'Application.luly10,

circuits particularly adapted for eliminating amplitude modulation effects in angular velocitymodulated carrier waves.

I As is well known to those skilled in the art,

the. usual form of amplitude-modulation limiter circuit employed in receiving systems for angular velocity-modulated carrier waves, such as, for example, frequency modulated carrier-waves (FM), depends at least in part upon the grid bias developed by the applied modulated carrier waves i h ly present invention relates toamplitude limiter networks, and more specifically to limiting in the grid circuit of the limiter tube. In this type of limiter 'circuit there is utilized a resistance capacity network in the grid circuit which I functions to developgrid bias for the signalgrid of the limiter'tube, andthe grid bias is directly dependent upon, the amplitude of the applied modulated waves. In this form of limiter cir- .-cuit there are two major, disadvantages. In the first place, the bias for the grid is developed in an RC circuit, that has atim'e constant. 'Ifhis fmeans that the bias can changeonly as-rapidly as the RC time constant permits it to change. Secondly, most limiter circuits operate .on a por-v The novel features which I believe to be characteristic of my invention areset forth in-particularity in' the appended claims; the invention itselfihowever, as toboth its organization *and method of operation will *best be understood by reference to the following description taken in connection with the drawings in which Ihave indicated diagrammatically several circuit organizations, whereby my invention maybe carried into efiect. V

In the drawings: i Fig. 1 illustrates thecharacteristics oi limiter circuits of tlieprior art,- 1

Fig. 2 illustrates the.

limiter, .1 a Fig. 3 illustrates one embodiment of the invention, I

Fig. 4 illustrates a, modificaticii,

ideal characteristic fora Fig. 5 shows the characteristics'of the limiter circuit inlFlg. l,

Fig. 6 illustrates another modification of the invention, I

Fig. '7 depicts the characteristics oi the ar rangement in Fig. 6. 4

til; 01' the input wave "that is'notsymmetrical 1 a ut-thei zero axis of that wave, and, hence,

ther'ejis secured a constant output over only a small: range of input signal amplitude;

7 It may, therefore, be'stated that it-isone or;

the main objects'of my present invention to provide an amplitude modulation limiter. circuit wherein the time constant network is eliminated;

and operation occurs about the zero axis oithe' applied modulated: carrier waves so that the.- range of input over which constant output ob- E tains isunrest-ricted.

the invention is multiegrid tube limiter circuits of the em-. jploying'an RC'time constant network in they to provide an amplitude modulation limiter circult ior frequency modulated waves wherein a pair of egectronic rectifiers are employed in re versed re ation and are biased. in opposite senses, and both rectiflers having a common input cir- Another object of the invention is to provide a limiter .stage having a high impedance input circuit wherein the limiter tube is oi the multigrid type, the grid circuit being free of any RC network, and operation being secured at the zero "Referring to the accompanying drawings, wherein like reference characters in the different .flgures designate similar elements, there is shown in Fig.1 in a purely qualitative manner the characteristics of the prior art circuit'which Y referred to above} As pointed outprevi'ously,

control grid circuit permit grid bias to bodeveloped in proportion ,to the-amplitude or the 4 applied modulated carrier waves. The RC network is either in the low potential side of the inputciicuit, or a condenser is inserted in series in the grid circuit and a grid leak resistor is con- 'nected directly fr m the control, grid to ground,

In either form of prior limiter circuit the operation is substantially similar. In Fig. lithere is shown the signal gridvoltage-plate current (E -I 5) characteristic of such prior limiter circuits. The--curve.l represents an assumed applied FM wave, whereas curve 2 denotes an applied FM wave oi. greateramplitude. It will be Stillanother object or the invention isto provide a limiter stage having a low impedance input circuitWherein the limiter tube includes a rectifier .device providing signal for the control grid of a separate electronic section.

seen from the diagram-in Fig. 1 thatthe limiter output is derived from a difierent portion of the input waves for each diflerent amplitude of input.

The greater the input .waveamplitude, the

narrower will be the pulse or plate current produced; further, the bias voltage developed by. the

input voltage is proportional to the amplitude 01' the inputwaves; ll his is clearly shown, a somewhat exaggerated manner, in Fig. 1'.

. for such purposes.

pulse of plate current designated i corresponds to that produced by'the applied input wave l,

whereas the pulse of plate current 2 results from the application of the input wave of a greater amplitude. The output pulse 2' is narrower Referring, now, to the curve shown in Fig. 2,

there is shown the difference in limiter action which is secured when the limiter operation takes place about the zero axis of the applied input waves. It is shown that the range of input over which constant output obtains is unrestricted aboveia low threshold value. The dotted vertical lines A and B represent'the limiting, or clipping, level. 'A series of curves 1, 2 and 3 are shown, and these are to be understood as applied FM waves of gradually increasing amplitude.

'Theseapplied waves are illustrated as swinging above and below the zero axis 0. If,'by some means to be described at a later point, the applied waves are symmetrically clipped at points A and B, then the fundamental components of the output wave would be very nearly the same for all values of amplitude of applied modulated carrier waves from curve A upwarda' If greater amplitude of applied waves were used the fundamental component would likewise bethe same. The curve C denotes the clipped wave which results fromthe limiting action, and it will be seen that this clipped .wave is symmetrical about the original axis. It will also be observed that if greater amplitude of applied waves were used,

no appreciable change in the wave form of curve C would be secured.- Comparison between curve C and the plate current pulses I and 2' of Fig. 1 shows the desirable characteristic of the former.

In Fig. 3 is shown one embodiment of the invention with which may be secured the desirable characteristic shown in Fig. 2. The limiting stage comprises a pair of rectiflers II and II. They are specifically shown, and merely by way of illustration,- as diodes. Preferably, they should be similar diodes, as those of a 6H6 type tube. The plate I 2 of diode III isconnectedtoone side of the resonant input circuit l3, while the cathode If is connected to the-.opposite side of the input circuit through a negative biasing voltage source l5. The opposite diode- II is com nected in reverse manner to input circuit I 3, and

the biasing source l biases the cathode 01' diode ll negatively with respect to the anode thereoi. just as in the case of diode IO. The input circuit I3 is of relatively high impedance. Assuming that the limiter is used in an FM receiver adapt ed to receive signals in the presently assigned 42 to 50 megacycle (mc.) band. it is well lmown that work I3 and I3 would have a pass band at least 150 kc. in width. The circuit 13 could be, for example, in the plate circuit of the last I. F. amplifier.

Since the FM detector following the limiter stage functions to produce the modulation voltage which was applied to the carrier at the transmitter, it is essential that the FM detector have no amplitude modulation effects on the carrier at the detector input circuit.- Hence, the output of the limiter stage in Fig. 3 is to be understood as feeding 9. FM detector circuit of any well known form, and since the limiting characteristic is that shown in Fig. 2, it will be seen that the applied waves at the FM detector input circuit will be free of any amplitude modulation eflfects. Therefore, the output of the detector will depend solely upon the frequency deviation of the carrier, or center, frequency of the applied FM waves.

The circuit shown in Fig. 3 functions to produce the clipped wave form C of Fig. 2 for the 1 following reasons. As the applied input waves increase from zero in a positive direction, no current flows in either diode until the peal: amplitude of the applied waves exceeds E volts,'E representing the bias voltage applied from either 2 of the sources l5 and I5. For values of applied waves exceeding E volts, the diode l0 that draws current acts as a short circuit, that is to saya low impedance, compared to the relatively high impedance oi. the input circuit l3. Hence, all of the voltage is dissipated in the input circuit I3. 0n the negative half of the cycle the other diode I l acts in the same manner so that the output wave has the shape 0 shown in'Fig. 2. It will be seen, therefore, that the arrangement of Fig.

3 functions to provide the ideal symmetrical lim-.

iting characteristic by virtue of the application of the FM waves to a high impedance input circult which has operatively associated therewith a pair of devices of uni-directional conductivity, the devices being inversely connected with respect to the high impedance input circuit, and

the devices being similarly biased with the result that for applied waves which exceed a predetermined amplitude the devicesfunction alternately as low impedance shunts across the high impedance input circuit.

In Fig. 4'there is shown a modification of the I arrangement of Fig. 3 wherein the limiter tube is a multi-grid tube 20 whose plate 2| is connected throughtheload element 22 to the positive terminal of the direct current energizing source.

The limiting output voltage developed acro'ss load I 22 is applied to the utilization 'output circuit, which may be the FM detector input circuit. The

the superheterodyne type of receiver is employed receiversfare well known in the art. the networks high impedance input circuit 13.

cathode'of the limiter tube 20' is connected to the signal grid 23 through a negative biasing source 24. The grid 2-3 and negative terminal biasing source 24 are connected to opposite sides of the The limiter tube 20 may be of the pentode type.

The operating characteristics of the circuit of t Fig. 4 are shown in Fig. 5. Considering the oper- .pliflers of the receiving system. Briefly, for such preceding circuit l3 would comprise a signal collector feeding a converter stage which would be followed by one or more I. .F. amplifiers. The I. F. value usually employed'is of the order of 4.3

,mc. Further, since' the FM channel width which is assigned is of the order of 150 kilocycles ationoi this form of invention,,it will-be understood that the grid 23 functions to'replace one of the diodes in the circuit shown in Fig. 3, and

the second diode is omitted. As the applied waves swing positive the grid 23 starts drawing current. When the amplitude of the applied waves exceeds E volts, the voltage in excess of E volts is dissipated in the high impedance input (kc.), then it will be understood that the netcircuit l3 as explained previously. Hence, the

I 2581fifl9 I ne ative bias biasing source 24 applies a normal of E volts to-thegrid 23. On the negative halt Y. of the cycle the waves are clipped by plate current cut-oflf. Considering Fig. 5,-it will be seen that the applied wavesare shown with respect to the zero axis of the Er-Jp characteristic. The

zero axis 0 is symmetrical relative, to that curve.

If one starts at point a, the value of the signal input is zero at this point; The plate current that flowsisoetermined by the bias E. As the input wave increases in the positive direction to point A',SQ does the. plate current'as shown.-As the input continues to increase ina positive direction, A the grid starts conducting (at- A) and hence shorts out theinput circuit, and theplate current doesnot increase further; The dotted wave shape g,'h and'i indicates what would happen if the input circuit wereof lo w impedance; With a high impedance input circuit the input wave voltage between A and Bras dissipated in' the input circuit.

From point B th e' value of the input wave decreasesito b which is zero, andthen increases in a negative direction to point e. Theplate current has followed this change as indicated. However,

at point 0 the plate current has decreased to zero value so that it can decrease nomore. The input wave. goes on increasing negatively, until it reaches its-maximum negative. point .11, at which point it decreases in amplitude to point d where platecurrent-again starts to flow. Between A and B thegridis. drawingcurrent, and-provides limiting, and between] e and d the "limiting is done by plate current cut-oil.

' Referring to the arrangement shown in Fig. 6,

the input circuit 13 is of a-low impedance in contwo. embodiments: Those skilledjin" the art are. fully aware of the'manner oi choosing the con-- stants'gf circuit l3 ;of Fig. 6 so as to'impart a low impedance thereto relative to the impedances of circuits I 3 of each of Figs. 3 and 4. The limiter tube 30 has two independent electronic sections, one of-these includes a cathode 3|, a control grld' 32 and a plate 33. The plate is connected through load element 30 'to the .positiv'e terminal of the direct currentsource. 'The cathode is connected to groifiid, and a resistor 35 is connected between the low potential side of input=circuit l3 and the grounded jcathode. The diode section of the tube comprises a portion of cathode 3| andan auxiiiary diode anode 00, the anode lll being connected to the high potential side of input circuit l3; The control grid 32 is directly connectedto the anode end of resistor 35.- During the negative half of the input cycle no current flows through diodell-IL,

In Fig. '7 there is shown the characteristic of the limiter stage of Fig. 6. Considering the operation of the arrangement of Fig. 6, the-diode fill-3i draws current only through the positive half of the applied FM waves. This produces a voltage drop across resistor that has the form of half sine waves which are applied to the grid 32. The device ill-3i is not a detector, but is merely a uni-lateral controldevice which has a series load impedance35. Across the latter ppears half-wavepulses of voltage of the ca or frequency," The horizontal wave forms designated iaRUdesignate the voltage pulses applied to grid 32 and developed acrpss resistor 38, If the applied FM waves increase in amplitude theiundamental component in the output is not'materially changed. 7

The .output wave shown in Fig. '7 is not sym- 5. metrical about the original zero axis of the wave,

but the original zero axis is one extremity of the output wave. The output wave, nevertheless,

is taken from the center portion of the input wave, and the fundamental componentin' the 10 output wave will not change materiallyfas the amplitude of the input wave isincreased after the threshold value of input amplitude has been reached. The diode iii-4| draws current during the positive half of theinput cycles. The current 6 flows through load 35, and hence a halt sine wave voltage appears across 35 and is applied to grid Preferably, in the circuits of both'Flgs. 4 andfi the screen voltage should be fed from a constant voltage source, or from a stiff supply. In this way'the cut-cit of the tube will not change with input signal amplitude. A triode section could be used instead of the pentode section in the tube of Fig. 6.

While I have indicated and described several systems for carrying my invention into eiiect, it ,will be apparent to one skilled in the art that my invention is by no means limited to the partlcular organizations shown and described, but

that many modifications may be made without departing from the scopeof my invention, as set forth in the appended claims.

What I claim is:

,l In a limiter stage for frequency modulated carrier waves, an electron discharge tube including at least a diode section and an independent electronic section having at least'a cathode, a

control grid and plate, a load element connected; to said plate, said load element being connected I 40 solely in the space current path of said inde-' tra-distinction to the input circuits of the first pendent electronic sedion and developing thereacross frequency modulated carrier voltage substantiallydree of amplitude variation, a wave input circuit of substantially low impedance con-. nected between the diode electrodes and including a resistor across which is developed voltage of solelycarrier frequency for said grid during positive half cycles of the waves at said input circuit, and' means connecting said grid to. a point on said'resistor which assumes an increasing negative potential with increasing amplitude of applied waves. I 2. In 5 limiterstage for frequency modulated carrier waves, an electron discharge tube includms at least a diode section and an electronicsection having" at least a cathode, acontrol grid, positive screen and plate, a load element connected to said plate, said load element being arranged in the spacecurrent path of the electronic section and developing thereacross limited irequency modulatedcarrier wave voltage, a wave input circuit of substantially low imped ance connectedbetween the dio de electrodes and including a resistor across which is developed carrier frequency voltage in the form c! halitube including at least 'a. diode section and an independent electronic section having at least a cathode, a control grid and plate, a load element arranged in the space current path of the electronic section and connected to said plate. a wave input circuit of substantially low impedance connected between the diode electrodes, a

resistor in series with: the input circuit across which is developed voltage of solely carrier frequency for said grid during positive half cycles of the waves at said input circuit; said resistor being connected solely in the space current path of said diode section, and means connecting said screen and plate, a load element connected to said plate, said load element being arranged solely in the space current path of said electronic section and developing thereacross amplitude limited modulated carrier wave voltage, a wave input circuit of substantially low impedance connected between the diode electrodes and including a series resistor across which is developed carrier frequency voltage in the form of half sine waves for said grid, means for maintaining the screen at an invariable positive potential, means to apply frequency modulated carrier waves to the input circuit, and means connect ing-said grid to a point on said resistor which assumes'an increasing negative potential with increasing amplitude of applied waves.

come a. RANKIN.

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