US2235549A - Modulator - Google Patents

Description

March 18, 1941. M 2,235,549

MODULATOR Filed Dec. 15, 1939 F-i l. 7 3 Z Fig.2. 9

Inventor: Robert B. Dome,

b Iva/W e) His Attorneg.

Patented Mar. 18, 1941 UNITED STATES llIODULATOR Robert B. Dome, Bridgeport, Conn., assignor to General Electric Company, a corporation of New York Application December 15, 1939, Serial No. 309,442

12 Claims.

This invention relates to apparatus for modulating an alternating current wave of radio frequency in accordance with a signal wave.

It is an object of my invention to provide improved and simplified circuits for modulating a carrier wave with a signal wave in a better and more convenient way than heretofore. It is a further object of my invention to provide a carrier modulating circuit which is suitable for modulating a carrier wave by a signal wave having a very broad range of frequencies.

In the attainment of my objects it is a principal feature of my invention to amplify the signal wave, with which a carrier wave is to be modulated, by a pair of amplifier tubes arranged in push-pull relation. One of these amplifier tubes is connected so as to reduce the voltage supplied to the anode of a carrier amplifier from a suitable source of operating potential to a minimum so that the carrier wave is amplified in minimum amount. The other of these signal amplifier tubes is arranged to decrease an impedance between the source of operating potential and the carrier amplifier tube to supply a maximum voltage to the carrier amplifier and hence produce a maximum amount of the carrier wave. Since the signal amplifier tubes are arranged in push-pull relation these operations occur alternately and may easily be adjusted to produce a faithfully modulated carrier.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a schematic diagram representing my invention; Fig. 2 represents certain characteristics thereof; and Figs. 3 and 4 represent certain modifications of my invention.

Referring to Fig. 1 of the drawing, there is illustrated a portion of a radio transmitter including a carrier wave amplifier and apparatus constructed according to my invention for modulating the carrier wave in accordance with a signal wave'having a very broad range of frequencies.

The carrier wave amplifier is a conventional one and is supplied from a source of waves of carrier frequency by a transformer In. The secondary of the transformer I9 is connected between the grid and cathode of 'an electron discharge ampli: fier device-1|.

In circuit with the grid and cathode of 'thedevice 'H and the secondary of the transformer Ill there is provided a grid leak resistor |2' by-passed by a condenser i3 which maintains a suitable bias potential between the grid and the cathode. The output circuit for the amplifier comprises a tuned circuit l4, l5, a 5

source of operating potential I 9, and a pair of resistors 2B and 2|. A condenser i8 is connected in shunt to the source l9 and resistors 26 and 2| to transmit high frequencies th'ereacross.v The tuned circuit l4, l5 may be coupled to any suitable load device, such as the antenna I1, which is connected to ground through a secondary winding l6 of a transformer, the primary of which is the inductance l5 of the tuned circuit l4, H.

The source of operating potential i 9 has means associated therewith for varying the potential applied to discharge device I I in accordance with a signal wave. This means comprises an impedance, represented as discharge device 22 connected in shunt with the source l9 and resistance 20, and a second impedance, represented by discharge device 23 connected in shunt with resist- .ance 2| These impedances are maintained at a constant value in the absence of a signal wave and are alternately reduced in value in response to alternate half cycles of the signal wave, so that the carrier wave operating potential first rises and then falls as first one and then the other of these impedances is reduced.

The discharge devices 22 and 23 have their cathodes connected together and to the point between the negative terminal of the source I9 and one terminal of resistance 2|. Each of these devices is provided with a grid circuit and these circuits are connected in push-pull relation for signal currents, which may be applied to the grids through coupling condensers 28 and 29. Suitable bias potential may be applied to the grids through inductances 24 and 25 and, if desired, suitable resistors'26 and 21 may be connected between the cathodes and the opposite sides of the signal source.

The input coupling systems 24, 28, 26 and 25, 29, 21 for the discharge devices 22 and 23, being of the resistance-capacitive type, may be readily designed for faithful transmission of extremely broad ranges of frequency, such as those used in television. This is also true of the anode coupling system of these devices, which comprises only the resistances 20 and 2| and the small radio frequency by-passing condenser IS.

The operation of this circuit may be more readily understood by reference to Fig. 2 in which the curve 30 illustrates the signal voltage applied to the grids of the device 23 and the curve 3| illustrates the signal voltage applied to the grid of the device 22. The curve 32 illustrates the voltage appearing across the condenser I8 when the voltages represented by the curves and 3| exist. When no signal voltage is applied to the grids of devices 2'2 and 23, as is indicated by the parts of the curves Bil and SI to the left, a constant voltage appears on the condenser l8 This. is represented by the level portion of thev curve 32to the left. As represented by the curves 30 and 3| when the voltage on the grid of the device 23 be-- comes more positive, the voltage on the grid of the device 22 becomes more negative. Tube- 23 therefore practically short-circuits the resistor -2I and the device 22 becomes practically an. open circuit; therefore, the voltage of the source IS'appears across the condenser l8 except for the voltage drop in the resistor 26. Thishighervoltage is represented by the rise in the curve 32-.-

As curves 30 and 3! also illustrate, when the voltage on the grid of deviceZS becomesmore negative, the voltage on the. grid'ofthe device--22- becomes more positive. Therefore, thezdevice- 23 becomes practically. an openucircuit and :the device fiibecomes practically ashort circuitu The potential of the source I9isalmostentirelyremoved from the device Hand the potential dropacross the resistor 20. becomes practically-equal to the potential of the source. I9. Since-the amplitude of the wave amplifiedin th'adevice-II varies as the operating. voltage. for that device varies, the amplitude of the. carrier wave follows the curve 32. I

Referring to Fig. 3; there is showna-modified arrangement for supplying" operating voltage, modulated in accordance with signals, to a-high' frequency amplifier. The high'frequency ampliher is identical with that shown inFig. 1' andfunctions in the same way. Its .partscorrespondto respective parts of the amplifier: illustrated by Fig. l and like reference characters have been applied thereto. When thereisno signal to vary the operating, voltage which .is applied across?v the condenser l8, a steady voltage: issupplied by a source 33 of direct current through ant-induc tance M to the junction between the condenser I8 and the coil i5 andxhence tothe anode" of amplifier Ii. The cathodeof device Ilis grounded and the negative terminal of the sources3'3 is grounded. A steady voltage thusapplied across the condenser I8 maintains a carrier= wave. of constant amplitude.

Means is provided for varying the amplitude of the carrier wave, which comprises an impede ance in series with the source: 33of operating potential and a second impedance in shuntto the source. The series impedance is the inductance 34 which is shunted by a condenser 36: and ,a discharge device 31 connected in series: On one half of the signal wave the resistance'of the device 3? is reduced and it, with the condenser'36, provides for a brief time a low impedance path around the inductance 34;

Although the condenser 36Jdoesformza-low impedance path momentarily, italso 'performszan additional function; The point between the:con'-v denser 3G and the cathode of'. the discharge;device ill is connected to. one-endbf an inductance 35, whose other end isccnnectedto ground; The average potentialacross condenser-35tmust re-- main at the potential oflthessource33s; There-.- fore, when the resistanceof the; devices31 isreduced, the. condenser 36;. not only: forms -.a lowa impedance path .arounditha inductance 234,: but: also momentarily adds; the. voltage due. to itsstored charge to the voltage of the source 33 9.10-- pearing on the anode of device II. Thus on that half cycle of signal voltage, the anode voltage of the device II rises nearly to twice the voltage of the source 33.

The second impedance connected in shunt to the source 33 comprises a second discharge device. 38. connected in shunt to the source 33 and the inductance 3%. On the other halt: of lthe signal wave the resistance of device 38' is reduced and substantially the entire voltage of the source 33appears across inductance 34 so that the anode. potential of amplifier device I! is reduced nearly to zero.

Eachof-thedevices 3'! and 38 is provided with r a grid circuit-and these circuits are connected by a transformer 39' to a source of signal voltage so that first one and then the other device becomesconductive. One secondary 40 of transformer 39 is connected at one end to the cathode andizatthe-:other end through a=condenser 42 to. the: grid ioi :device. 38;. This gridis connected to the cathode'through'an inductance 43anda sourcezMrof biasepotentia'l; Similarly the other secondary il-zof .the transformer 39 is connected at .one. end to the cathode: and .at the other end through azcondenser 45 to the'gridlof thedevice 31 The grid is connected to the cathode through aninductance 45:and:'a..source' 41. of grid bias potential.

The grid bias potential :sources. Mian'd 41 are arranged to maintain the :devices.'3'1 and 33 substantiallynonconductive 'in' the r absence 'of 'a r signal wave. The transformer windings are'so'poled that; when energized by aisignal'wave, first one and then the other" grid .of devices 3'! and" 3'81is' made-more positive so tliatthe devices'are: alternately conductive: Sincethe cathodes of the .devices 31' and 38- are at different, potentials, a transformerrisemployed to apply signal poten-. tialsito :the' grids.

Referring-to Fig. .4; axthird' modification ofimy inventionis shown wherein a source 48 ofpotentialand-a resistorJlS-are:connectedlin series re lation' between the: two terminals of the? con-i denser- I8. In-this=figure, as inFig; 3, the'higlr frequency amplifier illustrated is. identical with: that shown by Fig;- l and-.like reference charac ters havebeen applied to;like"p'arts-.u The "source'- 48 of potential must be sufiicientlyilargezto sup plyacross the condenser I 8 -not"- onlytheproper operating potential for the device I I i but also to supply the potential drop ithroughthe resistor 49 due to the flow of operating? current therethroug-h. An electron discharge devicefifl'has its anodeconnected to the positive terminal 'of the source 48 and I its cathode F'connected to' "the: cathode of-therdevice I." The 'anodeofa' second dischargedevice :5! is connected. to the cath'ode ofthe::device- I I Tandpthe cathode of the device 5| is connected'tothe 'junctionbetween'itheresistor 49 and the source 43, which is grounded. Suitable control circuits. for. the devices Strand act firstto 'reduceathe impedance. of." one device on one half cycle'rof a signaluwave' andAthen tore duce the. impedance of. the other device 1 on theotherrhalf. cycle.

A'circuit for-'control'lingthe impedances ofthese devicesisJcOnnecte'd ::to. .the-;control electrodes of theetwo devices. and comprises a: source of signal voltageivincluding a. transformer 1 52 which ha's' a pair of secondary windings -..53'an'd 54. O netr minal. :of. :each of: these :windirigsx iS'lCOIlIlECIJBd to arespective: control electrode of the tubes 5 Ii -and: 51. fan'dthe otherfterminal of" the winding' 'is "con nected through respective sources of bias potential 55 and 56 to the corresponding cathodes. The sources 55 and 56 of the bias voltages are adjusted so that the devices, 50 and in the absence of signal voltages are substantially nonconductive.

When signal voltages are applied through the transformer 52, first one device and then the other becomes conductive and changes the anode operating voltage for the device f|| above and below its steady value in the absence of signals. When the device 50 becomes conductive the voltage supplied to the anode of the device drops to a very low value and substantially all of the potential of the source 48 appears across the resistor 49. When the device 5| becomes conducting the potential drop across the resistor 49 substantially disappears and practically the entire potential of the source 48 appears on the anode of the device II. The resistance 49 and device 5| may be considered as an impedance in series with the source 48 whose magnitude decreases on one half cycle of the signal and the device 59 may be considered as the impedance in shunt to the source whose magnitude decreases on the other half cycle.

In each figure there are certain optimum values for certain of the elements which are instrumental in varying the operating voltage for the high frequency amplifier. For example, in the circuit illustrated by Fig. 1, the values of the resistors 29 and 2| should be so chosen that the depth of modulation produced by device 22 is substantially equal to that produced by the device 23. The values of the inductances 34 and 35 and the condenser 36 in the circuit illustrated by Fig. 3 should be made such as to give as nearly as possible equal depths of modulation on succeeding half cycles of the signal valve. In the circuit illustrated by Fig. 4 the resistor 49 should be of such size that, when no signal is applied to the circuit, a potential exists across it equal to half the potential of the source 48. Of course, the relative depth of modulation produced by the two tubes 22 and 23, 31 and 38, or 50 and 5|, may be varied over a considerable range by-adjusting the relative amounts of signal voltage applied to the respective grids.

While I have shown particular embodiments of my invention, it will, of course, be understood that I do not wish to be'limited thereto, since different modifications may be made both in the circuit arrangement and instrumentalities employed, and I aim by the appended claims to cover 'any such modifications as fall within the true spirit and scope of my invention.

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

1. In a power supply for an amplifier. through which a carrier wave is transmitted to be modulated by a signal, the combination of means .for applying an operating voltage to said amplifier, said means comprising a source of operating potential and an impedance in series with said source, means for by-passing said carrier wave around said first means, means in shunt to said impedance for transmitting current therearound on one half cycle of said signal to increase said voltage, and means in shunt to said source and said impedance to transmit current from said source through said impedance on the other half cycle of said signal to decrease said voltage, whereby the carrier wave generated in said amplifier is varied in amplitude in response to said signal.

2. In a power supply for an amplifier through which a high frequency wave is transmitted to be modulated by a signal, a source of unidirectional operating potential for said amplifier, an impedance in series with said source, means for by-passing said high frequency wave around said source and said impedance, means for reducing the magnitude of said impedance from a constant value in correspondence to increasing intensities of one half cycle of said signal, thereby to increase the current through said amplifier, and means for reducing the potential applied from said source to said amplifier from a constant value in correspondence to increasing intensities of the other half cycle of said signal, thereby to reduce the current through said amplifier, whereby said wave is modulated according to the varying intensities of said signal.

3. In a power supply for an amplifier through which a high frequency wave is transmitted to be modulated by a signal, a source of unidirectional operating potential for said amplifier, an impedance in series with said source, means for by-passing said wave around said source and said impedance, an electron discharge device connected in shunt to said impedance, means to make said device conductive on one half cycle of said signal, thereby to increase current through said amplifier, a second electron discharge device connected in circuit -with said source to reduce the potential applied from said source to said amplifier, and means to make said second device conductive on the other half cycle of said signal, thereby to decrease current through said amplifier, whereby said wave is modulated according to the varying intensities of said signal. 7

4. In a power supply for an amplifier through which a high frequency wave is transmitted to be modulated by a signal, a source of unidirectionaloperating potential for said amplifier, a pair of impedances connected in series with said source, an electron discharge device connected in shunt to the first of said impedances, means to make said device conductive on one half cycle of said signal, thereby to increase the current throughsaid amplifier, a second electron discharge device connected in shunt to said source and the second of said impedances, and means to make said second device conductive onthe other half cycle of said signal, thereby to decrease the current through said amplifier, whereby said wave is modulated according to the varying intensities of said signal.

5. In a power supply for an amplifier through which a high frequency wave is transmitted to be modulated by a signal, a source of unidirec tional operating potential for said amplifier, a resistance in series with said source, an electron discharge device connected in shunt to said resistance, means to make said device conductive on one half cycle of said signal, thereby to increase current through said amplifier, a second electron discharge device connected in circuit with said source to reduce the potential applied from said source to said amplifier, and means to make said second device conductive on the other half cycle of said signal, thereby to decrease current through said amplifier, whereby said wave is modulated according to the varying intensities of said signal.

6. In a power supply for an amplifier through which a high frequency wave is transmitted to be modulated by a signal, a source of unidireci device conductive on-one: half cycle'ofzsaid signal,.

thereby to increase. current through: saidampli fier, asecond electron dischargedevice connected:- in shunt to said source-and the secondloi said resistances,- meanstomake said second 'deviceconductive on the othenhalf cycleof said signal, thereby to decrease current throughsaid"ampli'-- fier, whereby said wave is-modulated according to the varying intensi-tiesoi saidsignal.

7. In a power supply for an amplifier through I which a hiighfiequencywave-is transmitted tohemodulated-myfir-signal; a capacitance in circuit with said amplifier-for by-passing 'said 1 wave from said'powersupply, and means-inshunt to said capacitance for maintaining operating voltage for said amplifier across said capacitance, said means comprising a source of unidirectional operating potential for said amplifier, an impedance in series with said source, means for reducing the magnitude of said impedance from a-constant value in correspondence to'increasing intensities of one half cycle ofsaid signal, thereby to increase current through said amplifier; a second impedance in circuit with said source to reduce the potential applied from said source to said amplifier, and means for reducing the ma nitude of said second impedance from a constantvalue in correspondence to increasing intensities of the other half cycle of said signal, thereby to decrease current through said amplifier,

-whereby said wave is modulated accordingto the varying intensities of said signal.

8. In a power supply for an amplifier through which a high frequency wave is transmitted to be modulated by a signal, a capacitance in-circuit with'said amplifier for by-passing'said wave from said power supply, and means in shunt to said capacitance for maintaining operating voltage for said amplifier across said capacitance; said means comprising a source'oftunidirectional opcrating potential for said amplifier, an'impedance in series with said sourceyan electron discharge device connected in shunt: to. said: impedance, means to make said device conductive on one-' half cycle of said signal, thereby to increase: current through said amplifier, a second electron discharge device connected to said source to re-- duce the potential applied from saidv source to said amplifier, and means to make said second device conductive on-the other half cycle of said signal, thereby to decrease currentv through said amplifier, whereby said Wave is modulated ac-- cording to the varying intensitiesof, saidsignal.

9. Ina power supply for an, amplifier through which a high frequency wave is transmitted to be modulated by a signal, a capacitance in circuit with said amplifier for by-passing saidwave fromsaid power supply, and meansainshunt to said capacitance for maintaining operatingvolta agefor said amplifier acrosssaidcondenser, said means comprising a source of unidirectional opplifier, whereby saidlwaveismodulated accordingitoithe varying. intensities of said signal.

L; lnea-hpowensupply for an amplifier through whiohiaihighcfrequency wave is transmitted to be modulated-by; as signal, a: source of unidirectional operating. potential for: said amplifier; a condenser, means fonconnecting saidsource and said condenser in conductive charging relation with each: other to; maintain said, condenser charged to; thezaveragea potential offrsaid source, means connected inz-circuit with said source for reducing the. potential appliedi from': said source to i said amplifier: on, oneehalf fcycle' of said signal; there-: by to reduce the current through'said amplifier,

and. meanssforo connecting; said source and" said condensert'efiectivelyi-in series to said amplifier on the; otlienchalfi cycle. of said-signal; thereby to increase lth'fi cnrrentzthroughsaid' amplifier, whereby 'said' wavezisxmodulated according to the varyingzintensiti'eszofisaid (signal:

11;, In; combination; .anz electrondischarge amplifienfor: amplifying a; carrier: wave, said amplifierz'ihaving'ananode.iand azcathode', a source of" variable:-v operating; potential connected between said? anodes. and: cathode; through an, impedance,

means: for; bypassing: said. carrier, wave around said sourcearrd saidiimpedance, asource oiisignal current, means: responsive to, one-half cycle of sadd'signal current to shunt saidti'mpedance thereby'to3increase:the;operating:potentialapplied-between:- said anode": ands cathode; and: means? responsive toz-the r opposite: half '1 cycle to shunt: said,

source; andwimpedance z-in series thereby: to reducethe: operating;- potential; applied, between said anode, and; cathode;

12. The combination with angamplifierhaving an anode. and; a; cathode: and: adapted to. amplify a carrier.- wave: inan:amounti determined by" operating; voltage 1 applied betweensaid anode and.

cathode; of: means ,for: impressing, operating potentialbetween: said: anode, and: cathode, said meansvincluding; substantial, impedance, a signal source, mean'safoir reducing said impedance on".

one-half cycle of saidrsignalso that the operating potential on. said, anode is increased, and means in shunt; to s-aid; first means whose-impedance; is; reduced: onzt-hez.v other half: cyclev ofsaid signal. so; than the; operating potential between said anode andicathodeiisi reduced; whereby said carrier, wave; is: modulated-1 in response to said signal;

ROBERT B; DOME.

CERTIFICATE OF CORRECTION.

Patent No. 2,255,5lp9. March 18, l9hl.

ROBERT B. DOME.

It is hereby certified that error appears in the printed of the above numbered patent requiring correc column, line 59, for the 'word "valve" read -wave-; and that the said Letters Patent shouldbe read with this correction therein that the s ame inay conform to the record of the case in the Patent Office.

Signed and sealed this 29th day of April, A. D. l9LLl.

Henry Van Arsdale,

(Seal) Acting Commissioner of Patents.

specification tion as follows Page 5, first

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