US2183865A - Modulated carrier wave transmitter - Google Patents

Description

Dec. 19, 1939. GREEN Er AL 2,183,865

MODULATED CARRIER WAVE TRANSMITTER -Qjr gm v INVETORS E. GREEN AND ATTORNEY 'l/GH 'FREQUENCY `STA GE Dec. 19, 1939.V E. GREENE- Al.y 2,183,865 l MODULATED CARRIER WAVE TRANSMITTR' HIGH FEEOUENCXJTAGE ."H. CLOl/GH ATTORNEY MODUL/rma l POTE N TAL f INVENTORS E. GREEN Ma Dec. 19, 1939. v E. GREENE-rm. 2,183,865

MODULATED CARRIER WAVE TRANSMITTER Filed Feb. 1o. 1957 A 4 sheets-sheet 4 AAAAAAA HIGH FEEQUEA/CY STAGE llllll MobuLAT/Na PO TEN T/AL H. moz/GH r ATTORNEY Patented Dec. 19,- 1939 UNIT-Eo STATES PATENT OFFICE MODULATED CARRIER WAVE TRANSMITTER Application February 1o, 1937, serai No. 124,994

1n Great Britain February 27, 193s 7, Claims.

This invention relates to modulated carrier wave transmitters.

Figure 1 of the drawings accompanying this specification, shows diagrammatically a well 5 known form of resistance capacity coupled modulator arrangement such as is normally employed for a high -power modulator system of the choke type. In Figure 1 the high or carrier frequency stage to be modulated is represented by a resistance R and LI is the modulation choke, high tension supply being made at the terminals +HT and -HT. The main or iinalA tube is the tube VI and this tube is preceded by two tubes V2 V3 in cascade. Modulation from a source represented at G is applied to the grid of the tube V3 which is resistance capacity coupled by means of the elements R4, C2 and R2 to the grid of the tube V2 which in turn is lresistance capacity coupled by means of the elements R3, CI and Ri to the grid of the following tube VI. Grid bias is applied to the three tubes VI, V2 and V3 from a potentiometer resistance which is shunted as shown across a. suitable source of direct current potential shown as constituted by a generator bl.

' The diiliculty of a known arrangement as shown in Figure 1 and of similar arrangements isthat the main modulator tube VI must be operated in the region of zero grid current for if any attempt bemade to drive the said tube VVI into grid current the said current will dow through the resistance RI thus altering the grid bias and producing distortion. The limitation that the tube Vi must be operated in the region of zero grid current of 'course involves inediciency. The principal object of the present invention is to avoid this defect and enable large modulation inputs to be handled without serious distortion.

'4o Although in the preceding description' the stages including the tubes Vl, V2 and V3 have been shown as singlel tube stages, of course any of these tubes (and, in practice generally the tube Vl) may be replaced by a number of tubes in parallel. i

According to this invention the iinal modulator -tube of a modulating system is coupled to the precedingapparatus by an interposed coupling tube stage not designed to provide amplification 5o but so arranged as to behave as a generator of low internal resistance for supplying grid current..

In this way thefnal modulator tube may Ibe ailowed to run into grid current without producing appreciable distortion.

` The invention is illustrated in Figures 2 to 1 l (ci. 17e-171.5)

inclusive of the drawings accompanying this specication which show diagrammatically various` embodiments thereof. Each embodiment includes low impedance means for supplying grid current to the modulator tube to permit the same 5 to draw grid current without causing appreciable distortion. Figure 11 is a simpliiied diagram used to illustrate the manner in .which the grid current is supplied by the low impedance means.

Figure 2 shows an arrangement of the general 10 type illustrated in Figure 1 but modified to bring it into accord with this invention. In Figure 2 it will beseen that the tube VI is preceded b yl a coupling tube Vl' vwhich is not arranged to provide amplication but is the tube provided in ac- I5 cordance with this invention to' permit the tube VI to'be run into a grid current.y 'I'he main high tension supply is made at the terminals -l-HTI and HTL and a second high tension supply is made atthe terminals -l-HT2 and -HT2. 20

The positive terminal -l-HT2 is connected to the anode of the tubeVl' the cathode of which is connected to the grid of the tube Vl and also through a'large choke L2 to the junction point of the two direct current sources bl and b2, the 2 5 positive terminal of the source b2 being connected to the negative terminal bl. The positiveV terminal of the source bl is grounded andconnected to the cathode of the tube VI and the negative terminal of the source b2 is connected 30 through the resistance RI` to the grid of the tube VI. The preceding stages yincluding the tubes V2 and V3 are arranged in much the same way as in Figure 1 but receive their bias from a potentiometer shunted across a further source 35 b3. shunt condensers are provided acrossthe Y sources bl and b2. i

It will be seen that in this arrangement the grid bias tothe tube VI is fed'from the source bl through the choke L2, thecathode of tube Vl' 40 being connected to the grid end of the said choke. The source bl also provides part of the high tension supply to the tube Vl', the remainder being supplied from the aum'liary Source connectedy at -l-HT2 and -HT2. The cathode of 45 the tube VI' can be energized from any suitable insulated machine or transformer (not shown). In operation the tube `Vi' provides no amplification of voltage but behaves as a generator of low internal resistance for supplying grid curto rent for the main tube VI. It will be noticed that this grid current does not ow through the choke L2, or through the source bl but is supplied from the auxiliary high tensionsupply only through the tubey VI. As a consequence the w 2 'l aisance tube VI can be operated in the region of grid. current thus giving increased emciency but, at

the same time there will be substantially no distortion due to the flow of grid current. vThis operation will be clear from the following mathematical discussion thereof made in connection with Figure 11, which shows a tube operating in the manner Vi' is operated. In a triode valve if the input is applied between 1 grid and anode, the resistance of the equivalent generator is RL n+1 l5 and the magnification factor is EL m-i-l This statement is proved as follows: 20 In Figure 11 let Impedance of load. R.=A. C. resistance of trode. m= 'magnicatin factor of triode.

',=A. C.. component of plate current.

E =A. C. voltage between grid and cathode. E A. C. voltage between grid` and anode.

Then relative to input between grid and cath- 30 ode we know that the valve behaves as a generator of internal EMF=mEg and resistance R.. This is expressed by the equation -85 But if we are considering the input betweenl grid 'rms equation is of the same form as 1) with lin place of m in place of R.

EMFm+1E and internal resistance modulation system is of the series type instead v of .the choke type, the final tube Vi being on the low potential side of the high vfrequency stage 1o represented by the resistance R. Corresponding v parts are indicated' in Figures 2 and 3 by corresponding references and accordingly detailed de- 4 scriptlon `of Figure 3' will, it is thought, be unnecessary. It willbe observed that the stages 1g including the `tubes V2 and V3 receive high ten- In the modication ,shown in Figure 3 the sion potential from a separate source whoselterminals are indicated at +HT3 and -HTL 'This is usually more convenient.

Figure 4-shows another series modulation system embodying the present invention but in this 5 case the final tube VI is on the high potential side of the carrier frequency stage represented by the resistance R. In this case the choke L2 of Figures 2 and 3 is replaced by a resistance R5 to provide the grid bias for the main modulator l0 tube VI and an additional tube Vl" gives coupling betweeny the tube VI and the tube V2. The elements R3 Ci RI couple the anode of the tube V1 to the grid of the tube VI" and R6 is the anode resistance of the tube Vl. In Figure 4 the source of anode potential for the tube VI' is represented by a machine HT2 instead of by terminals and the additional direct current source b4 provides grid bias for the tube VI". Variable grid bias may be applied to the grid of the tube VI" in order to provide floating carrier action, as known per se. As this obtaining of floating carrier action forms' no part of the present invention, it will not be described herein, and the apparatus for producing it, other `han the tube VI" is not illustrated in Figure 4.

Figure 5 is a diagram showing part of a modification of an arrangement as illustrated in Figure 4, only such parts being shown in Figure 5 as are necessary to an understanding of the modification in question. As will bc obvious, the' steady anode potential applied to the tube Vl' is equal to the difference between the potentials applied at -i-HTI and -i-HTZ.I The supply source (not shown) connected at +HT2 can usefully be 35 employed to feedearlier stages (not shown in Figure 5) sov that the current demand on this source -can be reduced owing to the opposition of feed currents. The cathode choke or autotransformer CH has three taps X, Yand Z there- 40 on as shown. The intercept X-Y is chosen sufficiently high to insure substantially no diminution in volume of the lowest frequency required to be-passed by the system. If the tap Z for l the grid of the tube Vi be positioned (as shown) to the right (in Figure 5) of point Y audio frequency voltage magnification will occur, but if the grid loading (grid current or capacity current) of the tube VI is high it may be desirable to move the point Z to Y or even to some position between X and Y.

In some circumstances it may be desired to have the currents in Vl' and V' in phase opposition or at least with a phase difference in excess of 90. For this result the point Z must not 55 a be to the right (inFigure 5) of X but to the left thereof. Where, however, this result is required it is preferred to use a double wound transformer CHT in place of the auto-transformer CH of Figure 5, the essential connections 30 being then as shown in Figure 6. The grid of the tube VI' may be driven in any convenient manner (e. g., as shown in Figure 5) its potential,

inthe zero modulation condition, being somewhat below the potential supplied at +HT2. 05 If desired, in the arrangement of Figures 5 andy 6 "floating bias" for iloating carrier action may be added, e. g., as shown in Figure 7 which shows one way of adding floating bias to the ar- I rangement cf-Figure 5. In Figure 'I there is ap- 70 plied between the input terminals 1F of the added tube VF unidirectional potentials derived by rectifying the audio frequency. In Figure 7 the condenser 'CF should be large enough to by-pass the audio frequency cathode currents of the tube Vl but not large enough to impose too much delay on the change of anode potential of Vl` resultant upon a change in rectified audio frequency potential applied at IF. Obviously a separate source +HT2 is not required in Figure 7. The choke CH may be replaced by a resistance if magncation is not required and if complete carrier extinction is required inthe quiescent conditions suitable bias must be applied either in the grid lead to the tube VI or in -the cathode lead to the tube VF.

The invention is of great advantage as applied to quiescent and other push-pull systems. Onex push-pull circuit in accordance with this inven tion is illustrated in Figure 8 and it will be observed that Figure 8 is substantially a pushpull duplication of the circuit of Figure 2 the only major differencev beyond such duplication being that the output from the final modulator stage including the two tubes VI, is fed through a transformer T. In Figure 8 similar references are appliedto similar parts in the two halves of the push-pull system. In some cases the two chokesv L2 maybe wound on a common core so that they will function as a transformer. In either case the grid connection of the tubes VI may be made either above or below the cathode connection of the tubes Vl so as to provide a `step-up or step-down transformer action as may be required.

Figure 9 shows a further push-pull arrangement in accordance with this invention. In Figure 9 the primary of the output transformer T is connected between the cathodes of the tubes VI instead of, as in Figure 8, between the anodes thereof. This arrangement hasV the advantage of improving rectilinearity. The only other diierence of substancevbetween Figure 9 and Figure 8 is that the tubes preceding the tubes Vi receive anode potential from` thesource HT2 through chokes L3 as indicated. The arrangement of Y the stage preceding the .stage including the tubes Vl' may be regarded as approximately a pushpull duplication of that stage of Figure 4 which includes the tube VI" and accordingly ,the said tubes (in Figure 9) preceding the tubes VI' are denoted by the references Vl".

The invention isrnot limited to the precise circuit arrangements shown. For example, iixed bias for the main modulator tube may be applied in the grid lead of that tube instead of in the cathode 'lead oi the tube immediately preceding it. Again any other methods' well known per se maybe resortedlto for obtaining desired biases on thev various tubes and desired variations in anode potential from tube to tube-for example,

iigure, it is thought will be found self-explanatory. l

Where a plurality of main modulator tubes in parallel are employed individual biases therefor may be obtainedin various diierent ways. For example, each grid may be tapped upon a resistance in the cathode lead of the immediately preceding tube; or they may be tapped upon a separate bias potentiometer resistance (connected at one end to a point in the cathode lead of the immediately preceding tube-e. g., to the cathode) which is shunted by a bias source; or they may be tapped upon a resistance connected between the cathode of the immediately preceding tube and the anodev potential source.

In any of the illustrated circuits-iloating carrier action maybe obtained by superimposing floating bias in any convenient way known per se. Figure 'l is but one'exampleof acircult in 1. In a transmitting system Aa radio frequency stage to be modulated, a source of modulating potentials a modulator t'ubehaving-input and output electrodes, means connecting theloutput electrodes of said modulator tube to said radio:

frequency stage to modulate'the same, a source of direct current potential, a coupling tube having an anode, a cathode, and a control grid, an impedance connecting the cathode of said coupling tube to one terminal of said source of irect current` potential, a coupling between said impedance and the 'input electrodes of said modulator tube, a connection between the other terminal of 4said source of direct current potential and the anode of said coupling tube, said impedance being of a value high enough to materially impede the ow of modulating potentials and means forapplying modulating potentials to the control grid and cathode of said coupling tube.

2. In a transmitter system, a high frequency stage to be modulated, a modulator .tube having input and output electrodes, means couplingthe output electrodes of said modulator tube in series with said high frequentcy stage and with av source of direct current potential, said high frequency stage being in the high potential side of said connection, a source of modulating-potentials, a` couplingv tube having a control grid, a

cathode, and an anode, a second source of potential, an impedance connecting the cathode of said coupling tube-to one'terminal of said second source of potential, a connection between the anode of said coupling tube and a point on said second source of potentiala connection" between the input electrodes of said modulator tube and said impedance, said impedance being of a value high enough to materially impede the ow of modulating potentialsand means for impressing biasing potential between the control grid and cathodeof said coupling tube and modulatingpotentials from said source on the-control grid and cathodeof said coupling tube.

3. A system as recited in claim 1 wherein an additional source of direct current potential controlled in accordance with said modulating potentials is coupled with said coupling tube to provide a floating carrier action. j

4. In a transmitter system, a high frequency stagey to be modulated, a source of. modulating potentials, a pair of electron discharge devices having output electrodes coupled to said high l vfrequency stage to modulate the same. said dispedances being of values high enough to ma-f terially impede the flow of modulating potentials and means for applying modulating potentialsv from said source of modulating potentials to the control grids and cathodes of said coupling tubes.

5.Asystemasrecitedinc1aim4whereintho output electrodes of said modulator tubes are connected in push-pull by the primary winding `oi a transformer, the secondary winding of which is coupled to said high frequency stage.

6. A system as recited in claim 4 wherein the l output electrodes of said modulator tubes induce anode and cathodeelectrodes and wherein the cathode electrodes of said modulator tubes are connected together by the primary windingsof a transformer the secondary -winding of which lis coupled to said high frequency stage.

Y 7. In a transmitting system, a high frequency transmitter stage, a source of modulating potentials, a modulation frequency relay tube having input electrodes coupled through a modulation frequency ampliiier system to said source of modulating potentials and output electrodes coupled tosaid high frequency transmitter stage to modulate the same in accordance with modulating potentials from said source and means for preventing -peak modulation potentials which produce current iiow in the input circuit of said modulator stage from causing appreciable distortion in the modulated output of said trannmitter comprising a coupling tube having a control grid, an anode, and a cathode, means for connecting the control grid and cathode of saidy coupling tube through said amplifier system to said source of modulating potentials, an im whereby said coupling tube actsas a generator4 of low internal resistance supplying grid current to said relay tube input circuit.

ERNEST GREEN. NEWBOME HENRY CLOUGH.

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