US2083336A - Modulator - Google Patents

Description

June 8, F. C; LUNNON ET AL MODLATOR Filed July 2l, 1952 Fav 20 faz/Raf af m; j 5h Pam/rm ffm/Uff? of' am af? .faz/mf of asf.

FREDERICK c. LUNNON BfHARoLQJ. wAssELL ATTORNEY Patented June 8, 1937 UNITED STATES PATENT OFFICE Hughes Wassell,

Chelmsford,

England, as-

signors to Radio Corporation of America, a corporation of Delaware Application July kZ1, 1932, Serial No. 623,750

In Great Britain August 18, 1931 Claims.

so-called absorber keying is eifected in a radio or other high frequency circuit arrangement of the so-called absorber type by keying means arranged to alter the circuit conditions of a valve in such manner that said Valve is connected in circuit as an absorber valve during spacing but acts as an amplier valve during marking.

In the present specification the term absorber modulating arrangements is employed to refer to an arrangement of the kind in which a transmitter, which is modulated, is kept under load during spacing periods by means of one or more thermionic valves which are arranged to provide a load' upon the source of anode potential when the transmitter is spacing,

The present invention provides an improvement in the invention disclosed in said United States application Serial No. 520,587, and the modified or improved arrangement is adapted for telephony,y as well as for telegraph.

When a transmitter is employed, for example, in duplex telephony, it is often desirable to suppress the transmitted carrier during intervals in speech so as to avoid setting up parasitic noises inthe' receiver. The present invention is particularly adapted for application to duplex telephone systems for this purpose though the said invention is not, of course, limited to this particular application for this particular purpose.

According to this invention carrier transmission is substantially suppressed during spacing, e. g. .during intervals in speech,'like 'in a modulated radio or other high frequency transmitter, said suppression of carrier being effected automatically by means operated in dependence upon the modulation input, the modulating arrangement being such that during the said suppression periods there is utilized as an absorber valve a valve which is utilized as an ampliiier valve during other periods. y

The invention is illustrated in Figs. 1 and 2 of the drawing accompanying this specification, which drawing shows the nal magnier stage of a transmitter in accordance with thepresent invention. In the drawing of the specication, parts corresponding to parts illustratedl in the (Cl. Z50-17) drawing accompanying United States application Serial N o. 520,587 are indicated with the same reference numerals as in the said drawing of Unitedv States application Serial No. 520,587. VThe earlier stages ofthe transmitter, of which the final stage is shown in the drawing of the present application, may be as illustrated in the drawing accom-y panying the specification of United States application Serial No. 520,587. y

Referring to Figure 14 of the drawing, the final amplier valve V3 of a transmitter, whose self capacity may be neutralized 'as shown, may be coupled to the preceding stage I0 by means of a coupling condenser lcs and receives anode potential from a source l2. The stage l0 may be an amplifier of high frequency loscillations and may include a source of high frequency oscillations. The grid circuit of the valve V3 includes the grid electrode, the impedance Z1, the anodel to cathode impedance of valve MV and the impedance between the grid and anode tied together and the cathode of GL in parallel as shown and extends yto a tapping point Ps upon a resistance Z. The circuit is completed by connecting the resistance Z to the cathode of Vs. The tapping point P; and 25 resistance Z correspond to the similarly marked tapping point and resistance in the arrangement illustrated in the drawing accompanying United States aplication Serial No; 520,587.

The impedances of the Valves MVand GL are 30 reversed as shown. The purpose of the valve GL is primarily to permit thenegative potential set upacross the resistance Z described more in detail hereinafter to be applied to'the grid of the valve V3 by way of tapfPs on Z. This negative potential is required during spacing periods or periods between voice frequency modulations in order to set the valve V3 at such a point on its characteristic that it will take the required dead loss feed and thus act as an absorber valve. Obviously the said negative potentialr cannot reach the grid of valve V3 through the anode to cathode impedance of the modulating valve MV because the said valve is connected the wrong way. The required negative potential might, of course, be supplied by Way of a high resistance shunting MV, butin this case the resistance 4would have to be extremely high or it would disturb the modulating characteristic of MV and mightcause distortion. Moreover, in the presence of secondary emission from the grid of the valve Va during spacing conditions, a positive potential might arise on the grid of V3 sufficientlyk high' lto overcome the negative potential supplied bythe resistance'Z and thus cause excessivecurrent/to 4o y 1 reverse direction. l

flow in the anode circuit of V3. The use of the one-way rectifier or valve GL overcomes the disadvantages which would arise from the use of a resistance at this point. The Valve GL serves the same purpose as a high resistance leak across the modulating valve MV without introducing the dangermentioned above.

The tube MV permitsthe impedance of the grid in the grid circuit of the valve V3. The resistance Z may be shunted as shown by a loop circuit comprising the anode to cathode impedance oi a valve K and a source of anode potential B in series. Modulating potentials maybe applied to the input electrodesof the valve -MV from, for exampleuthe secondary of a microphone transformer -MT, whose primary Amay be energized from a`rr1icrophone M as shown. The microphone M mayalso apply potentials to thevinput terminalsof a speech amplifier and rectifier ATD. 'Iherectifier ATD :may also Iinclude a so-called hang over or delay circuitconsisting of resistance and capacity inknown arrangement to provide orprloduce a predetermined time constant. The, cn'utput'of the device may be applied between the grid and the `cathode of the valve K1, which 'valve may, if desired, be replaced by a plurality of -valvesvin parallel. The grid and anode of the valve GL `are connected together as fs'hown and the `said valve is so arranged as in effect' to `constitute a rectifying diode allowing negative potential to reach the-grid of the valve y:z-ybut yet maintaining a high resistance in the IIyrhe action of Ythe -whole arrangement is as follows: f" vw Y W Whilelspeech is taking place rectied speech potentialsare impressed -on thegrid of vthe valve K1 lvia the time constant'device ATD, these` rectiiied Ypotentials being of such value and the settin'g'f of the valve/Kibeing vsuoli that the application of thesaidyrectiiied potentials results in biasing the valve K1 to cut-off and thereby prevntingthes'ource of potential B from Ysending current thijough-the resistance Z.` 'I'he grids of the various stages in the transmitter including thestage .Va showny are therefore connected to earth through ltheir respective tappings on the resistance Z. The potential on` the grids are s'uchfthat'osvcillations are developed in the preceding stages, andamplied and 'modulated at speechv/frequenciesin the stage V3 andthe transmit-terZ-accordingly functions in the normal manner.A rOn 4cessation-of speech, however, the recvoltageapplied ,tothefgridof the valve K1 will die'away at -a rate depending upon the time constant of the hangover circuit incorporated in" the` .time constant vdevice ATD. This time constant should bek so chosen that ther voltage onthefgrild ofthe u valve,-K1 is maintained during the intervals between words and phrases'but ceaseswhen an appreciable pause occurs.

When the rectified voltage is removed from the grid-pf valve K1' said valve becomes conductive, a! current riiow `occurs through the` resistance Z duefLto'the battery or other source B', and the various stages Yinjthe amplifier will be either biased,.ljl y,potentials supplied,v to said :stages ,from

the leads connected to points P0 and P2 on resistance Z, to cut-off and draw no current or set to draw dead loss anode current thereby acting as absorber valves in a manner similar to that described in United States application Serial No. 520,587 to maintain the load on the main source constant to the `desired degree to improve regulation.

The signal modulated energy appearing in tank Vcircuit 20 may be utilized in any known manner.

It may be transferred to an antenna for radiation, to a load circuit, or to transmission lines.

The drawing illustrates in Figure 2 an arrangement wherein the present invention is applied to telegraphy.

The arrangement may be regarded as a modication of the arrangement shown in Figure l, only such parts being shown in Figure 2 as are necessary to illustrate the nature of the modication.

Referring to Fig. 2, the arrangement of Fig. 1 is modified :by substituting for -the lmicrophone M, microphone transformer MT, and associated amplifier and rectifier and timeycontrol device ATD, va keying relay or equivalent vdevice M', a resistance R, and a battery or other suitable source of potential B". 'I'he action of the arrangement in Fig. 2 is `similar tothe arrangement illustrated in Fig. lof the drawing so far as the suppression of the carrier is concerned, and, as

will be apreciated, the operative diiferencels that the required negative biasing potential on the valve K is obtained, not from speech voltages, but from the source of vpotential B". R is a. limiting resistance of high value. The arrangement illustrated in the Iaccompanying drawing results in a. reduction of key clicksan important improvement more particularly where duplex and twoway telegraphytis in'question-andalso presents the-advantage that a'high speed of telegraphy is possible since the Akeying contacts only have to handle minute currents rand `therefore the said key may be constituted by la high speed relay.

Having thus described our invention and the operationthereof, whatwe claim is:

1. `A modulationsystem of the type in which oscillations 'are produced and applied to a thermionic repeater valve in which they are modulated and in which'the repeater isrenderedinoperative when no modulating- Apotentials are impressed `thereon,'and in which vpreceding stages ofthe transmitter system are biased to draw current when said repeater'is inoperative to thereby regulate the load `applied to .a common -source of 'potential'including a't'hermionic valve having its `anode "to cathode impedance connected in series'with vthe cathode 'to control grid impedance of said 'repeater by way of a potentiometer, a

connected in parallel'with the anode to cathode impedance of4 said thermionicvalve, a rectii'ying device having its output electrodes connected by way of a source of Apotential in parallel to said vpotentiometer, and a source 'of modulating potentials'connectedI to the input electrodes of said rectifyingdeviceand tothe control grid and cathode of lsaid thermionic Y valve.

2. A'system'as recited in claim 1 in which said device, which is conductive in one direction only, comprises the internal impedance of a thermionic valve 'which permits negative `potential to ,flow from saidsource connected with said `potentiometer ytothe con-troly grid-of said repeater, `and i-n=\which a `time constant device is interposed between said source of modulating potentials and the input electrodes of said rectifying device.

3. A device as recited in claim 1 in which a time control device is interposed between said source of modulating potentials and the input electrodes of said rectifying device, and in which said potentiometer is tapped at 'appropriate points to provide biasing potentials for the control grids of valves in the transmitter preceding the repeater valve recited.

4. A modulating system of the type in which oscillations are produced and applied to a thermionic repeater in which they are modulated and in which the repeater is rendered inoperative when no modulating potentials are impressed thereon including, a thermionic Valve having its anode to cathode impedance connected in series with the cathode to control grid impedance of said repeater by way of a portion of the resistance of a potentiometer, a thermionic discharge device which is conductive in oney direction only connected in` parallel with the anode to cathode impedance of said thermionic valve, a second thermionic device having its output electrodes connected in parallel to the resistance of said potentiometer, and a source of modulating potentials connected to the input electrodes of said last named thermionic device and to the control grid and cathode of said thermionic valve.

5. In a signalling system, a thermionic repeater of high frequency oscillations having a control grid and a cathode connected in a circuit to be energized by said oscillations, a, lirst thermionic valve having a control electrode and having its internal impedance, in series with a source of direct current potential connected between the control grid and cathode of said repeater for normally biasing said repeater to a point at which no current flows therein, a source of modulating potentials, a second thermionic valve having a control electrode, a circuit including the impedance of said second thermionic Valve connected between the grid and cathode of said thermionic repeater, a connection between the control electrode of said second valve and said source of modulating potentials for impressing said modulating potentials on said second valve to modulate said repeater, and a connection between said source of modulating potentials and the control electrode of said first thermionic Valve to control the same in accordance with said modulating potentials to overcome the normal bias applied to said repeater.

FREDERICK CHARLES LUNNON. HAROLD JOSEPH HUGHES WASSELL.

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