US2481925A - Pulse modulator - Google Patents

Description

H. R. HEGBAR PULSE MODULATOR 2 Sheets-Sheet l Filed June 29, 1943 Cttomeg .W wN w mwms GQ M .w .W2 Il Il @JW xk NN QN w o o JWSQ A; MN .S ,MN Tl kN bww/Ll mw IFNNw/Ll Sept. 13, 1949. H. R. HEGBAR PULSE MODULATOR 2 Shee'ts-Sheet 2 b PP Imventor ser Filed June 29, 1945 @um NN @N Gttorneg Patented Sept.. 13, 1949 PULSE MODULATGR VHoward R. Hegbar, Princeton,

N. J., assignor to Radio Corporation of America, a. corporation of Delaware Application June 29, 1943, Serial No. 492,761

(Cl. 25o-27) 2 Claims.

My invention relates to the production of electrical pulses and particularly to systems wherein stored energy is to be discharged through a plurality of .parallel connected electric discharge devices such as gas or vapor tubes.

When two or more tubes of the Thyratron type or the like are operated in parallel, there usually is difculty in obtaining simultaneous ignition. If the paralleled tubes do not have identical grid control characteristics and aretriggered with a grid voltage having a nite slope on the leading edge, one tube will fire and thus reduce the plate voltage on the other, which will either iire late or will not fire at all during the cycle.

An object of the present invention is to provide an improved method of and means for obtaining a simultaneous discharge through a plurality of parallel connected electric discharge tubes.

Another object oi the invention is to provide an improved method of and means for obtaining simultaneous ignition of parallel connected gas or vapor tubes.

A further object of the invention is to provide an improved method of and means for producing high voltage electrical pulses in response to the occurrence of control or triggering pulses.

rIhe invention will be described particularly with respect to the modulation of high frequency oscillators such as magnetrons which, in pulseecho apparatus, require large peak power at a small duty cycle. In many applications, particularly in lightweight pulse-echo apparatus, it is desirable that the modulator be operated from a low voltage power supply. In such a modulator a wave-forming electrical network may be discharged through a gas or vapor tube, and the resulting pulse may be put through a pulse transformer to obtain the desired output voltage. When the required peak power output of the modulator exceeds the electrical capability of the gas or vapor tube, either a larger tube must be selected or a means must be provided for parallel operation of two or more tubes.

In accordance with my invention, the desired parallel operation may be obtained by utilizing two circuit actions either alone or in combination, the latter being preferable. One action is that of regeneration or mutual ignition wherein the ignition of any tube or tubes immediately causes an increase in second-grid voltage of the unfiredv tube or tubes To aid this regeneration, the second action may be introduced which consists of maintaining the anode Voltage of the unred tube or tubes at a value many times the magnitude of the static discharge voltage. This sec,

ond action is not obtained when tube anodes and cathodes are directly connected, because then the ignition of one tube will cause the anodelcathode voltage of the rest to fall to the low discharge voltage.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure 1 is a circuit diagram illustrating an embodiment of the invention wherein the fea-Y ture of cross-connection formutual ignition is used alone.

.Figure 2 is a circuit diagram of another embodiment of the invention wherein there is added to the cross-connection feature means for maintaining the anode voltages at values high enough to facilitate ignition of the parallel connected tubes.

Figures 3 and 4 are circuit diagrams of em bodiments of the invention wherein three and four tubes, respectively, are cross-connected to obtain simultaneous ignition, and

Figure 5 is a circuit diagram of an embodiment',

of the invention where a bias is applied to the control grids of the tubes.

In the several iigures, similar parts are indicated by similar reference characters.

Fig. 1 shows an embodiment of the invention comprising a wave-forming network I0 which may be in the form of a low pass lter having series inductance coils II and shunt capacitors I2. The capacitors I2 are charged to a high potential from a direct current source I3 which is connected to the network4 I0 through a choke coil I4. The choke coil I Il has such high impedance at the pulse frequency that this end of the network IU looks into substantially an open circuit.

It is well known that a rectangularfvoltage' are Thyratrons type 2050) and through the pri-I mary I8 of a transformer I9 coupled to the load' to produce the voltage pulses I5. The load in this instance is a magnetron 2| which is to be keyed by the appliedpulses for the production and transmission of pulses of high frequency radio energy. The secondary oi the transformer I 9 may be a bilar winding consisting of two coils 25 and 20a through which heating current is supplied to the filament of the magnetron from a filament heating transformer 25.

The tubes I and I1 each have a cathode, an anode, a control grid and a screen grid. The anodes are directly connected to each other but the cathodes are connected to each other through low impedance elements 22 and 23 in the cathode leads of the tubes I6 and I1, respectively. The elements 22 and 23 preferably are inductors but may be resistors. In the example shown, the inductance coils 22 and 23 each had an inductance of about 0.5 microhenry, the resistance of the coils being of the order of l/oo, ohm.

The tubes I6 and I1 are rendered conducting periodically by applying to their control grids periodically recurring trigger pulses 24 which are supplied from a suitable pulse generator 26. In the specific example being described, the pulses 24 and the output pulses I5 have a repetition rate of 1000 per second. The pulses I5 have a duration of 2 microseconds. The pulses 25 may be applied through a coupling capacitor 21 and through grid- current limiting resistors 23 and 29. Simultaneous ignition of the tubes IS and I1 is obtained by connecting the cathode of tube I6 to the screen grid of the tube I1 through a connection 3l and by connecting the cathode oi the tube I1 to the screen grid of the tube it through a connection. 32. It will be apparent that as soon as one of the tubes ignites the resulting voltage drop in the impedance element in its cathode lead will drive the screen grid of the other tube more positive and immediately cause it to ignite also.

It may be noted that in practice it is preferred to tune the choke coil [4 and the capacitors I2 of the network IIJ to resonateat 1/2 the repetition frequency of the pulses for the purpose of obtaining double the power supply voltage on the network I0 at the time it is discharged.

Referring to Fig. 2, I have shown the use of two wave-forming networks 35 and 31 which are discharged. by the tubes I'I and I1, respectively, both networks being discharged through the primary I 8 of the load transformer I9, This provides the second action previously mentioned, namely, that of maintaining the anode of an unred tube at a comparatively high voltage after another tube has ignited. In addition, it is preferred to utilize the cross-connection 3l and 32 as previously described. The cathode impedance elements may be resistors, as indicated at 22a and 23a. if desired.

Thev advantage in using two networks as shown in Fig. 2 will be evident from the followingr considerations. Taking specic values, by way of example, assume each of the networks 35 and 31 has a characteristic impedance of 33 ohms. Since these network impedances are in parallel with each other when the tubes IB and I1 ignite, the load impedance should be 1%:165 ohms. W e may now determine what the voltage on the anode of tube I1 will be if only the tube I5 ignites.

Let E0 be the voltage to which each of the networks 36 and 31 is charged. Then the current through the load when only tube IB ignites will be andthe drop across the load will be E mil-55mm Ohms=% E.,

The voltage on the anode of tube I1 then will be En* 1A3EI0=%E0. This is a much higher voltage than for the corresponding case when the circuit of Fig. 1 is employed. Because of this, there is less diiiculty in making the tubes I5 and I1 ignite practically simultaneously.

In Fig. 3 the regenerative or cross-connection feature is shown applied to three vapor tubes I5, I1 and 4I connected in parallel. The cathode lead of the tube 4I includes an impedance element 42. A grid-current limiting resistor is indicated at 43. Only one wave shaping network l0 is illustrated but separate networks for the .several vapor tubes may be employed as illustrated in Fig. 2. In order that the ignition of any one tube will apply a positive Voltage to the screen grid of all the other tubes, the screen grids of the tubes I6, I1 and lll are connected to the center points of three potentiometers 46, d1 and i8, respectively, through conductors 49, 5I and 52, respectively. Each potentiometer consists of two resistors in the example illustrated, and is connected between the cathodes of the two tubes other than the one having its screen grid connected thereto. For instance, potentiometer 48 is connected between the cathodes of tubes I6 and I1, and the connection 52 from its midpoint goes to the screen grid of tube 4I.

In Fig. 4 the same regenerative principle is shown applied. to a still larger number of parallel connected tubes. Here there are four tubes I5, I1, 4I and 55 in parallel. The cathode lead impedance element and the grid-current limiting resistor for the tube 53 are indicated by reference characters 54 and 56, respectively. The method of connecting the screen grids to potentiometers is the same as described for Fig. 3, but where there are four tubes each potentiometer consists of three resistors, two of the resistors 4being connected to the cathodes of two tubes as before and the third resistor being connected from the potentiometer midpoint to the Icathode of a third tube. Also, as in Fig. 3, the potentiometer midpoint is connected to the screen grid of the remaining tube.

For example, one potentiometer is indicated at 51 and. comprises resistors 51a, 51D and 51o. The resistors '51a and Eilb are connected between the cathodes of tubes IS and I1, while the resistor 51o is connected between the midpoint of resistors 51e- 51h and the cathode of the tube 4I. The midpoint of potentiometer 51 is connected through a conductor 58 to the screen grid of the remaining tube 53. If any one of the tubes I6. I1 and 4I ignites before the tube 53 ignites, the midpoint of potentiometer 51 will go positive and apply a positive potential to the screen grid of tube 53 so that it will ignite immediately.

The other three-resistor potentlometers 6I, 62

' and 53 are connected in a similar manner to the u, Thus tube cathodes and to the screen grids whereby the screen grid of any tube will go more positive in response to the ignition of any other tube.

the four tubes will ignite substantially simultaneously in response to the application of a triggering pulse.

Fig. 5 shows a cross-connected circuit similar to that of Figs. l and 2 wherein the cross-con nections include capacitors 1I and 12 so that a bias voltage may be applied to the screen grids through resistors 13 and 14. This bias voltage maybe ci negative polarity, for example, to aid in deionization, or a different bias voltage may be appliedv for some other purpose.

I claim as my invention:

1. In combination a plurality of electric discharge tubes of the gas or vapor type, each tube having a cathode, an anode, a control grid and an auxiliary grid, a plurality of impedance elements connected between ground and said cathodes, respectively, a plurality of wave-forming networks, means for connecting said anodes to said networks, respectively, voltage supply means for said networks, a load connected in series with said tubes and said networks, means for applying a voltage appearing at the cathode end of any one of said impedance elements to the auxiliary grids of the other tubes, and means for applying a triggering voltage to said control grids.

2. In combination in a pulse producing circuit, a plurality of electric discharge tubes of the gas or vapor type, each tube having a cathode, an anode and a grid electrode, a plurality of impedance elements connected between a common point and said cathodes, respectively, a plurality of wave-forming networks, means for connecting said anodes to said networks, respectively, voltage supply means for said networks, a load connected in series with said tubes and said networks, means for triggering said discharge tubes at a certain repetition rate, and means for applying a voltage appearing at the cathode end of any one of said impedance elements to the grid electrodes of the other tubes.

HOWARD R. HEGBAR.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Name Date Mettag Aug. 10, 1943 Moyer Dec. 28, 1943 Lord Feb. 5, 1946 Tonks July 30, 1946 Number

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