US2885554A - Pulse generator - Google Patents

Description

y 1959 c. R. KENNY PULSE GENERATOR Filed Oct. 22, 1956 INVENTOR.

CHARLES R. KENNY 7%? ATTOR NEY TIMING PULSE SOURCE Unie PULSE GENERATOR Charles R. Kenny, Purdys Station, N.Y., assignor to General Precision Laboratory Incorporated, a corporation of New York Application October 22, 1956, Serial No. 617,348

11 Claims. (Cl. 250-36) This invention relates to pulse generating circuits, particularly to such circuits which are required to cause a rectangular current pulse to pass through a load device which has reactive components.

Pulse generators are well known in the prior art and include those in which pulses are formed with the aid of an artificial line and those in which a reactance is first charged and then discharged through the load. Such pulse generators are capable of generating substantially rectangular voltage pulses which, if applied to a resistive load, will cause substantially rectangular current pulses to flow. However, if the load contains reactive components, the charge accumulated during the pulse causes current to flow through the load even though the voltage of the pulse generator is returned abruptly to zero.

An example of a reactive load through which it is often desired to pass a rectangular current pulse is the familiar microwave magnetron widely used in radar equipment. In such equipment a substantially rectangular timing pulse is applied to a pulse generating circuit which in turn applies a pulse to the magnetron causing it to generate pulses of microwaves. The microwave energy is radiated into space, reflected and received by a receiver. The receiver is gated ofF during transmission and should be gated on again as soon as possible after transmission has ceased in order to receive echoes for as large a portion of the cycle as possible. Ideally the starting and stopping of the generation of oscillations by the magnetron should be coincident with the leading and trailing edges respectively of the rectangular timing pulse applied to the pulse generating circuit in which case the receiver off time may be limited to the duration of the timing pulse. However, in practice the magnetron may continue to oscillate for a substantial period of time after the trailing edge of the rectangular timing pulse has passed because the voltage between magnetron electrodes persists instead of falling sharply to zero. This persistence of the voltage is caused by the stray capacitance between electrodes which becomes charged during the pulse period and which discharges through the magnetron after the pulse has passed, thus prolonging the generation of oscillations and making it necessary to extend the receiver off time beyond the duration of the timing pulse.

An object of the invention is to cause a substantially rectangular pulse of current to traverse a reactive load.

Another object of this invention is to counteract the effects of the reactance of a load circuit to which pulses are applied.

A more specific object of the invention is to reduce the time required to terminate the operation of a magnetron oscillator.

Another object of the invention is to make the shape of the envelope of the oscillations of a magnetron more nearly rectangular.

Yet another object of the invention is to render a magnetron operative for a period of time more nearly 2,885,554 Patented May 5, 1959 equal to the duration of a timing pulse applied to a pulse generating circuit.

A subsidiary object of the invention is to reduce the operating voltage between the electrodes of a magnetron to zero quickly.

Briefly stated, the invention comprises a switch, such as an electronic tube, connected in parallel with the load. During pulses from the pulsing circuit the switch is non-conductive and therefore does not affect the usual mode of operation. Upon termination of the pulse from the pulse generating circuit, the switch is rendered conductive thereby providing a low impedance path through which the charge on the reactive components is quickly dissipated.

For a clearer understanding of the invention reference may be made to the following detailed description and the accompanying drawing in which:

Figure 1 is a schematic diagram of one embodiment of the invention;

Figure 2 is a group of curves useful in explaining the invention;

Figure 3 is a schematic diagram of another embodiment of the invention; and

Figure 4 is a schematic diagram of yet another embodiment of the invention.

As an illustrative example, the invention will be described as applied to a pulsed magnetron circuit although it will be understood that the invention is also applicable to other types of load circuits.

Referring now to Fig. 1 there is shown a magnetron 11 having a grounded anode 12 and a cathode 13 connected to a conductor 14. The magnetron has associated with it stray capacitance indicated by dotted lines on the drawing and denoted by the reference character 15. A source 16 of rectangular voltage pulses, for the purpose of timing the desired duration of oscillation of the magnetron 11, is connected to the grid 17 of a pulser tube 18. The tube 18 is shown as a tetrode and its cathode '19 is grounded while its anode 21 is connected through an inductor 22 to a source of positive voltage which may for example be on the order of 4,000 volts.

' The anode 21 is also connected to one plate 23 of a capacitor 24 the other plate 25 of which is connected through a resistor 26 to the conductor 14.

The circuit so far described, ignoring for a moment the resistor 26, operates in a conventional manner. During the interval between pulses the capacitor 24 is charged until its plate 23 is at substantially the potential of the positive voltage supply, since the tube 18 at this time is biased to out 01?. The charging circuit includes the conductor 14 and a diode 27 having its anode 28 connected to the conductor 14 and its cathode 29 grounded. The plate 25 of capacitor 24 at this time is at substantially ground potential.

A pulse from the timing source 16 renders the tube 18 conductive suddenly driving the potential of the plate 23 of the capacitor 24 to ground potential which causes the plate 25 to drop to --4,000 volts. The capacitor 24 will therefore discharge through the magnetron 11 cansing it to oscillate vigorously. When the pulse from timing source 16 has passed, the tube 18 will again become nonconductive. The capacitor 24 will of course cease dicharging immediately and recharging will start to replace the charge lost during the pulse period. However, the magnetron 11 will not stop oscillating immediately since, during the pulse period, the stray capacitance 15 will have become charged and will now discharge through the magnetron allowing oscillations to continue.

Figure 2 illustrates the action at this time. The curve 31 represents the timing pulse from the source 16 while the curve 32 represents the potential of the cathode 13 of the magnetron 11. At the end of the pulse period the potential of cathode 13 does not return to zero immediately but returns slowly as indicated by the dotted curve 33 because of the discharge of the stray capacitance 15.

In orderto reduce thetime required to terminate-the oscillation of the magnetron, the invention employs a triode 35 having its anode 36 connected to the anode 12 and its cathode 37 connected to the conductor 14. The grid 38 is connected through a resistor 39 to the junction of plate 25 and resistor 26. The resistor 39 is shunted by a capacitor 41.

During the pulse interval, the discharge circuit of capacitor 24 includes the resistor 26 and the voltage drop across resistor 26 biases the tube 35 to cut off so that it has no effect at this time. At the termination of the pulse, capacitor 24 starts to recharge thereby reversing the direction of the current flow through resistor 26 and rendering tube 35 conductive. The capacitor 15 may now discharge through the low impedance of tube 35 instead of through the magnetron 11 and its charge is quickly dissipated thereby terminating the generation of oscillations by the magnetron 11.

Although the circuit of Fig. l operates satisfactorily, the circuit illustrated by Fig. 3 has been found to be more effective. The circuit of Fig. 3 is similar to that of Fig. 1 and like parts are denoted by the same reference characters. In Fig. 3 there is shown a transformer 46 having three windings 47, 48 and 49. The plate 25 of the capacitor 24 is connected through the winding 47 to the conductor 14 so that both the charge and discharge circuits include winding 47. Winding, 47 is shunted by a pair of diodes 51 and 52 poled so as to be conductive for discharge currents from capacitor 24. The triode 35 has its anode 36 connected to the anode 12 of the magnetron as before but its cathode 27 is connected to the same terminal of Winding 47 as is connected to the plate 25 of capacitor 24. The winding 48 has one terminal connected to the cathode 37 and the other terminal connected to the grid 38.

When a pulse from the timing source 16 renders tube 18 conductive, the capacitor 24 discharges, most of the current passing through diodes 51 and 52 to the magnetron causing it to oscillate. Part of the discharge current passes through winding 47 thereby inducing a voltage in winding 48 which biases tube 35 to cut 01f. At the termination of the pulse, the tube 35 is rendered conductive by a combination of two factors. As before, the capacitor 24 starts to charge and the charging current which flows through winding 47 induces a voltage in winding 48 which biases the grid 38 positive. At the same time, the stray capacitance 15 discharges through the tube 35 and this discharge current also flows through the winding 47 in the same direction as the charging current for capacitor 24 and assists in inducing a voltage in winding 48 which renders tube 35 conductive. Thus, as long as there is substantially any charge left on the stray capacitance 15, the tube 35 will remain conductive thus permitting the complete and rapid discharge of capacitor 15.

There is also shown in Fig. 3 a resonant circuit comprising the winding 49 which is shunted by capacitor 54. One terminal of winding 49 is connected to the grid 38 of the triode 35. The action of this resonant circuit is not completely understood but it has been found to be beneficial. During the pulse period, when tube 35 is cut off, the resonant circuit is thought to have little or no effect. However, upon termination of the pulse, it is believed that the energy stored in the resonant circuit assists in maintaining tube 35 conductive for lower stray capacity potentials than would otherwise be possible. In any event, its use has been found toreduce-the time required to terminate oscillation of the magnetron 11.

Figure 2 also shows the advantages of the present invention. In a circuit such as that illustrated in Figs. 1 and 3, omitting the tube 35 and its associated circuit, it

was found that when a one microsecondtiming. pulsewas applied that oscillations of the magnetron persisted for 2 /2 microseconds after the passage of the pulse while with the invention as illustrated in Fig. 3 the cathode potential of the magnetron returned to zero as shown by the solid curve 56 and oscillations were terminated 0.4 microsecond after the passage of the timing pulse.

Figure 4 shows a simplified version of the invention which has been found to operate very satisfactorily. In

this embodiment, the winding 49 and the capacitor 54 are not used and the diodes 51 and 52 are replaced by a resistor 61 shunting the winding 47. The secondary winding 48 is connected between grid 33 and cathode 37 as in Fig. 3, and the remainder of the circuit is identical to that of Fig. 3. The resistor 61 of course reduces the current through winding 47 both during pulses and during the interval between pulses but a proper selection of the turns ratio between windings 48 and 47 enables tube 35 to be rendered conductive promptly at the end of the pulse period. In one specific-embodiment a turns ratio of 2:1 has been found satisfactory.

Although specific embodiments of the invention have been described, many modifications will occur to those skilled in the art. It is therefore desired that the invention be limited only by the true scope of the appended claims.

What is claimed is:

1. A pulse generating circuit comprising, a capacitor, a network, a charging circuit for said capacitor including said network, a load, a discharge path for said capacitor including said capacitor, said network and said load connected in series, means for discharging said capacitor periodically through said discharge path, switch means,

a discharge circuit for said load including said switch means, and means for controlling the conductivity of said switch means in accordance with the direction of current flow through said network.

2. A pulse generating circuit comprising, a capacitor, 2. network, a charging circuit for said capacitor including said network, first switch means, means for rendering said first switch means conductive periodically, a reactive load, a discharge circuit for said capacitor comprising the series combination of said capacitor, said first switch means, said load and said network, second switch means connected to provide a discharge path for energy stored in any reactive components of said load, and means for controlling the conductivity of said second switch means in accordance with the direction of current flow through said network.

3. A pulse generating circuit comprising a capacitor, a transformer having first and second windings, a charging circuit for said capacitor including said first winding, first switch means, means for rendering said first switch means conductive periodically, a load, a discharge circuit for said capacitor including said first winding, said load and said first switch means, second switch means connected to provide a discharge path for energy stored in any reactive components of said load, and means for controlling the conductivity of said second switch means in accordance with the voltage of said second winding.

4. Apparatus according to claim 3 further comprising a resistor connected across said first winding.

5. Apparatus according to claim 3 further comprising unidirectional conducting means connected across said first winding.

6. Apparatus according to claim 5 in which said second switch means is an electronic tube having a cathode, an anode and a control electrode, and in which the terminals of said second winding are connected to said cathode and said control electrode respectively.

7. Apparatus according to claim 6 further comprising a third winding on said transformer, a capacitor shunting said third winding, and a connection from one terminal of said third winding to said control electrode.

-8. A source of pulsed microwave oscillations comprising,z.a.magnetron,. a capacitor, a transformer having firstand second windings, a charging circuit for said capacitor comprising a voltage source, an inductor, said capacitor, said first winding of said transformer and a diode, a first electronic tube having a cathode, an anode and a control electrode, said first tube being adapted to be rendered conductive periodically by the application of a series of voltage pulses to its control electrode, a discharge circuit for said capacitor comprising said capacitor, said first winding, said magnetron, and the cathode-anode circuit of said first tube, a second electronic tube having a cathode, an anode and a control electrode, an auxiliary circuit in parallel with said magnetron, said auxiliary circuit including the anode-cathode circuit of said second tube and said first winding, and means for connecting the two terminals of said second winding of said transformer to the cathode and control electrode of said second tube.

9. Apparatus according to claim 8 further comprising a resistor connected across said first winding.

10. Apparatus according to claim 8 further comprising a unidirectional conducting device connected across said first winding.

11. A source of pulsed microwave oscillations comprising, a magnetron, a first capacitor, a transformer having first, second and third windings, a charging circuit for said first capacitor comprising a voltage source, an inductor, said capacitor, said first winding of said transformer and a diode, a first electronic tube having a cathode, an anode and a control electrode, said first tube being adapted to be rendered conductive periodically by the application of a series of voltage pulses to its control electrode, a discharge circuit for said first capacitor comprising said first capacitor, said first winding, said magnetron, and the cathode-anode circuit of said first tube, a second electronic tube having a cathode, an anode, and a control electrode, an auxiliary circuit in parallel with said magnetron, said auxiliary circuit including the anode-cathode circut of said second tube and said first winding, means for connecting the two terminals of said second winding of said transformer to the cathode and control electrode of said second tube, a unidirectional conducting device connected across said first winding, a second capacitor connected across said third winding, and means for connecting one terminal of said third winding to said control electrode of said second tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,415,302 Maxwell Feb. 4, 1947 2,431,952 Maxwell Dec. 2, 1947 2,562,450 De Lano July 31, 1951

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