GB2122444A - Pulse generating circuit - Google Patents

Abstract

A pulse generating circuit includes a pulse forming network (10) of the delay line type, and a power supply (11) having a resonant charging circuit operable to charge the network. A first switch (14) is connected in series with a load (12) across the pulse forming network (10), and a second switch (15) is connected in parallel with the load (12) to terminate the pulse when required. One of the first and second switches must be of a type which permits only the unidirectional flow of current through it. The circuit stores unused energy thus permitting a reduction in the power supply voltage. <IMAGE>

Description

SPECIFICATION Pulse generating circuit This invention relates to pulse generating circuits, and in particularto such circuits in which a pulse forming network or delay line is charged from a supply and subsequently discharged through a load to generate a pulse.

Pulse generating circits of the above type are known which use an artificial delay line or pulse forming network built up from inductors and capacitors which simulatethe properties of a delay line. Such circuits have been used for many years for generating pulses by providing a discharge path from the pulse-forming networkthrough a matched load. In addition various proposals have been madeforterminating the pulse so produced to give a pulse of controllable and variable duration. Itis possiblewhenterminating such a pulse to return a proportion ofthe undissipated energy to the power supply.

It is an object of the invention to provide a pulse generating circuit for providing a variable-width pulse from a fixed length pulse-forming network, and to store unused energy on the pulse-forming networkso thatthere is a significant reduction in the required driving voltage ofthe primary power supply.

According to the present invention there is provided a pulse generating circuitwhich includes a pulse forming network ofthe delay line type, a power supply including a resonant charging circuit operableto chargethe pulse forming network to a required voltage, a first switch connected in series with a load across the pulse forming network and operable to initiate an energy pulse through the load, and a second switch connected in parallel with the load and operable to terminate the energy pulse, at least one of thefirstand second switches being a device permitting only unidirectional current flow through it.

The invention will now be described with reference to the accompanying drawings, in which: Figure lisa schematic circuit diagram of a first embodiment; and Figures 2 and 3 show similar diagrams of alternative embodiments ofthe invention.

Referring nowto Figure 1, a pulse forming network 10 is shown as an arrangement of inductors Land capacitors C. The network may be charged to a required potential of the necessary polarity by a charging circuit 11 having a resonant charging action.

This charging circuit will not be described in detail since such circuits are well known. The load 12 is connected between one terminal ofthe network 10 and a common line 13, which may be earthed. Afirst switch 14 is connected between the other side ofthe network 10 and the common line 13,whilsta second switch 15 is connected in parallel with the load 12. At least one ofthetwo switches must be of a type which, when closed, will only allow currentto flow in one direction.

The load will usually be of relatively low impedance compared with the dynamic impedance ofthe re sonant circuit, in which case when the network 10 is charged from the supply 11 the charging current will flowthrough the load if switch 15 is open. If the load is not resistive, or is of high impedance, the switch 15 may be closed during charging ofthe network. In this Iattercasetheswitch 15 will be opened oncethe network is charged.

An energy pulse through the load 12 is initiated by closing the switch 14, causing the network to dis charge through the load. The direction of currentflow during this discharge is determined by the polarity of the charging network. The energy pulse in the load 12 is terminated prematurely by closing switch 15. With a short circuit across the network,the energy still present in the network causes a surge if current and a consequent reversal of voltage polarity on the net work capacitors. This process is nearly lossless. Since at least one of the two switches 14 and 15 will only permit current flow in one direction, the reverse charge of the network cannot be dissipated since this would require currentto flow in the opposite direction.

The power supply 11 then re-charges the network as before.

Ifthe network has a long pulse length compared with the required pulse duration, then a large propor tion of the energy stored in the network is retained, thus reducing the inputvoltage required by the resonant charging circuit. If the supply voltage is V5, and the voltage remaining on the network is VR, then the voltage Vc to which the network charges is given by VC=2V5+VR Hence for a particularvalue of Vc, V5 is reduced as VR is kept large. By way of example, if the pulse length of the network is twice the required pulse duration, then the su pply voltage is reduced to about 15% of that necessary if all the stored energy is dissipated.Clearly the voltage stored depends upon the pulse duration, and hence the powersupplyvoltage must be arranged to meet the charging requirements afterthe longest pulse required.

The circuit of Figure 1 shows the arrangement which is necessary if both the load and the switch 14 require to have one side earthed. For othertypes of load and switch which do not havethis requirement, slightly different circuit arrangements may be used within the scope of the invention. Figure 2 shows the case where I only the load has one side connected to the common line 13, otherwise the circuit is exactly the same as that of Figure 1.

The switches 14 and 15 may take several different forms, so long as at least one of them is able to pass only a unidirectional current as stated above. Switch 14 may, for example, be a transistor, a thyristor or a thyratron, all of which satisfy the above requirement.

Similarly switch 15 may be a thyristor, a thyratron, or a saturable reactor. Figure 3 illustrates the embodiment referred to above with a magnetron oscillator as the load 15, a thyristor as switch 14, and a saturable reactor as switch 15.

Switch 15 is the critical component, in that its operation determines the precise duration of the pulse. Clearly some form of timing and control circuit is necessaryto operate the two switches. If switch 15 hasto be closed to allowthe networkto charge, then it must be of a type which allows currentflow in either direction, sincethe charging current and the discharging currentwill flow in opposite directions.

Claims (7)

1. Apulsegenerating circuitwhich includes pulse forming network of the delay line type, a power supply including a resonant charging circuit operable to charge the pulse forming networkto a required voltage, a first switch connected in series with a load across the pulse forming network and operable to initiate an energy pulse through the load, and a second switch connected in parallel with the load and operable priorto the end ofthe pulse generated by the pulse forming network to terminate the energy pulse, at least one of the first and second switches being a device permitting only unidirectional current flow through it such that energy which is not dissipated in an energy pulse is retained in the pulse forming networktoform part ofthe energy available fora subsequent energy pulse.

2. Acircuitasclaimed in Claim 1 in which the power supply is connected to the junction between the first switch and the load.

3. Acircuitas claimed in eitherof Claims 1 or2 in which the or each switch permitting only the unidirectional flow of current is a thyristor.

4. Acircuitas claimed in any one of Claims 1 to 3 in which one ofthe first and second switch permits the bidirectional flow of current.

5. Acircuitas claimed in Claim 4 in which said one switch is a saturable-core reactor.

6. Acircuit as claimed in any one of Claims 1 to 5 in which the load is a magnetion oscillator.

7. A pulse generating circuit substantially as herein described with reference to any one ofthe accompanying drawings.