GB2135547A - Pulse circuits - Google Patents

Abstract

A pulse circuit for use with a radar transmitter is arranged so that a sequence of very closely spaced radar pulses can be generated, with the interpulse spacing being shorter than the normal recovery periods of the associated pulse generators. A number of separate pulse generators are linked to a magnetron via a common pulse transformer; a fast acting diode 10 in each pulse generator isolates it from the remaining pulse generators, and the charge left on the capacitance 20 of the magnetron at the cessation of a pulse is used to enable the diode to revert rapidly to its blocking state. This discharge of stray capacitance also helps the magnetron to recover more quickly. Winding 13 is used to reset the saturable reactor 11 when its magnetic flux becomes saturated. <IMAGE>

Description

SPECIFICATION Pulse circuits This invention relates to pulse circuits which are capable of generating high power pulses of short duration. A circuit of this kind can be used to provide the operating power for a high power oscillator such as a magnetron which forms part of a radar transmitter operating at radar frequencies. Such a pulse circuit is sometimes termed a radar pulse modulator. A radar transmitter radiates pulses having a very low mark to space ratio; that is to say, transmitted short pulses are usually spaced apart in time by relatively long intervals during which echoes of the pulses are returned by intercepted radar targets to a radar receiver.The useful range of a radar is related to the power transmitted during the short pulse periods and it is therefore very important to maximise the power of the pulses whilst ensuring that the pulses turn on and turn off cleanly without the generation of excessive noise.

Although it is possible to meet these requirements with the transmission of a single pulse followed by a long inter-pulse period during which the pulse modulator can recover and reset before the transmission of the next pulse, certain radar applications require the transmission of a closely spaced burst or group of pulses, and it is very difficult to satisfactorily transmit such a burst or sequence of very closely spaced high power pulses even though each burst is well separated in time from adjacent bursts.

The present invention seeks to provide a pulse circuit in which this difficulty is reduced.

According to this invention a pulse circuit includes a common pulse transformer arranged to receive a sequence of low voltage high current pulses in turn from a plurality of respective pulse generators and to transform them into high voltage pulses for utilisation by a load; means associated with each pulse generator for determining the duration of the pulse which it generates; a diode associated with each pulse generator which serves to isolate it from the other pulse generators and which is arranged to at least partially cancel residual charge present on the stray load capacitance on the cessation of the pulse which it generates.

In practice the load may be a high power oscillator, such as a magnetron which converts each pulse into a series of microwave oscillations.

When a magnetron is turned off, so as to terminate the microwave pulse which it generates, the magnetron and its associated stray capacitance retain a certain amount of electric charge, and the magnetron cannot usually be satisfactorily reused until the charge has been removed or allowed to leak away. Furthermore, the pulse generator requires a finite time to switch off, recover and be recharged. For this reason it has proved difficult to generate, with a single pulse generator, a burst or sequence of pulses in which the pulses follow each other very rapidly. In the present invention it is arranged that the load and several pulse generators interact so as to transfer-charge from the load to the pulse generator on the cessation of a pulse.This has the effect of permitting the load to be reused very rapidly by other pulse generators, and the transferred charge is used to neutralise the charge stored on an output diode of the pulse generator so that it can very rapidly recover its blocking state and present the required high impedance to the pulse transformer. In this way, all of the pulse generators can utilise the same pulse transformer, and this provides a very great economy particularly if each burst contains a significant number of individual pulses. The transformer has a magnetic core material which is capable of supporting the entire required burst of pulses without the need to reset. Subsequently, the magnetic core material is reset by the application of a suitable pulse to a bias winding of the transformer before the next sequence of pulses is generated.

The invention permits a sequence of well defined high power pulses to be generated with each pulse in the sequence having fast rising and falling edges which are clean so that each pulse in the sequence is distinct and does not merge into adjacent pulses. As each pulse in the sequence is generated by a separate pulse generator the length of the sequence can be increased or decreased by inserting or removing pulse generators (or by energising only a selected number of the available generators). Furthermore it is quite feasible to generate different pulse widths and/or amplitudes from each pulse generator.

The invention is further described by way of example with reference to the accompanying drawings in which Figure 1 shows a pulse circuit in accordance with the invention and Figure 2 is an explanatory diagram.

Figure 1 shows those parts of a radar transmitter which are relevant to an understanding of the present invention. The radar transmitter transmits a burst or sequence of very short pulses having a very high carrier frequency, which is usually in the microwave band. In this example each burst or sequence of pulses consists of four pulses each having a duration of 0.5zbS with 1 yS spacing between them. To achieve such a rapid burst of pulses four separate pulse generators 1, 2, 3 and 4 are connected via a common transformer 5 to a magnetron 6. All four pulse generators are identical and derive their power from a common power supply 7 operating at, typically, 600 volts D.C. Each pulse generator generates a single pulse having the required shape and duration and an amplitude of about 600 volts.The voltage level of each pulse is transformed by the transformer 5 up to about 30,000 volts so that it can be used to directly drive the magnetron 6. As is known a magnetron is a relatively efficient and satisfactory generator of microwave power, but it requires the application of a very high operating voltage. The magnetron is such as to oscillate at microwave frequencies whenever a sufficiently high voltage is applied to it and the shape of the pulses which it generates and the efficiency with which they are generated are primarily dependant on the nature of the pulses generated at the pulse generators 1 to 4 and the way in which they are transformed from a low voltage to a high voltage by the transformer 5. Typically the magnetron 6 is ultimately connected to an antenna at which the generate pulses are radiated for radar or other purposes.

Referring to Figure 1 in more detail, each pulse generator contains a pulse forming network 8 which is represented in conventional manner as an inductor with distributed capacitance. The pulse forming network 8 is connected in series with a switch 9, which in practice is an electronic device such as a thyristor. The switch 9 is connected in series with the primary winding 16 of the transformer 5 via a diode 10. A saturable reactor 11 is connected in series with a further diode 12, and the combination of saturable reactor 11 and diode 12 are connected in shunt with the previously mentioned diode 10 and the primary winding 1 6. A small winding 1 3 is provided for resetting the saturable reactor 11 when its magnetic flux becomes saturated.The secondary winding 18 of the transformer 5 is connected to the magnetron 6 as shown, and a diode 14 is connected in series with a resistor 1 5, both being in shunt with the magnetron 6.

In operation all four pulse forming networks 8 are charged from the power supply 7 whilst respective switches 1 9 are rendered conductive and then sequentially discharged by closing the respective switches 9 in a predetermined sequence. The resulting sequence of relatively low voltage pulses are applied in turn to the transformer 5. The sequence of events is more clearly illustrated in Figure 2 in which the voltage stored on a charged pulse forming network is represented by the value E1 for the pulse forming network 1. Similar notation is used for the remaining three pulse generators 2, 3 and 4. It will be seen from the wave forms that the pulse forming networks are charged sequentially, commencing at instants t1, t2, t3 and t4 .This is merely to ease the power requirements of the power supply 7, and the instants at which the charging of the pulse forming networks commence is not critical.

Whilst the pulse forming network 8 is being charged the switch 9 is open so as to be in its non-conductive state. When the switch 9 is closed at time t5, the pulse forming network is very rapidly discharged through the diode 10 and the primary winding 1 6 of the transformer 5. The saturable reactor 11 is provided to cleanly terminate the pulse after a predetermined interval, has elapsed and it is arranged to saturate at time t6. The saturable reactor 11 initially exhibits a relatively high impedance as it behaves as a conventional inductor but when it saturates, its impedance drops very rapidly and it then behaves as a low impedance so that it in effect almost short circuits the output of the pulse generator 1, and consequently the pulse on the secondary winding 1 8 decays.

A reset pulse is subsequentially supplied to the winding 13 to reset the magnetic state of the saturable reactor to its initial condition, so that it is ready to operate when the next sequence of pulses is required.

Whilst the low voltage pulse is present on the primary winding 1 6 of the transformer 5 a very large current is flowing through that winding and the transformer acts in conventional manner to produce a corresponding very high voltage pulse having moderate current which causes the magnetron 6 to go into oscillation. When the pulse terminates due to the action of the saturable reactor 11, radio frequency oscillation of the magnetron will substantially cease when the voltage falls just slightly below the threshold value, and the remaining energy in the output current, is in the stray capacitance 20, which is charged to the threshold value. The internal capacitance is represented by the capacitor 20.

Normally this charge can decay fairly slowly during a relatively long interpulse period in a conventional radar without causing any difficulty.

However in the present application, this is not possible since both the pulse generator and the magnetron must be restored to their initial condition very rapidly so that the magnetron responds to the next pulse from the pulse generator 2. When the pulse is rapidly terminated due to the saturation of the reactor 11 the charge is stored in the stray capacitance 20 can be used to ensure very rapid forward recovery of the diode 1 0. Because diodes inevitably store charge to a greater or lesser extent, even if they are nominally very fast recovery diodes, this process would normally take some little time.The coupling of the diode 10 to the magnetron 6 by the transformer 5 enables the charge stored on the magnetron 6, to be automatically used to recover the charge retained by the diode 1 0. This action not only permits the magnetron to receive the next pulse from the pulse generator 2 with very little delay, but it also permits the diode 10 to revert to its high impedance state very quickly. This means that the pulse generator 1 is not affected by, and does not distort, the pulses which are subsequentially generated by the remaining pulse generators 2, 3, and 4.

The diode 14 and the resistance 1 5 are used to minimise back swing voltage associated with the rapid discharge of the stray capacitance 20. The diode 12 is required to allow the core of the reactor 11 to be reset without influencing the operation of other pulse generators 2, 3, 4 or the pulse transformer 5.

Each generator may contain a number of pulse forming networks connected in parallel to increase the power of each pulse. Although each such network may have its own switch 9, and reactor 11, it is preferred to use a single common diode 10 within a given pulse generator.

In this example all of the pulse generators are identified, and the action of each is the same and the net result is that the magnetron 6 is able to generate four very closely spaced pulses as is represented at the bottom line of Figure 2. The pulses can be spaced much more closely together than would be possible if a single pulse generator were used, and it was fully recycled between pulses.

Claims (7)

1. A pulse circuit including a common pulse transformer arranged to receive a sequence of low voltage high current pulses in turn from a plurality of respective pulse generators and to transform them into high voltage pulses for utilisation by a load; means associated with each pulse generator for determining the duration of the pulse which it generates; a diode associated with each pulse generator which serves to isolate it from the other pulse generators and which is arranged to at least partially cancel residual charge present on the stray load capacitance on the cessation of the pulse which it generates.

2. A pulse circuit as claimed in claim 1 and wherein each pulse generator includes a reactive pulse forming network arranged to be charged periodically, the low voltage high current pulse being generated when the network is rapidly discharged.

3. A pulse circuit as claimed in claim 2 and wherein said diode is connected so that the low voltage high current pulse flow through it when its associated pulse forming network is discharged.

4. A pulse circuit as claimed in claim 3 and wherein said diode is connected in a series path between said pulse forming network and the primary winding of said transformer.

5. A pulse circuit as claimed in any of the preceding claims, and wherein said means for determining the duration of the pulse comprises saturable reactor effectively connected in shunt with the primary winding of the transformer.

6. A pulse circuit as claimed in claim 5 and wherein a diode is connected in series with said saturable reactor, so that the resetting of the reactor does not adversely affect other pulse generators and said pulse transformer.

7. A pulse circuit substantially as illustrated in and described with reference to Figure 1 of the accompanying drawings.