GB2382249A

GB2382249A - High power pulse generation

Info

Publication number: GB2382249A
Application number: GB9207846A
Authority: GB
Inventors: Trevor Howard Robinson; Paul Anthony James Garner; Christopher John Hodge
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1992-04-07
Filing date: 1992-04-07
Publication date: 2003-05-21
Anticipated expiration: 2012-04-07
Also published as: GB2382249B; GB9207846D0

Abstract

A system for generating a series of ultra fast, high voltage pulses comprises one or more high voltage pulse generators and a plurality of output lines each terminating at an antenna. Delay means are provided for spacing apart the pulses emitted from the or each antenna so that the spectrum of the series of pulses has a predetermined shape. The system may be used in a radar system to interrogate a plane or missile to reveal its identity or as a weapon in which the pulse sequence is selected so that the energy is concentrated to couple efficiently with the internal circuitry within a target.

Description

HIGH POWER PULSE GENERATION The invention relates to the generation of high power pulses for use, for example, in a radar system to interrogate an object such as a plane or missile in order to reveal its identity.

This invention provides a system for generating a series of ultra fast, high voltage pulses comprising one or more high voltage pulse generators, a plurality of output lines each terminating at an antenna; and delay means for spacing apart the pulses emitted from the or each antenna so that the spectrum of the series of emitted pulses has a predetermined shape.

Different types of planes and missiles tend to have differing resonant frequencies or frequency bands-i. e. they tend to interact with and reflect some frequencies much better than others. In a conventional radar system, a single frequency C. W. or, more usually, pulsed C. W. signal is emitted and differing objects are detected and tracked by the combined reflection of that single frequency, or narrow band, signal. In a system according to the invention, a series of ultra-short pulses can be emitted whose spectrum

can be selected to suit the object which it is attempting to identify.

By ultra-fast we mean that a half-cycle of the emitted pulses has a width typically of between 100 pS-1 nS. By high voltage, We mean a voltage typically from tens of kV to greater than 1 MV, and having peak powers from a few MW to greater than 10 GW. The delay between emitted pulses may vary, but is typically between 100 pS and 20 nS so that the spectrum of the series of pulses has main peaks at anywhere between 100 MHz and a few GHz. It is to be understood that the invention is not intended to be limited to the values indicated above.

It is much preferred that the delay means is adjustable so that spectrum of the series can be varied.

In one embodiment a common high voltage pulse generator is operative to provide a main pulse which is divided amongst a plurality of output lines, a separate delay means being provided in each output line.

Such an arrangement has an advantage in that the level of jitter, or uncertainty in the timing of different pulses in the series, is low because each has come from a common

source. The delay means may comprise, in such a case, a transmission line whose effective length can be adjusted.

The means for generating the main high voltage pulse may comprise a primary circuit for generating a relatively long low voltage pulse, secondary circuit means comprising a dielectric filled pulse forming line connected to an electrode of a dielectric switch and which is operative to store the energy from the relatively long low voltage pulse and to discharge the energy across the dielectric switch when the voltage reaches a predetermined level, thereby to produce a main high voltage pulse for supply to the plurality of outputs.

In an alternative embodiment, a separate high voltage pulse generator is provided for each output line, each generator being connected to a common control means operative to trigger each generator when required, whereby the delay between pulses on different lines can be selected.

Each generator in such a case must be of a type having low levels of jitter, otherwise the pulses will not combine together to form a series having the desired spectral characteristics.

In another aspect the invention provides a method of generating a series of ultra fast, high voltage pulses and including the steps of passing a pulse along each of a plurality of output lines which terminate at an antenna, and selecting the delay between different pulses on different output lines so that the spectrum of the series of the pulses has a predetermined shape.

The method may include the step of selecting the delay between different pulses so that the spectrum has a peak substantially coincident with a frequency or frequency band which will give a return characteristic of a target. The method may further include the steps of sequentially adjusting the delays on each of the output lines so as to emit a plurality of series of pulses, each series having a spectrum of a differing shape.

In order that the invention may be better understood, various embodiments thereof will now be described with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a schematic block diagram of a system according to one embodiment of the invention;

Figures l (a) and (b) show schematically pulses at various parts of the system of Figure 1.

Figure 2 is a schematic view of an adjustable delay for use in the system of Figure 1 ; Figure 3 is a schematic block diagram of a system according to another embodiment of the invention ; and Figures 4a-h show typical series of pulses that can be generated from a system according to the invention, together with their respective frequency spectra.

The system shown in Figure 1 comprises a primary circuit 1 including a thyratron circuit 2 for producing relatively long pulses, typically of a few hundred nanoseconds duration, as is shown in FIgure 1 (a) and of a voltage of 50-lOOkV. The transformer 3 steps up the voltage of the pulse to a level of, say, 500kV and this is then sent to a secondary circuit 4 and used to charge up a dielectric filled pulse forming line 5. This may comprise a tube coaxially mounted within a tank filled with a dielectric such as oil or water. When sufficient energy has been stored within the pulse forming line and the voltage is at a sufficiently high level, the pulse discharges between the

electrodes (not shown) of a high voltage dielectric switch 6. The dielectric may be oil pumped through the switch or high pressure gas. The pulse at this stage typically has a width of a few nanoseconds, or less, the characteristics of which are shown in Figure l (b). After being discharged across the switch, the pulse is divided by the multi output circuit 7 and emitted on a plurality, as shown six, of output lines 8. Each output line 8 includes an adjustable delay 9, a ferrite loaded shock line 10 (optional) and terminates at a TEM horn or antenna 11.

The adjustable delay 9, as shown schematically in Figure 2, comprises a trombone or a pair of concentrically arranged tubes 20,21 slidably received one within the other, whereby the effective length can be adjusted and thereby the length of time taken for a pulse to travel along the arrangement. As shown, the outermost tube 21 includes a rack 22 connected to a pinion 23 of a stepper motor 24. The trombone may be replaced by any appropriate transmission line arrangement, whose effective electrical or mechanical length can be adjusted. It is envisaged that time delay may also be accomplished electrically using an arrangement similar to a shock line. Each adjustable delay 9 is connected to a control 12, such as a microprocessor or logic based circuit, operative to adjust the delay on each of the

delay lines. In an alternative embodiment, no such control is present and the delay lines may be manually adjustable. In yet a further alternative, where there is no need to vary the delays on the output, the delay means may each comprise a fixed length of transmission line. The ferrite loaded shock line is operative to sharpen the rise time of a pulse in the order of 10 nS or so to between 100 pS and 1 nS.

Figure 3 shows an alternative embodiment in which, instead of a common pulse generator feeding a plurality of outputs, one pulse generator 30 is provided for each output line 31. Each pulse generator 30 feeds a TEM horn 32 via, as shown, a ferrite loaded shock line 33, and is connected to a common control means 34 which is operative to trigger each pulse generator when required to generate a pulse. For optimum performance, the level of jitter of each pulse should be low, say of the order of 10% of the rise time of the pulse i. e. less than about 10-20 pS for a pulse having a rise time of 100-200 pS. In such a case, separate pulse generators as shown can be used. In the illustrated embodiment, including a separate shock line, each pulse generator 30 typically comprises a separate thyratron or similar device. Each pulse generator 30 may instead comprise solid state devices such as a plurality of e. g. field effect transistors operated in the avalanche mode and

incorporated into shock line form, in which case the separate shock line 33 shown may be omitted, as such devices can be constructed capable of generating sufficiently short pulses. If especially large power outputs are required, each separate output line 31 may be replaced by a bank of lines each having solid state devices as described above and triggered in parallel so as to emit pulses in phase with one another. The delay between different banks may be adjusted as previously described to obtain the desired spectral characteristics. In a further alternative each pulse generator 30 may comprise semiconductor lasers integrated with high voltage Si or GaAs photoconductive switch elements which when triggered, generate an output pulse.

The advantage of using a plurality of pulse generators, as shown in Figure 3, is that the control of each of them can be accomplished relatively easily using low current, low voltage electronic circuitry.

As described above, the delay between pulses can be adjusted to provide a desired spectrum. In a modification the width of a complete cycle can also be changed by altering or adjusting the length of the pulse forming line in the case of the Figure 1 embodiment to give a further means of selecting the spectral characteristics.

Figures 4a-h show the comparison of a variety of wave forms, as emitted, that can be generated utilising a system according to the invention. Figure 4a shows a single pulse, or monocycle, together with its frequency spectrum. Such a pulse could be produced by sending all pulses down all the output lines in phase with one another. Figure 4b shows the effect of adjusting the delay so that three out of the six outputs are delayed by an amount equal to the length of one pulse. It can be seen that, as compared to Figure 4a, more power is concentrated towards the main IGHz peak. Figure 4c shows the effect when the delay is adjusted on each line so that five pulses are emitted in series spaced apart from one another by an amount equal to the length of one pulse, so that the series resembles a more continuous wave. Again, the IGHz peak is raised still further. Figures 4d and 4e show the effect of radiating three pulses in series, but, in the case of Figure 4e, inserting an extra delay between the pulses equal to the length of one monocycle. A comparison of the spectra for the two cases shows that in Figure 4e (ii) the 500 MHz level is only 1.7 dB below the IGHz level, and the 1.5 GHz level is increased almost 10 dB compared with Figure 4d (iii). Figure 4f shows the effect of inserting a delay equal to two monocycles between each pulse so that the 670 MHz and IGHz levels are substantially the same, but the

330 MHz level is almost 30dB up as compared to that shown in 4d (ii). Figures 4g and h show the effect of inserting a one monocycle delay between each of five pulses.

A system according to the invention can use a series of pulses whose spectra can be manipulated or selected so as to be suitable for identifying a target. For example it may be required to step through a plurality of different pulse sequences so as to test for the presence of, e. g., a particular type of missile or aeroplane. For example, a particular missile may have body dimensions and positioning of wing geometry or tail fins that give strong returns at certain frequencies and the series of pulses can be adjusted to test for such features. An alternative use of the system could be as a weapon in which the pulse sequence is selected so that the energy, for example, is concentrated into a frequency band which will couple efficiently with internal circuitry within the target.

CLAIMS 1. A system, for generating a series of ultra fast high voltage pulses, comprising one or more high voltage pulse generators, a plurality of output lines each terminating at an antenna; and delay means for spacing apart the pulses emitted from the or each antenna so that the spectrum of the series of emitted pulses has a predetermined shape.

2. A system, according to Claim 1, wherein the delay means are adjustable so that the spectrum of the series can be varied.

3. A system, according to Claim 1 or 2, wherein a common high voltage pulse generator is operative to provide a main pulse which is divided amongst a plurality of output lines, a separate delay means being provided in each output line.

4. A system, according to Claim 2 or 3, wherein the adjustable delay means comprises a transmission line whose effective length can be adjusted.

5. A system, according to any of Claims 2 to 4, wherein the adjustable delay means comprises a pair of concentrically arranged tubes one of which is slidably received within the other.

6. A system, according to Claim 3, in which the means for generating the main high voltage pulse comprises a primary circuit for generating a relatively long low voltage pulse, secondary circuit means comprising a dielectric filled pulse forming line connected to an electrode of a dielectric switch and which is operative to store the energy from the relatively long low voltage pulse and to discharge the energy across the dielectric switch when the voltage reaches a predetermined level, thereby to produce a main high voltage pulse for supply to the plurality of outputs.

7. A system, according to Claim 1, wherein a separate high voltage pulse generator is provided for each output line, each generator being connected to a common control means operative to trigger each generator when required, whereby the delay between pulses on different output lines can be selected.

8. A system, according to Claim 7, wherein each pulse generator comprises a plurality of transistors arranged as

in a shock line.

9. A system, according to Claim 7, wherein each pulse generator comprises a semiconductor laser integrated with a photoconductive switch element.

10. A system, according to any preceding claim, comprising control means for selecting the delay on each of the output lines.

11. A method of generating a series of ultra fast, high voltage pulses and including the steps of passing a pulse along each of a plurality of output lines which terminate at an antenna, and selecting the delay between different pulses on different output lines so that the spectrum of the series of pulses has a predetermined shape.

12. A method, according to Claim 11, including the step of selecting the delay between different pulses so that each pulse within the series is spaced from an adjacent pulse by an amount substantially equal to the width of that pulse so that the series resembles a near continuous wave.

13. A method, according to Claim 11, including the step of selecting the delay between different pulses so that

each pulse within the series is spaced from an adjacent pulse by an amount greater than the width of that pulse.

14. A method, according to any of Claims 11 to 13, including the step of selecting the delay between different pulses so that the spectrum of the series has a peak substantially coincident with a frequency or frequency band which will give a return characteristic of a target.

15. A method, according to any of Claims 11 to 13, including the step of selecting the delay between different pulses so that the spectrum has a peak substantially coincident with a frequency or frequency band which will couple efficiently into a target.

16. A method, according to any of Claims 11 to 15, including the steps of sequentially adjusting the delays on each of the output lines so as to emit a plurality of series of pulses, each series having a spectrum of a differing shape.

17. A method, according to any of Claims 11 to 16, including the step of generating a main high voltage pulse and splitting that main high voltage pulse amongst the plurality of output lines.

18. A system for generating a series of ultra fast, high voltage pulses substantially as described or as shown in any one of the accompanying drawings.

19. A method of generating a series of ultra fast high voltage pulses substantially as described or as shown in any one of the accompanying drawings.

Claims

1. A system, for generating a series of ultra fast high voltage pulses, comprising one or more high voltage pulse generators, a plurality of output lines each connected to the or a respective generator and each terminating at an antenna; and means for delaying the pulses emitted from the or each antenna so that the spectrum of the series of emitted pulses, one from each output line, has a predetermined shape.

in a shock line.

10. A system, according to any preceding claim comprising control means for selecting the delay on each of the output lines.

11. A method of generating a series of ultra fast, high voltage pulses and including the steps of passing a pulse along each of a plurality of output lines which terminate at an antenna, and selecting the delay between different pulses on different output lines so that the spectrum of the series of pulses, one from each output line, has a predetermined shape.

18. A system for generating a series of ultra fast, high voltage pulses substantially as described or as shown in Figures 1 or 3 of the accompanying drawings.

19. A method of generating a series of ultra fast high voltage pulses substantially as described or as shown in Figures 1 or 3 of the accompanying drawings.