GB2164219A

GB2164219A - Radar modulator

Info

Publication number: GB2164219A
Application number: GB08427100A
Authority: GB
Inventors: Robert Richardson; Antony Tom Barker
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1984-09-01
Filing date: 1984-10-26
Publication date: 1986-03-12
Also published as: GB2164219B

Abstract

A modulator for a radar transmitter comprises of voltage step-up transformer 7 which has a number of primary windings. An equal number of solid state switching devices are arranged to switch simultaneous pulses of current through an associated primary of the transformer so as to produce an additive effect in the secondary resulting in a single high voltage pulse at an output of the secondary. The latter is connected to an output tube of the transmitter. <IMAGE>

Description

SPECIFICATION A modulator for a microwave generator This invention relates to a modulator for a radar transmitter.

Radar transmitter modulators conventionally fall into two types namely line-type modulators and hard-tube modulators. In either case the modulator includes a switch which applies pulses of current to the radar output tube. The output tube invariable requires a high voltage to be applied to it and for this reason the aforementioned switch has conventionally been a vacuum or gas tube capable of withstanding high voltages.

The vacuum or gas tube in a radar transmitter modulator gives rise to certain problems. For example it has limited life and requires a sophisticated fast acting protection device to short circuit the energy store in the event of a flash arc occurring within the tube.

This invention provides modulatorfora radar transmitter comprising a voltage step-up transformer and a plurality of solid state switching devices each arranged to switch simultaneous pulses of current through an associated primary of the transformer to produce an additive effect in the transformer resulting in a single high voltage pulse at an output of a secondary of the transformer, the secondary being connected to an output tube of the transmitter.

By employing the invention the aforementioned problems can to a large extent be overcome. Firstly, solid state switching devices having good reliability and a long life expectation are relatively easily available. Secondly, because each switching device has its own transformer primary, a malfunctions in one of them will not effect satisfactory operation of the others and the transmitter will continue to operate, though at slightly reduced power.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying block diagram of a modulator constructed in accordance with the invention.

The illustrated modulator comprises a number of identical switching modules of which two are shown at 1 and 2 respectively. Each of these receives power from a common terminal 3 and a trigger signal from a common terminal 4. Each module generates an identical control pulse and amplifies it so as to produce at its output terminals 1A, 1 B or 2A, 2B a high current pulse. This is passed through an associated primary 5 or 6 of a voltage step-up transformer 7.

The simultaneous pulses in the transformer primaries 5 and 6 produce an additive magnetic effect which induces a high voltage across the secondary 8 and this is used to drive a microwave tube shown at 9.

Filament power for the microwave tube 9 is provided at 10 and a suitable control circuit 11 feeds pulses at defined times through an additional transformer primary 12 so as to reset the magnetic core of the latter to its original magnetic condition.

Each module 1 or 2, comprises a switched mode power supply unit 12 which provides a suitable voltage across a capacitor C. In response to a trigger signal from terminal 4 a control circuit 13 feeds a pulse to a base drive circuit 14 which simultaneously switches on a number of bipolar transistors connected in parallel, two being shown at 15 and 16. This allows the capacitor 12 to be connected across the primary winding via the transistors.

The fuses 17 isolate faulty transistors should any fail it being understood in this connection that although only two have been specifically illustrated a much larger number e.g. 10 are in fact provided. If a complete module fails the diodes shown at 18 isolate it during a pulse to prevent it having an adverse effect on the circuit.

Resistors 19 limit the current through each transistor to protect the latter in the event of a spark in the microwave tube which might otherwise give rise to excessive current through the transistors.

Although transistors are shown by way of example it is equally possible to use any other input controllable semiconductor such as gate turn-off thyristor or low Field Effort transistor.

An important aspect of the illustrated system is that no fault occurring in a module, for example a short circuit occurring across the output terminals, can affect the performance of the other modules.

This is because of the isolation between modules achieved by having a separate primary and a separate power source for each module; and by the diodes 18. Another important feature is that, because all the modules are identical, a faulty module can easily be replaced by a spare module provided for that purpose. It is preferred to arrange a spare module so that it can automatically be switched into operation in the event of failure of one of the other modules. For this purpose the circuit 13 provides an output signal indicated by the word "status" on the drawing which indicates a central controller (not shown) whether the module is functioning correctly. This can be done simply by detecting the presence of high tension applied to the transistors.In the event of a malfunction the central control unit sends a command along the input labelled "control" to shut down the module automatically whilst a replacement is switched in its place. The replacement module would be connected to an additional primary circuit of the transformer not normally used.

The overall modulator performance is dependent on the ability to transmit the pulses generated by the individual modules to the transformer without loss of fidelity. Bearing in mind that the distance involved in this connection could be up to about 8 feet it is desirable to use a transmission line of which the characteristic impedance is matched both to the output of the individual module and to the input of the transformer primary. With a matched transmission line the transmission of the pulses will only be delayed by the electrical length of the line and will not lose their fidelity.

The module impedance is likely to be very low, for example less than 3D, and no readily available cables or connectors are commercially available for this purpose. This problem has been overcome with excellent results by providing all the connections between the modules and the transformer by a single, large printed circuit board. The circuit board carries a number of pairs of conductive tracks, the tracks of each pair being directly opposite each other on opposite sides of the board. This allows the characteristic impedance to be controlled to be the desired value. Suitable edge connectors can be provided at opposite edges of the board to provide the necessary connection with the individual modules and with the transformer.

Claims

1. A modulator for a radar transmitter comprising a voltage step-up transformer having a number of primary windings and an equal number of solid state switching devices each arranged to switch simultaneous pulses of current to an associated primary to produce an additive effect resulting in a single high voltage pulse at an output of a secondary of the transformer, the secondary being connected to an output tube of the transmitter.

2. A modulator according to claim 1 in which each switching device includes a plurality of solid state switches connected in parallel and arranged to operate in unison.

3. A modulator according to claim 1 or 2 in which each switching device is identical.

4. A modulator according to any preceding claim including an additional solid state switching device serving as a spare, means for detecting a fault in one of the other switching devices and means for switching a spare into operation in response to such a detection.

5. A modulator according to any preceding claim in which matched transmission lines join the individual switching devices to the transformer primaries, these transmission lines each being formed by a pair of conductive tracks on opposite sides of a common insulating board.

6. A modulator according to any preceding claim in which each switching device includes its own power supply unit.

7. A modulator according to any preceding claim in which each switching device is physically discrete and is adapted to be released from the rest of the modulator for replacement.

8. A modulator substantially as described with reference to the accompanying drawing and substantially as illustrated therein.