AP 3302 Pt. 3
Section 2
CHAPTER 10
Electronic Switching Circuits
Introduction
We have already seen that in a pulsed radar equipment many circuits have to be switched on and off very rapidly. Very often one circuit has to be brought into operation at exactly the same instant of time as some function occurs in another circuit. Alternatively, we may require a circuit to become operative an exact number of microseconds after some function has occurred elsewhere in the equipment. Because of the very short time intervals involved - often of the order of nanoseconds (i.e. l0-9 seconds) - mechanical switches and relays cannot be used. Electronic switches are therefore necessary.
Switching
is carried out at various stages throughout a radar equipment. In the initial
stages of a trigger unit (e.g. at the master timing unit) the switching is very
often at millivolt and microampere levels. At intermediate stages, switching
up to several hundreds of volts at a few milliamperes may be required. At the
final transmitter stages the power levels are very high and switches capable
of handling several thousands of volts at currents of the order of tens of amperes
are necessary. The final switching stage of one ground radar transmitter switches
at a voltage of 38,000V and at a current of about 130 amperes.
Electronic Switches
The earliest electronic switch was the thermionic valve. A valve is easily cut on and off, and the speed at which the action occurs is high. Fig lb illustrates the switching action. Hard valve switches have certain limitations, among them the fact that the power they can readily handle is limited. Where fast switches capable of passing very high currents at high voltages are required a gas-filled valve (such as a thyratron) may be used.
A transistor may also be used as a switch in much the same way as a valve by applying suitable voltages to the base-emitter circuit (Fig lc). A semi-conductor diode is another good electronic switch (Fig ld).
Tunnel Diode as a Switch
All the devices mentioned are limited in their switching speed. In valves, transit time is the main limitation and in semiconductor devices it is the hole-storage effect (see Part 1B, p 322). Where very high switching speeds at very low power levels are required a tunnel diode may be used This is a semi-conductor device capable of nearly instantaneous switching. With the tunnel diode, switching times of less than one nanosecond have been achieved.
The tunnel diode looks like a normal p-n junction diode. But the p and n materials are more heavily 'doped', having a larger number of acceptor and donor impurities, and the junction is extremely thin. Because of this, electrons can ‘tunnel’ through the very thin junction at low values of forward-bias voltage.
