Main Radar Home

Site Map

Radar theory Home

AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

Contact the Editor

AP 3302 Pt. 3

Section 2


Electronic Switching Circuits

b.      When the valve is conducting it has a very low internal resistance.

c.      It can pass currents of the order of amperes.

Fig 1 la shows the basic block schematic outline of one type of modulator used in radar transmitters. The energy for the pulse, which is to be dissipated eventually in the load (e.g. a magnetron oscillator), is obtained from the energy source - a high-power d.c. supply. This energy is stored in the storage device at a slow rate during the periods between pulses, the charging impedance deciding the rate at which energy is supplied to the storage device. At the required instant of time the switch is closed and the stored energy is rapidly dissipated in the load. Since a high level of energy is being expended in a very short period of time, a pulse of very high peak power is produced (Power = Energy/Time)

A basic arrangement using this system is outlined in Fig 1 lb. The thyratron is cut off during the periods between pulses and during this time C charges at a relatively slow rate through R and the load. When the trigger input pulse arrives the thyratron ‘fires’ and, because of its low resistance, the capacitor discharges rapidly through the load and the thyratron. The energy acquired by C during the period between pulses is rapidly dissipated in the load and a high-power, short-duration pulse is produced. The capacitor discharges until the anode voltage of the thyratron is such that the valve cuts off (about + 15V). When this happens, C again commences to charge through R towards the e.h.t. supply and after a short time the circuit is again ready for the next input trigger pulse.

A capacitor has the disadvantage of discharging exponentially with time and, because the thyratron cannot be cut off at its grid by the trigger pulse, the pulse developed across the load has a poor shape. For this reason a capacitor is not normally used as the energy storage device. In most radar equipments a delay line or pulse-forming network is used instead. The delay line is considered in detail in Section 5. All we need to know at present is that it can be used in much the same way as the capacitor of Fig 11 but, compared with the capacitor, it produces a much better shaped pulse of higher energy content. The pulse produced by a delay line modulator has steep leading and trailing edges and an accurate pre-determined pulse duration. 


Previous page

To top of this page

Next Page

Constructed by Dick Barrett

(To e-mail me remove "ban_spam_" from my address)

©Copyright 2000 - 2002 Dick Barrett

The right of Dick Barrett to be identified as author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.