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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Limiting Circuits


In radar it is often necessary to remove the positive or negative extremes of an input signal. This may be done to improve the shape or to 'clip' unwanted parts of an input waveform. A circuit which does this is known as a limiting circuit or limiter; an alternative name is a clipping circuit (Fig 1).

Diode Limiters

A diode limiter is the simplest form of limiting circuit and consists of a diode and a single fixed resistor. There are two types of diode limiter - series and parallel limiters. These in turn are each sub-divided into positive and negative limiters. Thus we have four distinct circuits (Fig 2). The diode may be either a thermionic valve or a semiconductor type.

Action of Diode Limiter

The resistance RD between the anode and cathode of a diode is very high (say 10MW) when the anode is negative with respect to its cathode (diode cut off). However when the anode is made positive with respect to its cathode the diode conducts and RD then falls to a low value (say 1kW) (see Fig 3).

Thus if we apply an alternating voltage V of peak value 100V to any of the limiter circuits shown in Fig 2, each circuit operates as a voltage divider consisting of a fixed resistor R, normally about l00kW, in series with the diode resistance RD which is either very low (when diode conducting) or very high (when diode cut off).


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