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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 1

Square Waves

Introduction

In Section 1 we learned the basic principles of operation of a pulse-modulated radar system and saw how the system may be built up in the form of a block schematic diagram where each block represents a group of special circuits. Before we can tackle the job of maintaining such equipment we must know more than the purpose of each block; it is important to understand how it produces the required results. We must therefore 'look inside' each block and learn about the special circuits which work together to make up a complete radar installation (Fig 1). In this section we shall look at some of these basic radar circuits.

We know that a radar transmitter must be switched on and off many times each second to produce a series of pulses of r.f. energy. We also know that the indicator timebase circuits must be switched on at the instant each transmitter pulse begins and switched off after a time equal to the maximum required radar range. As we progress we shall find that many other circuits must be switched on at the instant the transmitter fires or an exact number of microseconds later.

Mechanical switches and relays cannot operate with the precision which is essential for this type of circuit switching. For accurate synchronization within the equipment the switching must be done electronically with square waves of voltage. We shall see later that almost all pulsed radar circuits are concerned with the production or transmission of such waveforms. Let us therefore begin by learning what is meant by the term 'square wave'.

The Simple Square Wave

A waveform is a graph which shows how a voltage or a current varies over a given period of time. A square wave is a voltage or a current change in which the waveform has square, i.e. right-angled, corners.

Fig 2a shows a simple circuit of a battery, a switch and a bulb, in which the negative terminal of the cell is earthed. The voltage at the positive terminal is therefore + 1.5V with respect to earth. If now the circuit is switched alternately on and off at intervals of two seconds, the wave-form of voltage applied to the bulb is a square wave as shown in Fig 2b.


 

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