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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

square wave is the difference between the two voltage levels. The polarity may be stated as either positive-going or negative-going with respect to one of the fixed voltage levels. In most cases the reference level will suggest itself naturally, e.g. we often use h.t.+ as a reference level.

Period and Frequency of Square Waves

The period of the square waves used in radar equipment is usually expressed in microseconds. For example the period of the square wave shown in Fig 5 is 200 microseconds. The frequency of this waveform is therefore 5,000 cycles per second.

Symmetry

So far we have considered only the case where the time interval during which the circuit is switched on is equal to that during which it is switched off. The two parts of the waveform are then of equal duration and the square wave is said to be symmetrical (see Fig 6a).

If the time interval during which the switch is open is longer or shorter than the time during which it is closed the two parts of the waveform are no longer of equal duration. The resultant waveform is often called an asymmetrical square wave or a rectangular wave (Fig 6b and e).

These various terms are often used indiscriminately and any of the waveforms shown in Fig 6 may be loosely referred to as either square waves or rectangular waves.

Pulse Duration and PRF

The rectangular wave shown in Fig 6c is the waveform most commonly met with in pulsed radar. To indicate the length of each part of one cycle we call either part a pulse and give the pulse duration in microseconds.

A pulse may be defined by the terms 'narrow' and 'wide' (see Fig 7). A narrow pulse is one in which the pulse duration is very short compared with the period of the rectangular wave.


 

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