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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 1

Square Waves

To maintain the same percentage distortion for a pulse of shorter duration the leading edge of the pulse must rise in a much shorter period of time. For example if the pulse duration is reduced to 0.5 us the leading edge of the pulse must rise in a time t not greater than 0.05 us. The frequency of the highest harmonic within the pulse is now:-

1/4t = 1/(4 x 0.05 x 10-6) = 5 Mc/s

The r.f. and i.f. bandwidths in the receiver must now be increased to 10 Mc/s. Thus as the pulse duration is reduced and the leading edge of the pulse made steeper the bandwidth of the circuits handling the pulse must increase accordingly. We shall see in a later chapter how wideband amplifiers are obtained.

Summary

    a. A square wave of voltage starts from a fixed voltage level, changes to a different voltage in a negligibly short time, remains at this new voltage for a certain time and then changes to the original voltage level in a negligibly short time, remaining there for some time.

    b. The amplitude of a square wave is the difference between the two voltage levels between which the waveform varies.

    c. The polarity is described by taking one of the voltage levels as a reference and stating whether the wave goes positive or negative with respect to that level.

    d. The period of a square wave is the time taken by one complete cycle of variation.

    e. The frequency of a square wave is the number of cycles occurring in one second.

    J. A symmetrical square wave is one in which the two parts of the cycle are of equal duration.

    g. An asymmetrical square wave (a rectangular wave) is one in which the two parts of the cycle are of unequal duration.

    h. Either part of one cycle of a square wave can be called a pulse and its duration is called the pulse duration (sometimes pulse length or pulse width). The terms 'wide' and 'narrow' are also used to describe a pulse.

    j. The number of pulses occurring each second defines the pulse repetition frequency.

    k. For accurate timing and range measurement, pulses with steep leading edges are necessary. Because of the large number of harmonics associated with a steeply rising pulse wideband circuits are then needed.


 

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