Radar System Engineering

SEC. 16.5] STABILITY REQUIREMENTS 641
echoes to beat in amplitude in the region of overlap, even when no coherent
reference signal is present. Since the expression for the phase shift is
identical with the previous expression, Eq. (4) also applies here.
Extra cycles will also appear in the video pulses if the magnetron frequency
varies during the transmitted pulse. Such variation occurs
chiefly at the beginning and end of the pulse. If Afl is the maximum
departure in frequency from the value in the central part of the pulse,
the number of beat cycles is certainly less than T Afl. With the same
criterion as before we therefore should have
For a l-psec pulse this gives Aj, < ~ Me/see. However, since most of the
effect is concentrated near the ends of the pulse, a much larger variation
is probably permissible. Since the frequency pattern within the pulse
does not change from pulse to pulse, the quality of the cancellation is not
affected. All that is involved is a slight loss in signal-to-noise ratio for
moving targets.
Cancellation Equipment. —Consider next the stability requirements on
the cancellation equipment. The detectability of a signal on the PPI can
be roughly measured by the area under the voltage pulse. Thus if the
trigger and signal delay lines differ in delay time by an amount A71, there
will be two uncanceled spikes of width A71 for each echo. In order to
cancel, for example, to 4 per cent, we must therefore make A~l less than
2 per cent of the pulse length. The delay lines should be capable of
matching each other within this tolerance for at least an hour at a time.
Likewise the amplitudes of the signals in the delayed and undelayed
channels should match to 4 per cent for an hour at a time.
Modulator. -Pulse-to-pulse variation in the repetition rate can produce
the same effect as unmatched delay lines. Thus the modulator
should fire relative to the trigger with a variation of not more than 2 per
cent of the pulse length, or+ ~sec in the case of a l-psec pulse.
Finally, we have to consider the effect of variation in pulse length. If
this should change by an amount ATfrom one pulse to the next, there will
be an uncanceled spike of width A,. Thus we should not allow A,/r to
exceed 4 per cent.
Variation in pulse length can also cause an indirect effect on the phase
of the coherent oscillator when the latter is not in tune with the i-f echo
signals. Let us assume, for example, that the locking pulse causes the
coherent oscillator to execute forced vibrations at the intermediate frequency
for the duration of the pulse, after which it reverts to its natural
frequency. Then the variation in phase of the coherent oscillator is given
by A, . Af (in cycles), where Aj is the amount by which the coherent
(5)