Radar System Engineering

634 MOVING-TARGET INDICATION [SEC. 16.3
the amplifier have separate diode detectors arranged to give opposite
signal polarities, so that the delayed and the undelayed signal can be
adjusted in amplitude to cancel each other when added. Since the video
signal from a moving target is bipolar (cf. Fig. 16”7), a rectifier is included
in the circuit ahead of the PPI.
In order to get good cancellation, the signals in the two channels of
the amplifier must match very closely in time. This means that the
m
_
Oscillator
and
amplifier
-—-— -——— ——-—— ---
r Two.channel amplifier
I
I
Carrier
Delay line + amplifler
I
I
-1 C?rrier
I
amplifier
I ,/
L’___ -_____ .–––– –-l
Video signals from
MTI receiver
MTI video out to PPI +
FIG. 16.11.—Block diagram of delay-line circuits,
pulse-repetition period has to match the delay time of the supersonic line
with great accuracy. The velocity of sound in mercury varies with the
temperature by about one part in 3000 per degree centigrade, so that
temperature variations will cause the delay time to drift. This can be
compensated for by providing a trigger generator whose PRF is altered
to take account of changes in the delay time. A simple way of doing this
is to use another supersonic delay line as the timing element in the trigger
~ Triggertosystem
1
I
I
1
15 Mclsec
resonant
circuit
t
Blocking
oscillator
+ Trigger * 15 Me/see
delay line amplifier
t
.
Video
amplifier
+ Detector
FIG. 16,12.—Trigger generator.
generator, as shown in Fig. 16.12. The circuit works as follows. When
the blocking oscillator fires, it delivers a trigger to the system and also
shock-excites a 15-Me/see resonant circuit. The oscillations so set up
are passed through the delay line, and thereafter amplified and detected.
The delayed signal is used as a trigger; it is applied to the blocking oscillator,
which then fires and starts a new cycle. By making one of the
delay lines variable in length (which requires a different design from that
of Fig. 16.10) the delays in the trigger and signal circuits can be matched