Radar System Engineering

676 MOVING-TARGET INDICATION ~SEC. 16.20
delay-line driving circuit is desensitized by a trigger from the stable
oscillator. This trigger does three other things: it generates a sample
video pulse which travels down the line; it operates a coincidence circuit
which examines the time of arrival from the comparison amplifier of the
preceding sample pulse; and finally after suitable delay it fires the
transmitter and resensitize the echo circuits. The coincidence circuit
supplies a variable d-c bias to the oscillator which keeps the frequency of
that oscillator in step with the supersonic delay time as measured by the
sample
pulses.
This arrangement has the disadvantage that it takes a number of tubes
and, like all multistage feedback circuits, is difficult to analyze in case of
trouble. It has the advantage that, with small additional elaborations,
the sample pulse residue after cancellation can be used to control the gain
of one channel of the comparison amplifier. This type of AGC corrects
for changes in both the amplifier and the delay line.
Regenerative Trigger Circuits. —Three distinct methods have been
proposed for trigger regeneration. They differ in the way in which the
fraction of a microsecond inevitably lost in the trigger amplifier is made up
in the delayed signal channel. The compensating delay must be inserted
after the point at which the trigger pulse has left that channel but before
cancellation.
The first method is to add the required delay either at carrier or
video frequency. Unfortunately, no electrical delay lines are available
which are capable of reproducing a microsecond pulse with less than 1 per
cent distortion.
The second method involves the use of an auxiliary delay-line receiving
crystal slightly closer to the transmitting crystal than is the regular
receiving crystal. The auxiliary crystal receives the trigger pulse. The
finaf time adjustment can be accomplished in the trigger circuit either
mechanically by an actual shift of the crystal or electrically by the use of
a short variable delay. The disadvantages of this method lie in the
excessive attenuation in the trigger channel, the increased difficulty of
delay-line construction, and the loss of design flexibility due to the
necessity of mixing the trigger and the video signals at the transmitting
end of the delay line while preventing their interaction at the receiving
end.
The third method of trigger regeneration, already described in Sec.
16.3, is the use of an extra delay line in close association with the signal
delay line as illustrated in Fig. 1633. This line is thermally lagged by
the box of Fig. 16.35. The method allows freedom in the choice of line
constants and of electrical coupling, with some resultant circuit simplification
as compared with t he three-crystal delay line. H owever, the amount
of thermal correction is not as great as in the three-crystal line, The