Radar System Engineering

SEC. 16.20] DELAY-LINE TRIGGER CIRCUITS 675
a PPI. The most difficult of these operations is amplification. If
statistical noise and weak moving-target signals are to be fully visible,
they must reach the final video rectifier at a level sufficient to cause linear
operation of that detector. Since the noise level at the cancellation
detector may be as much as 40 db beneath the signal peak, a gain of
several hundred may be required between cancellation and rectification.
This video amplification is difficult because of the bipolarity and wide
dynamic range of the signals. Unless special precautions are taken,
strong signals from moving targets may draw grid current and bias the
amplifier to the point where weaker moving-target signals are lost. The
bipolar nature of the signals prevents the application of the clamping
technique, which is useful in avoiding a similar trouble with unipolar
video amplifiers. Blind spots and overshoots can be adequately minimized
by careful choice of time constants, grid bias, and plate load
resistors, although several additional amplifier tubes may be needed as a
result.1
Full-wave rectification before application to a unipolar device such
as a PPI is roughly equivalent to doubling the PRF~and is correspondingly
to be recommended. Either crystal or diode rectifiers may be used.
Limiting can take place both before and after rectification.
Figure 16.35 shows the two-channel amplifier on the left, with the
delay-line box in the middle, and the video-modulated oscillator and
amplifier for driving the delay line on the right.
16.20. Delay-line Trigger Circuits.-For proper cancellation the
PRF of the radar transmitter must match the supersonic delay to ~ of a
pulse length. Free-running oscillators for generating the trigger have
been built to fire the transmitter at a suitably constant rate. Because of
the large thermal coefficient of delay in mercury, normal fluctuations in
ambient temperature necessitate manual readjust ment of such oscillators
at intervals ranging from 10 min to 1 hr. Since this amount of attention
cannot be tolerated in most applications, several methods have been
devised for automatically maintaining time synchronism between the
PRF and the supersonic delay.
Time synchronism is maintained by generating trigger pulses at a
PRF determined either by the signal delay line or by an auxiliary delay
line. The methods for doing this can be divided into two classes
regenerative and degenerative. The regenerative method has already
been described briefly in Sec. 16.3.
The Degenerative Trigger Circuit.—A highly stable oscillator generates
the transmitter trigger, and a correction is applied to the oscillator if it
fails to match the delay line. This correction is obtained as follows,
Several microseconds before the transmitted pulse, the echo input to the
1Nonblocking amplifier design is discussedin Sees. 58 and 10.4of Vol. 18.