Radar System Engineering

708 RADAR ltELA Y [SEC. 1743
wave from the flopover is used to open the sine pulse switch for 25 ~sec
or so at the expected time of arrival of the sine pulse. 1
The delayed pulse reaches the far end of the delay line exactly at the
middle of the switching wave. The remainder of the circuit is designed
to force thk “comparison” pulse into time coincidence with the sine
pulse by properly controlling the delay through adjustment of the output
voltage. Various methods can be used to accomplish this adjustment.
The operation of the method illustrated is described below. The
comparison pulse triggers a flip-flop. The square waves from the two
plates, one positive and one negative, are fed to the control grids of a pair
of pentodes (Fig. 17. 13c), turning one on and the other off. The screen
grids, normally at cathode potential, receive the positive sine pulse. If
the pulse arrives when the control.grid of a given tube is on, that tube will
produce a negative pulse on its plate. The signals are integrated in
opposite polarity on condenser Cl by means of the diodes. A negative
signal from Vj drives a negative charge through V3. to Cl. A signal from
V, drives a negative charge to ground through VW The potential on C,
is the output signal. It is returned (perhaps after amplification) to the
comparison-pulse delay circuit which it controls. Thus, if the switching
occurs before the arrival of the sine pulse, VI conducts, Cl becomes
more positive, and the delay is increased. If the switching is too late, Vt
conducts and the delay is decreased. The process continues on successive
cycles until the delay is such that the sine pulse” straddles” the instant of
switching. The delay circuit will then follow the variations in the delay
of the sine pulse so that the output voltage varies in the desired manner.
Figure 17.14 illustrates a system in which, although it is assumed that
the modulator can be triggered, means are provided to distinguish
between the cosine pulse and the modulator pulse for other reasons.
The delay circuits in the synchronizer may be any of the varieties
described in Sees. 13.7 and 13.12, the most precise and trouble-free
results being obtained by the use of circuits similar to but not so refined
as the circuit shown in Fig. 13.36. If this type is used, the sine or cosine
voltage is used to bias the cathode of the diode, and the bias of the anode
is adjusted to give the desired delay A at zero scanner angle. The value
of B is determined jointly by the slope of the sawtooth and the amplitude
of the sine and cosine voltages from the scanner.
The basic pulse, sine pulse, and cosine pulse are passed to the usual
three-pulse coder along with the modulator trigger signal, which is derived
from a fixed-delay circuit triggered by the cosine pulse. In order that the
trigger signal shall be properly timed when decoded at the receiver, the
1 Since this interval is too narrow to allow rapid “looking in” initially or after
tracking has been lost, the circuits are arranged in a mamcr not shown so that in the
abaence of pulses the grid remaine on.