Radar System Engineering

694 RADAR RELAY [SEC. 17.5
generator to switch vzb is avoided by the action of the positive pulse from
V*.. During the pulse the grid current of V,b charges C, sufficiently to
block off the tube long enough to allow generation of the desired sawtooth.
The a-c voltage from a generator on the scanner acts through the cathode
follower VM to vary the bias of the diode sinusoidally and therefore vary
the delay by steps which approximate a sinusoidal variation of about one
microsecond amplitude. The mean value is set at 5 to 10 psec. The
delayed pulse is mixed with the coding for the basic pulse on the common
cathode resistor R and sent to the master mixer. It is not necessary to
code the azimuth pulse since its function is merely to transmit the
sinusoidal frequency; any interference strong enough to mask this pulse
would render the whole system useless in any case.
At the receiving station the basic pulse is decoded by the coincidence
circuit of Fig. 17.3.1 The resulting pulse, taken from point Y of that
diagram, blocks the grid of vsb and initiates the action of the delay
circuit comprising the sawtooth generator Vbb, the cathode follower Vti,
and the blocking oscillator VT.. The tube V,a is delayed in firing by its
excessive bias. When VTCfires, its cathode drives the l-psec delay line,
and the delayed pulse returns through VTb to turn off the blocking
oscillator. Thus successive I-ysec pulses, adjacent in time, appear at the
grids of V8band VE~ respectively. Their boundary time is made coincident
with the mean time of arrival of the azimuth pulse by adjustment
of the slope of the sawtooth in the delay circuit. The amplified pulse
train from point X (plate of Vti) of Fig. 17.3 is applied to the cathodes of
Va. and VW. Because of the bias the tubes can conduct only during the
coincidence of the azimuth pulse with the pulses on the grids. Condenser
C, is charged negatively by the signal from v8b acting through diode VW
and positively by the signal from V% which forces negative charge to
ground through Vg~. If the coincidence time is equal on VSCand V%,
the net result is zero. However, as the azimuth pulse moves back and
forth in time with respect to the switching pulses, the coincidence times
vary in an out-of-phase manner so that the potential of Cl has a stepwise
variation with a sinusoidal envelope (Waveform I). This alternating
signal is passed through cathode follower VGband a bandpass filter in
order to produce the desired sinusoid for driving the synchronous motor.
In order to minimize the number of transmitted pulses, the basic pulse
can be related to the modulator pulse in any of several ways. If the first
few microseconds of the radar cycle are not too important and the
modulator is not self-synchronous, its trigger can be the third pulse of the
coded group. No range error will result, but the azimuth pulse will
appear on the indicators at the receiving station at a range corresponding
1The pulses can be separated from the video signals by amplitude selection,
time separation as in Fig. 17.4.
or by