Radar System Engineering

SEC. 17.5] THE METHOD OF INCREMENTAL ANGLE 691
signals at the receiving station since otherwise video signals might occasionally
produce a spurious trigger. If the transmitter provides higher
power inthepulses than inthe video signals, amplitude selection can be
used, and the coded pulses can simply be passed to the transmitter.
Otherwise, the coded pulses must be distinguished in time, and the extra
equipment indicated by the dotted boxes must be used. The video
switch (Sec. 13.9) is arranged to pass video signals only when the flip-flop
(Sec. 13.7) is on, and pulses only when it is off. The flip-flop is fired by the
modulator trigger, so that video signals are passed until shortly before the
next cycle, at which time the flip-flop opens the switch for pulses until
the modulator pulse occurs again.
At the receiving station, signals from the receiver pass to a switch
which is open when a pulse code is expected. Following this switch is a
decoder similar to that of Fig. 17.3. The resulting single pulses serve to
trigger the indicators, to control the signal-selecting switch, and to provide
the rotation. The switching action is similar to that of V,aj Vlb, Vti, and
Vw of Fig. 17.3, the length of the flip-flop being slightly less than that
of the flip-flop at the transmitter so that video signals are always excluded
from the decoder.
To produce the mechanical rotation, the decoded pulses first actuate
a scaling circuit, such as that of Fig. 13.20, which reduces their frequency
to twice that appropriate to a synchronous motor. The resulting puIses,
by triggering a scale-of-two circuit similar to that of Fig. 13. 16b, produce a
symmetrical square wave. A broadband a-f filter removes the higher
harmonics, leaving a sine wave at the fundamental frequency which, after
amplification, powers a synchronous motor that drives a data transmitter.
Proper initial phasing of the data-transmitter shaft can be made by
methods analogous to those of display-sector selection (Chap. 13);
alternatively, a controlled phase shifter can be inserted between the a-f
filter and the amplifier. A convenient index for use in this alignment can
be provided by transmitting one or more angle markers along with the
video signals, as illustrated in Fig. 17.4.
This method is restricted in its use. The requirement of a continuous
scan at a nearly constant speed mentioned in Sec. 17.3 is made even
more rigorous by the synchronization with the modulator, which must
usually operate at a definite PRF. Any requirement for variation in the
scanning rate or the PRF introduces serious complexities because of the
fixed relation between these two quantities. Another draw-back is that
rephasing must be done whenever the signals are interrupted or seriously
interfered with,
More flexibility is provided by a third means for relaying incremental
angle data. If the scanner and the modulator cannot be synchronized but
the scanning is nevertheless reasonably uniform, the periodic signal can be