Radar System Engineering

SEC. 13.14] B-SCOPE DESIG.V 531
will pass back across the tube when the scanner is pointing 180° away
from the sector being viewed; it is thus necessary to blank the tube during
that interval. This isaccomplished bythe circuit of VT., v7b, Vs,andl’g.
By means of a resistance network across two phases of the differential
synchro a voltagel is obtained which has a modulation phase 90° removed
from that of the azimuthal sweep voltage. After rectification by V,,
this signal controls the action of the flopover Vs. The latter triggersin
opposite directions at two points which are symmetrical in scan angle
with respect to the center of the sector displayed. The angular interval
between these points is adjustable by means of the bias applied through
the secondary of the isolating transformer ahead of the rectifier. The
plate of VS. controls the potential of the CRT cathode through d-c
restorer V7. and its parallel resistor in such a way that the cathode-ray
tube is turned on only when Vti is conducting.
The range sweep and associated delay circuits are shown in the bottom
row. The simple delay multivibrator is not intended for use in range
determinations, but can be replaced by some such arrangement as that
of Fig. 13.42 if accurate range measurements are to be made.
Since only discrete sweep lengths are involved, the range-sweep
flip-flop is switched along with the sawtooth generator in order to eliminate
the vacuum tube otherwise required for an automatic “turn-off”
circuit. Since the amplitude of the sawtooth is small compared to the
supply voltage, it is sufficiently linear without special precautions. If
continuous sweep-length control is desired, the circuits of Fig. 13.42
should be substituted, the feedback voltages for the turn-off circuit and
for the sawtooth generator being taken from the cathode of V15~.
The sawtooth passes through the “phase-splitting” amplifier Vl,b,
which provides signals of both polarities for the push-pull sweep amplifiers.
The cathode feedback of the final arriplifiers is sufficient to give
reasonably linear current amplification, since the push-pull action tends
to compensate for tube nonlinearities. The high-frequency response is
not good enough, however, to provide a linear displacement at the beginning
of very fast sweeps. For this reason a “step” is introduced at the
beginning of the sawtooth by placing a small resistor in series with the
charging condenser of the sawtooth generator. The sudden transfer of
current from the switching tube Vlti to the condenser circuit results in
the abrupt appearance of a voltage drop across the resistor, thus providing
the step.
The above circuits can be used to produce a “micro-B” display by
the expedient of gearing together the sweep-delay and the sector-width
controls, and providing the proper normalization. A given normaliza.
tion will be correct for only one sweep speed.
1Exceptfor this requirement,this synchrocould have a single-phaserotor.