Radar System Engineering

532 THE hzYCEI VI.VG SYSTEM—INDICA TORS [SEC. 13.15
In applications involving extremely rapid sector scans (several scans
per second), synchros cannot be used and a variable condenser must be
substituted for the scanner synchro. The oscillator frequency must
then be much higher. No problem of sector selection is involved, since
the rapid scanning covers only a narrow sector, which is completely dkplayed
at all times.
A typical narrow-band video amplifier and a marker signal mixer
are included for completeness. Incoming video signals of 8 to 10 volts
are amplified by V6 and applied to the cathode of the CRT. The tube
VT. provides d-c restoration to assure the proper operating level. Electronic
range and angle indices from sources outside of the diagrams can
be mixed into the double cathode follower Vg and applied to the CRT grid.
It is assumed that both range and angle indices have positive polarity
and that the potential of the angle-index lead is zero in the absence of a
signal.
If the center of the sector is fixed in azimuth, a great simplification can
be brought about by driving amplifier Vl, Vb directly from a potentiometer
which replaces the remainder of the top row in the diagrams. The
azimuth blanking circuit ( v7b, V8, V9) Can also be replaced by a simPle
cam-and-switch arrangement. If the scanner executes only a sector
scan, even this is unnecessary.
13.15. Plan-position Indicator.-As has been indicated in earlier
sections (e.g., Sec. 12”2), PPI displays can be produced by any of several
methods, which, in general, may be classified under three principal
headings.
1. The rotating-coil method, in which the range-sweep current is
passed through a coil which is rotated about the tube neck in
synchronism with the antenna motion. If “ off-centering” is
desired, it is provided by means of a fixed coil outside of the moving
one.
2. The “ resolved time base” method, in which a sawtooth waveform
is passed through a 2-phase resolver, 1practically always a synchro,
and the resulting sweep “components” are utilized to energize
orthogonal deflecting coils or plates.
3. The method of “ pre-time-base resolution” in which slowly varying
sine and cosine voltages obtained from a d-c excited sinusoidal
potentiometer or by rectification of signals from an a-c resolver are
used to generate the necessary sawtooth. This method is used
principally in cases where the scan is too rapid for the rotating-coil
method and the antenna is so far away that the transmission of the
1In some cases a 3-phase synchro is used, and the resulting signals are passed
through a resistor network which reduces the three phases to two.