Radar System Engineering

SEC. 13.17] RESOLVED-CURRENT PPI 541
practice because of the finite time for sweep “recovery.” By using a
trapezoidal waveform, in order that the time spent at maximum ‘‘ negative”
current become appreciable, sweep duration can be increased to
about 60 per cent in practice. This is easily accomplished through the
medium of a resistor in series with the integrating condenser of the sawtooth
generator, as is explained in Sec. 13.14.
Thk form of resolved-current PPI has been successfully used in the
lightweight airborne equipment (AN/APS-10) described in Sees. 15.12
to 15.14. As is shown in Fig. 13.48, the synchro is driven directly by the
amplifier without a transformer. The synchro secondary and the deflection
coil (which is wound in a standard synchro stator form) have 3-phase
rather than 2-phase windings. The equipment is light in weight, comparatively
simple and economical. Its principal drawbacks are the
limited duty ratio, the precision required in the triggering circuit, and
the limited off-centering attainable. No off-centering is used with the
3-phase system of Fig. 13.48; when
it is desired, 2-phase windings are
+m
more suitable.
The Use oj Special Wavejorms.
The method that will now be described
is similar to that just V2
treated in that positive and negative
signals are “balanced.” It
+200V
differs, however, in that the wave- ‘L
form includes a quiescent period,
immediately preceding each cl
v,
Waveform
sweep, during which the waveform
——.
T --- c~
has its average value, so that
—-- c,
0
there is no necessity for precise %1
control of the trigger (Fig. 13.45). {_E = = =
The required waveform in the pri- -
FIG. 13.49.—Balanced-waveform
mary is derived by the introduc- using an inductance.
generator
tion, immediately following the
sweep proper, of a signal of opposite polarity whose time integral is
exactly equal to that of the sweep. Both sinusoidal and rectangular
waves have been used, but only the former will be discussed here.
A waveform of the proper type can be generated by the Miller-type
circuit (Sec. 13. 10) shown in Fig. 13.49. When the grid of VI is released
by opening the switch, it quickly rises to cutoff and a negative sawtooth
begins on the plate. The form of this wave is practically independent of
the load impedance, which is in this case made up of the inductance L, V2,
and R2 in series, and the various capacities. The a-c component of current
in the coil is at any instant given by