Radar System Engineering

522 THE RECEIVING SYSTEM—INDICATORS [SEC. 13.12
the same amount. Since no pulse is involved in the reverse regeneration,
the exact phase at which it appears is immaterial. The precision of the
circuit is largely determined by L and C, which should be temperaturecompensated
and well shielded. Since RI has some effect on frequency,
it should be of a reasonably stable type. Frequent y adjustment is best
made by slug-tuning the coil L. Once aligned, a well-designed unit
should maintain its calibration to better than 1 per cent.
Many variations of this circuit, mostly simplifications, have been
used. For example, if only a few markers are required on each pulse
cycle, Vz can be omitted, since the high-Q coils in the oscillating circuit
I T T J 2 L,, +300V
-4
Negative
square
wave
1 M 22k
input
2W
0.1 39k
6SN7
2W
o-l ---
r
w
&
Equallyspacedtime lnd[ces ‘Utput
FIG. 13.38.—Switchedgenerator for multiple time indices.
will ring with sufficient amplitude for several cyc,les. In many cases a
simpler pulse generator can be used.
When two or more marker frequencies are needed alternatively, the
resonant circuit is switched. Scaling-down is rarely used, since more
vacuum tubes would be involved and since it would be necessary to
recycle the counting circuits on each pulse cycle.
Delayed Sweeps.—Any of the timing circuits just discussed can be
used to provide the trigger for a delayed, expanded sweep, and in fact
thk function can be combined with that of accurate range determination.
The most usual method using continuous delays is illustrated in the block
diagram of Fig. 13.39. The precision variable-delay circuit directly
provides the trigger for the delayed sweep. An index marker is pr~
vialed at the center of the sweep by a second fixed-delay circuit, usually
a flip-flop, whose action is initiated by the sweep trigger. The timing
arrangements are obvious from the waveform diagram. The total range