Radar System Engineering

510 THE RECEIVING S YSTEhl—INDICA TORS [SEC.1310
cuits are properly matched, and the signal grids can therefore be in full
operation at all times. Since it is essential that the proper d-c levels be
maintained when the signal duty interval is large, d-c restorers must
follow any a-c coupling. This circuit is often used as a single-channel
switch, the signals being applied to one tube, and the other serving merely
to cancel the plateau. The circuit may also be adapted to suppressor
grid switching.
13.10. Sawtooth Generators.—The sawtooth waveforms used as a
basis for range sweeps and in certain varieties of precision timing circuits
are practically always generated by a wave-forming network that is
switched on and off by some sort of clamp.
The basic action of all such devices can be illustrated by the simple
circuit of Fig. 13.29. When S, a single clamp of proper polarity, is closed,
an equilibrium state is reached in which X differs from ground potential
E
(1) Clym;;,g >T -+1 r
+oli +on.
R
!
~---- ----—- ==--—-.-
x
(2) Vx when
l----m
s
(J
.
Rg
c
RC=T
(3) V&:
ifi::--
Gnd
-- —_____________
T ----
E
FIG. 13.29.—Basicsawtooth generator.
only by the small drop across R~. If S is suddenly opened, the current
is “shifted from S to C, and the latter starts to charge exponentially at a
rate determined by R, C, and E. The rising wave of the sawtooth is an
exponential of time constant RC, as indicated in Waveform 2 of Fig.
1329. If the sawtooth has an amplitude small compared to E—that is,
if RC >> T—the rise is approximately linear (Waveform 3). Such sawtooth
generators are widely used in cases where no great precision is
required.
When various sweep speeds are to be used, it is necessary to change
the rate of rise of the sawtooth. This can be done in discrete fashion by
switching the condenser, the resistance, or, within limits, the charging
voltage. Continuous variation is most satisfactorily accomplished by
using a rheostat for R (unless the control is remote) or by varying E
through a potentiometer.
Figure 13.30 illustrates both positive and negative sawtooth generators
using single-triode clamps. If waves of both polarities are desired,
the two circuits of this figure can be combined, or a single tube can be