Radar System Engineering

SEC. 1310] SAWTOOTH GENERATORS 511
used to perform both functions in a somewhat less satisfactory manner.
More often, however, a single wave is generated and the second obtained
from it by a phase-inverting amplifier.
More sophisticated sawtooth generators are necessary when precision
is required. Allofthese endeavor toprotide great linearity inone of the
.,J1;GF
—
FIG. 13.30.—Sawtooth generators using triode clamps.
following ways: (1) by substitutinga “constant current” device for R;
(2) by adding the condenser voltage toE, thereby keeping the current
through R constant; (3) by including a high-gain negative-feedback
amplifier, so that only an extremely small voltage need be developed
across the condenser.
1. Apentode can beusedas aconstant-current device. This method
has been little used in radar, however, partly because of general
complications and partly because
when a pentode is used
in a positive saw-tooth generator
it is necessary to provide
a floating screen-voltage
2.
supply.
Figure 13.31 illustrates the
use of a cathode follower to
keep the voltage drop across
R nearly constant. That
part above the switch indic
andR=
a t es t h r e e alternatives.
Neglecting the a-c d r o p
~
across C,, the positive feedback
raises point Y by an
FIG. 13.31 .—Bootstrap sawtooth generbtor.
Switch indicates alternate methods.
amount EXG, where G is the
gain of the cathode follower and E. the potential of the point X.
Thus the drop across R is given by
E–E, +E&=E– E,(l– G)