Radar System Engineering

SEC. 13.7] GENERA TION OF RECTANGULAR WAVEFORMS 497
“flip-flop.” It has wide application both in cases where the waveform
is directly used and in timing or “delay” circuits.
3. Those in which both wavefronts are externally induced. In this
class are the “squaring amplifier, ” which produces square waves
from sinusoids or other waveforms, and various triggered devices.
The latter can be divided into two classes: (a) the “ flopover” or
“ lockover, ‘‘ in which an external signal induces a change from one
stable state to another, and a reverse signal or one from a second
source reverses the operation (Fig. 13.15a); and (b) “scale-of-two”
circuits, in which successive triggers from a single source induce
alternate transitions between two stable stqtes (Fig. 13”15b). The
name “ scale-of-two” arises from the fact that if sharp pulses are
derived from those wavefronts of o~e polarity their number will be
half that of the original trigger pulses.
FC2 c,
y;
.-—
(2) Plate 1
(3) Grid 2
:0 t, t, t; t;
l-rl_rLB+
--~m-+~~f
I
(4) Plate 2
—rul_rB+
FIG. 13. 13.—Eccles-Jordan multivibrator.
Aside from the squaring amplifier, which will not be discussed here,
there are three principal forms of rectangular-wave generators.
The Ectiles-Jordan Circuit. —Figures 13”13 to 13”15 illustrate a group
of two-tube circuits in which the sharp transitions are produced by positive
feedback from each plate to the alternate grid. All are based upon
a circuit of Eccles and Jordan. 1
Figure 13.13 illustrates the free-running multivibrator, which is a
form of relaxation oscillator. In order to understand its action, assume
an initial condition (for example, at LOon the waveform diagram) with
the grid of V1 beyond cutoff and that of Vz at cathode potential. Then
V2 is temporarily quiescent with its plate at a low value; the plate of V,
is at B+, and grid g1 is rising exponential y toward bias potential as Cl
is discharged through RI. At time t,, gl reaches the cutoff point and VI
starts to amplify. The amplified signal is passed to Vz where it is further
amplified and fed back to VI. This regenerative action quickly lifts g,
to the grid-current point and drives gn far past cutoff so that the original
condition is reversed. The plate of VI is down and that of Vz is at B+.
1W. H. Ecclesand F. W. Jordan, Radio Rev., 1, 143 (1919).