Radar System Engineering

504 THE RECEIVING SYSTEM—INDICATORS [SEC. 13.9
positive excursion of A the resistance extracts a small charge, which is
replaced by the diode on the next negative excursion.’ The clampigg
can be done at the positive extreme by reversing the diode. If fidelity of
response is of importance, the RC product must be sufficiently large to
give proper low-frequency response.
The d-c restorer has many applications, only a few of which can he
listed here. It is frequently used to insure that a grid to which unipolar
‘%
Waveformat A
‘A Withoutdiode
-*-C
J-LrLrLc
R
.thdiode.~..
o
FIG. 13.20 .—D-c restorer,
video signals are applied is operating in the correct voltage range. ThkI
is particularly important in the intensity modulation of a cathode-ray
tube where the tube must be just cut off for zero signal amplitude. The
d-c restorer can be used to fix the starting point of a sweep by clamping
the deflecting plate of an electrostatic tube, or the grid of a direct-coupled
sweep amplifier, between sweeps. In the above examples, the clamping
fixes the point from which signals are measured. In others, the clamping
fixes the level to which an electrode is switched “ on” by a square wave,
as for example when intensifying a cathode-ray tube.
-Fqlo
F1~. 13.21,—Diode
Limitsrs.
Biased Diodes. —Figure 13.21 illustrates two self-evident methods of
limiting the excursion ~f a signal by clipping off the top at a level determined
by a d-c potential difference applied to the circuit. The circuit
of Fig. 13”22, on the other hand, transmits only that portion of the signal
in which the voltage exceeds the applied potential, so that in effect the
bottom is clipped. This arrangement is of considerable importance in
time-measuring applications. If a very linear sawtooth is applied to the
1Soxnetirneathe resistoris made slightly more negative than Vo in order to hold A
tightly against the diode during the clamping period.