Radar System Engineering

506 THE RECEIVING SYSTEI-INDICA TORS [SEC. 139
two diagrams indicating clamps of opposite polarities. They will, of
course, close only if the potential of point X is in the specified direction
from li’O,and even when closed they will offer no impediment to potential
changes in the opposite direction. The resistance R is included to counteract
any slight leakage charges.
The requirements for the source of the switching waveform are
somewhat rigid. It must provide a satisfactory value of EOand a square
wave of sufficient amplitude. Furthermore, since its impedance adds to
that of the clamp, this impedance must be sufficiently low to withstand
any ‘‘ pulling” by the circuit attached to X, and must be capable of discharging
sufficiently rapidly any capacity associated with X.
Frequently a satisfactory square-wave source already exists in the
equipment. If not, further complexities must be added to the clamp.
u-
Figure 13.23b shows a modification that can be used when the source has a
+
+R>> R,
w R
* x
l-r
+ ‘--
c
R,C>>T
I Eo
(.) .%2E, (b) Ex 5EO
FIG. 13.24.—One-way triode clamps. (a) Positive clamp; (b) negative clamp.
sufficiently low impedance but an improper d-c level. The tube VI
serves the same function as before and Vj acts as a d-c restorer. The
presence of the resistor RI increases the requirement for a low-impedance
source.
If the waveform source has high impedance, it is necessary to introduce
a cathode follower as shown in Fig. 13.23c. The clamping takes
place through the diode and RI; thus R, must not be too large. This in
turn requires considerable current through Vt during the clamping interval.
The proper d-c level for the switching signal can usually be chosen
by a proper choice of R,, R,, and the bias potential. If not, it may be
necessary to use condenser coupling and a d-c restorer on the cathodefollower
grid. The corresponding negative clamp is not used, since the
circuit of Fig. 13.24b accomplishes the same results more simply.
Figure 13.24 illustrates one-way clamps using a single triode. 1 In
both cases the clamping is done through the tube impedance as viewed
from X. In the positive clamp this impedance is unfortunately rather
large so that this clamp is not very ‘‘ tight .“ The negative one, on the
LThe well-known Rossi coincidence circuit is an example of a triode clamp.