Radar System Engineering

f!Ec. 17.4] METHODS OF COMBATING’ INTERFERENCE 687
bracketing pulses. They are not, however, usually made greater than a
few microseconds because of the bulkiness of longer delay lines.
A method of decoding the three pulses is shown in the upper part of
Fig. 17.3. Signals from the receiver are dlff erentiated by the grid circuit
of amplifier VICso that blocks of signals are not passed. If the incoming
pulses are large enough, the tube can be biased past cutoff in order to
exclude signals at lower levels (as, for example, when amplitude is to be
used as a basis of discrimination between pulses and video signals).
Weak interference can also be excluded in this way. V, acts as a limiting
1 t++.!lfnI I
~r H~----–-..J.!J-
i-
2U4+
v~~
c-nl----—Tl—T—
--- &JLL_____JLL__
LS6C
~&&Lllu_ ----~
FIG. 17.3.—Interference blanker and triple-pulse decoder.
amplifier. After passing through the cathode follower V~., each pulse
arrives twice at the grid of V3*, once with no delay and once with a
2-psec delay. Six pulses will therefore arrive at VW, and two of these, the
first delayed pulse and the second undelayed pulse, will be in coincidence,
as the waveform diagram shows. Thus five pulses, the second of which
has double amplitude, will arrive at the grid of VAafter a further delay of
4 ~sec. VAis so biased that only the large pulse lifts the grid past cutoff.
Since the total delay of this pulse is 6 psec, its arrival will coincide with
that of the third undelayed pulse on the suppressor of V,. At this time,
and this time only, the plate of V4 receives a signal and fires Vs.. This
event cannot occur if any of the pulses is missing; therefore interference
can produce a result only when three spurious pulses occur with approximately
the correct spacing. Differentiation at the input circuit prevents