Radar System Engineering

686 RADAR RELAY [SEC. 17.4
excluded bythe receiver. If, however, theseinterferingp ulsesarenottoo
numerous and strong, it is possible to provide almost complete protection
against them by using a “coded” signal to represent the pulse, or by
excluding all pulses that do not come within the narrow time interval
which follows the last useful pulse by the known repetition period, or by
doing both.
The most usual type of coding consists of a group of pulses succeeding
each other by precise, unequal time intervals. The responsive circuit at
the receiver is arranged to recognize only a group with precisely these
spacings. .4 similar combination is extremely unlikely to occur in the
interference.
+=
Triggerin
p~llr. ,,..
.
4
%llillr’~
Trig,
.ti
(I2L --- c
Wti +====
FIG. 17.2.—Triple-pulse coder.
A common type of coding and a simple method of producing it are
illustrated in Fig. 17.2. When the blocking oscillator V,. is triggered
by an incoming signal, it produces a pulse of less than l-psec duration.
The sharp positive pulse at 1?is passed through the 2-~sec delay line to the
grid of ~lb. The negative pulse on the plate of Vlb passes back to V,. and
retriggers it to produce a second pulse delayed by 2 psec (Waveforms A
and B). The cumulative effect of the two actions charges Cl sufficiently
to cut off Vla for a time so that further regeneration does not take place.
The first pulse is driven down the 4-psec delay line by ~lb and triggers V2~
after a total delay of 6 ~sec. The firing of V,. charges C2 sufficiently to
cut off the tube so that the second pulse, which arrives 2 ~sec later, cannot
trigger it. Thus three pulses occurring at O, 2, and 6 psec appear across
resistance R and pass to the mixer to be combined with the video and
other
signals.
Other combinations of time delay can, of course, be used. The
individual delays should not only be unequal, but within reason each
should be great enough to prevent radar or other interference pulses from