Pulse Compressions
Pulse compressions are used to increase the trans-
mitted average power while retaining the range res-
olution of a narrow pulsewidth. Many fire-control and
search radars use pulse compression. Pulse-com-
pression radar has additional advantages over normal
pulse radar—it has better discrimination of target
echoes in clutter and is less susceptible to jamming.
Two basic types of pulse compression are linear
FM and phase coding. Both encode the transmitted
pulse with information that is compressed (decoded)
in the receiver of the radar. Radars that use pulse com-
pression can compress pulses with durations of many
microseconds down to a tenth of a microsecond.
The ratio of transmitted pulsewidth to compressed
pulsewidth is called the pulse-compression ratio.
Ratios of up to 160:1 are currently in use.
PHASE-CODED PULSE COMPRESSIONS.—
Phase coding the transmitted pulse involves shifting
the phase of the transmitter RF during the pulsewidth.
A binary code is the normal method used to determine
the phase shift. With a binary code, the binary bits can
determine if the signal will be shifted to an in-phase
condition or a 1800 out-of-phase condition with re-
spect to the reference. Pulse compression of the
encoded waveform involves decoding the phase shifts
and comparing this to the stored code.
By making a bit-by-bit comparison of the received
signal to the transmitted signal, target detection can be
determined at the point when the bits match. This type
of circuit is known as a matched filter.
ANGLE-ERROR DETECTIONS.— With track-
ing radars that use phase-coded pulse compression,
extraction of the angle error is also required. Mono-
pulse radar receiver angle-error information is con-
tained in the phase difference between the sum (range)
and difference (angle) channels. Before the phase dif-
ference can be determined, the phase coding must be
removed.
The IF signal from both angle channels and range
channels is equalized in IF limiters (log-IF ampli-
fiers). The phase coding is removed by switching the
output phase of the signal decoder with the binary
phase code during the range gate interval. Phase-
coded IF signals in correspondence with the range
gate and the binary phase code will produce a decoded
signal. Signals more than 1 bit out of correspondence
will have their code changed.
The decoded signal will then be fed to the narrow-
band filter. The decoded signals have a much nar-
rower bandwidth than phase-coded signals and pass
through the narrowband filter. The narrowband filter
will ring when a decoded signal passes through and
produce a signal.
The output signal is then fed to a limiter to main-
tain equal signals in all three channels. The reference
phase of the range channel is compared to the angle
channel in a phase-sensitive detector. The output of
the phase-sensitive detector is a dc voltage represent-
ing the amount and direction of the phase error. The
angle-error voltage is then processed to correct the
antenna/director pointing error.
Figure 2-15 shows the basic process required to
extract the angle-error information.
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