Pulse Compressions

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. 2-28