SAW Pulse Compression
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SAW Pulse Compression

The Compressor or Receive section of the Pulse Compression Subsystem comprises the SAW compressor dispersive delay line(s) plus the device matching and amplification circuits required to meet the system dynamic range/signal to noise parameters.

Single compressors or multiple, matched compressor sets can be provided, e.g. for phased array or monopulse radars. The key subsystem parameters are explained below.

Compressed Pulse Width (t)

Compressed Pulse Design FeaturesThe pulse width is normally defined at its -3dB level. It is also common to specify the pulse width at the base of the pulse somewhat above the peak sidelobe level, say at -35dB, as this determines discrimination between a small target and an adjacent larger one.

Sidelobe Levels

Sidelobe levels fall into categories:

Close-In Sidelobes
These are caused by minor waveform distortions, including Doppler shift.

Far-Out Sidelobes
Caused by gating effects related to finite pulse length.
In order to simplify specification, close-in sidelobes are those lying within a range of ± 5 where is the -3dB compressed pulse width. The level of sidelobe suppression is dependent on the time bandwidth product, TB, and the weighting function applied. Values in the range 30 to 50dB have been achieved in current systems. Estimates of performance for specific parameters are provided through computer simulation and based on a considerable database of practical designs.

Loop Delay

As discussed in the Expander Section, the centre frequency delay is measured at the nominal centre frequency of the compressor. As long as expander and compressor delays are specified at the same absolute frequency, the addition of expander and compressor delays gives the loop delay. This corresponds to the time after the trigger point at which the compressed pulse peak is produced and it will normally be no less than twice the centre frequency delay (t0) specified in the expander. Since loop delay is affected by Doppler shift, it is normally specified at zero Doppler.

The maximum input signal to the compressor is normally specified to be 0dBm. Additional internal amplification can be provided if requested. Design practice ensures that signal levels in the compressor channel give maximum dynamic range.

Output Level

This is the peak level of the compressed pulse with a maximum working signal level applied at the input. Common practice is to have either unity gain to the signal or unity gain to noise. Remember, compression gain makes a big difference and it may be impractical to have unity gain to noise! 

Dynamic Range

SAW Compression

The dynamic range is usually specified as the ratio of the peak available signal output and the rms output noise level when the compressor input is terminated in a matched impedance. The required dynamic range should be specified by the systems engineer so that MESL can determine the required signal levels at the various points within the compressor channel. A design example is given in Figure 4.

Here, a processing gain of 20dB is assumed (see Additional Technical Notes). The equivalent noise bandwidth is approximately half the nominal bandwidth. For practical purposes, only noise from the post-compression amplifier is relevant and the bandwidth of the output filter is assumed to be equal to the compressor bandwidth channel since this is the optimum situation.

Input and Output Impedances

As stated for the expander, MESL subsystems are designed to operate in a 50W system with impedances which generally provide an operating VSWR of 1.5. Other values may be provided when required.


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