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Surface Acoustic Wave (SAW) Pulse Compression Devices

Fabricated on quartz for high temperature stability, SAW dispersive delay lines (DDL's) - sometimes described as 'chirp' filters - are used for pulse compression. Their delay from input to output varies continuously with frequency over their operating bandwidth, and this can be pre-programmed almost arbitrarily at the mask level. Two devices, one in the transmitter and one in the receiver, are programmed as matched filters.

They have complementary dispersive delay characteristics and yield a constant delay when used in cascade as their dispersions mutually cancel. A pulse input to the cascade consequently appears at the output virtually undistorted. However, the pulse emanating from the first device (the transmit pulse) is unrecognisable in terms of the input applied - its length may have been stretched by a factor of several hundred times because of inherent dispersive delay. It sounds almost too good to be true that the SAW device forming the other half of the cascade (i.e. the receive device) can pack this back up again to the near equivalent of the original!

Naturally, the first device is called an expander and the other a compressor.




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