A SAW Accumulating Correlator with CCD Readout

An accumulating correlator is described in which the multiplication and time-integration of many samples of two wideband surface acoustic wave (SAW) inputs are achieved in a charge-coupled device (CCD). The CCD is coupled to a LiNb03 delay line across a 350-nm gap by means of an array of 300 polysilicon taps. The SAW inputs counterpropagate on the delay line, and the cross-correlatio n is subsequently r eadout f rom the CCD. Electric fields accompanying the signal and reference SAWS are picked up by the taps, mixed by nonlinearities in the silicon substrate, and the dc mixing products are accumulated in the CCD; thus the charge in each well represents a discrete sample of the correlation between signal and reference. The spatial charge pattern is read out from the CCD to provide 300 discrete samples of a 7-LIS segment of the c orrelation function. T his unique SAW/CCD structure has produced a signal-processin g gain of 30 dB at a bandwidth of 20 MHz for a biphase-encoded waveform dominated by Gaussian noise; and it combines in a single device a number of significant advantages, These include analog-analog correlation, programmability, wide input bandwidth, selectable output data rate, and compactness.