Surface wave recirculation loops for signal processing

A repetitive signal can be correlated in a surface acoustic wave (SAW) recirculation loop, thus increasing its SNR. Various imperfections which limit the practical performance of the loop are examined here, particularly the delay line frequency response and spurious levels, and nonlinearity. The conclusions are related to a practical system in which a long pseudonoise signal is correlated by means of a SAW convolver followed by a SAW recirculation loop. The convolver correlates segments of the waveform, giving a repetitive sequence of correlation peaks which can then be summed in the loop. For the latter, a novel SAW delay line was developed using an ST-X quartz substrate, giving a 3-dB bandwidth of 22 percent at 140 MHz with 35-us delay. The system was used to correlate 2.1 ms of a 10 Mbit/s input signal, using 70 circulations in the loop. With an input SNR of -41 dB, an output SNR of +4 dB was obtained.