Surface-acoustic-wave matched filters for multifrequency shift keyed communication systems

The paper describes the design, fabrication and assessment of twelve s.a.w. matched filters on ST-quartz used to demodulate data in a four-level frequency shift keyed system. The system uses four frequencies separated by 250 kHz around 70 MHz, and the pulse length (4, 12, or 36 microsec) may be selected according to the input signal/noise ratio to ensure reliable detection of the matched filter outputs. A 12 microsec transducer made up of 4 microsec and 8 microsec sections was used as the basic design unit. Thinned output transducers and quarter-wave cancellers were used to reduce reflections. Individual frequency responses of each of the twelve filters showed relevant nulls better than 35 dB. When all four frequency responses of a filter bank were superimposed, the orthogonal channel rejection was 25 dB in the worst case, which is adequate for error-rate performance within 1 dB of theory. Computer simulation showed close agreement with measured responses. The model includes taper, random phase errors incurred at the artwork cutting stage, electromagnetic delay and, in the case of 36 microsec devices, piecing errors and curvature of the 15 cm substrate.