Surface-acoustic-wave matched filters for multifrequency shift keyed communication systems
This 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 f.s.k. system. The system uses four frequencies separated by 250 kHz around 70 MHz, and the pulse length (4, 12, or 36 μs) may be selected according to the input signal/noise ratio to ensure reliable detection of the matched filter outputs. A 12 μs transducer made up of 4 μs and 8 μs sections was used as the basic design unit. This provided the 4 μs and 12 μs devices, and piecing three 12 μs sections on the same substrate gave us the 36 μs devices. We have used thinned output transducers and quarter-wave cancellers 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 bankwere superimposed, the orthogonal channel rejection was 25 dB in the worst case, which is adequate for error-rateperformance 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 μs devices, piecing errors and curvature of the 15 cm substrate.
