Acoustic surface-wave transversal filters

Some basic concepts of transversal filters are reviewed. Transversal-filter structures provide a conceptually simple way for obtaining a wide range of transfer functions. However, the practical realization of such filters has, in the past, required very complex or bulky circuits because of the necessary delay line and because of the many taps and attenuators that may be needed. However, interdigital acoustic surface-wave transducers on piezoelectric substrates are seen to provide a very compact and attractive way to realize transversal filters. Acoustic waves in a solid have an extremely low velocity as compared with electromagnetic (EM) waves, so that the acoustic surface waves on the substrate provide a very compact delay line. Further, the desired set of weighted taps is readily achieved by a photoetched array of metallic fingers. Fringing electric fields from these fingers tap into the acoustic surface-wave delay line by way of the piezoelectric effect. Equivalent circuits for interdigital acoustic surface-wave transducers are discussed and equations for their transfer and input-impedance properties are presented. The application of interdigital transducers for use as conventional filters, pulse-compression filters, and general matched filters is briefly described.