Equivalent Circuit Parameters of Interdigital Transducers Derived from Dispersion Relations for Surface Acoustic Waves in Periodic Metal Gratings

Abstract

Abstrucr-Relations between exact field solutions for acoustic surface waves in structures of metal gratings on piezoelectric surfaces and an equivalent circuit model of interdigital transducers are estab- lished. One of the previously proposed equivalent circuit models is applied, and by matching the dispersion relation for the surface wave derived from this model with that obtained from the field analysis, analytic expressions for the equivalent circuit parameters are deter- mined. The approach used in this paper is to match the dispersion relations near resonance obtained from the field analysis with those obtained from the circuit model, and to use the field solutions far off resonance to determine the transducer capaci- tance. Principally, we are using the field solution for an infinite periodic array to develop a circuit model for the unit cell of the array. The model therefore fails to describe transducer end effects and neighbor coupling effects in a transducer with arbitrarily changing polarity on neighboring fingers ( IO) . The field solution for surface waves in split-strip periodic arrays is applied to analyze thinned transducers with several half wavelengths between neighboring finger pairs. Neighbor coupling can be estimated from the dependence of the circuit parameters on the distance between pairs. Another special case considered is the regular split-strip transducer with neigh- boring strips connected to one electrode in pairs of opposite polarity.