Although sophisticated theoretical models for SAW filters have been developed specifications now call for an even better understanding of second order effects than these models provide. In particular, the precise modelling of the wavefronts in the surface plane is important and 'free diffraction' has generally been assumed in earlier work. However these models, which ignore the effect of the transducer metallization pattern on the wavefronts, are not valid for strongly piezoelectric materials such as lithium niobate. Moreover the relationship between the wavefronts and the individual transducer admittance has not been considered. With the growing importance of low-loss filters accurate predictions of all terms of the admittance matrix of the filter are required. In this paper a new theoretical model for the admittance matrix, based on an expansion of the solution in the orthogonal modes of the structure, is presented. The model takes account of the degree of metallization of different regions of the surface, including the transducer bus-bars. Results are presented for typical filters and compared with earlier theory and experiment.
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