Use of Green's function approach for low loss SAW filter CAD system development

Abstract

Low loss filters based on Single Phase Unidirectional Transducers (SPUDT) could be characterized as a separate class of Surface Acoustic Wave (SAW) filters because they overcome the main SAW filters limitation-6 dB bidirectional insertion loss. There are two different physical phenomena which could be used to ensure low loss: acoustic reflection of SAW (used normally for substrate with small piezoelectric coupling coefficient k/sup 2/) and electric SAW regeneration (used for substrate with high k/sup 2/). Although both effects finally give contribution to reflectivity of SAW Interdigital Transducer (IDT), models used to account for them are completely different. For SPUDT on the substrates with small k/sup 2/ COM-theory is used, while for the substrate with high value of k/sup 2/ Green's Function Model (GFM) could be directly used. GFM being applicable to IDT with arbitrary topology has two main disadvantages, namely, it is difficult to find a set of consistent model parameters for practical use and computation time increases in power law with the growth of IDT length reaching very quickly unacceptable values. In this paper we present CAD system for calculation of low loss SAW filters parameters based on GFM. Floquet theorem being used for presentation of filter with long IDT, computation time decreases dramatically. The method developed works both for high and low k/sup 2/ value substrates. Results of calculation of SPUDT characteristics with the help of method proposed are presented.