Analysis of SAW transducer having aperiodic multi-electrode cells using a coupled FEM/BIE numerical model

Low loss SAW filters sometimes require a structure with a complex geometry in order to improve the electro acoustical response (coupling coefficient, reflection coefficient, and static capacitance). Most of the coupling of mode models and P matrix models use parameters obtained from a single electrode periodic transducer analysis. In the case of complex electro acoustical cells (like HanmaHunsinger cells), it is sometimes necessary to obtain the COM parameters from the analysis of the entire electro acoustical cell, which means assuming that the filter contains an infinite array of identical electro acoustical cells the elementary cell of which is made of several electrodes. At the 2011 IEEE Ultrasonics Symposium, an original coupled Finite Element Model/Boundary Integral Equation (FEM/BIE) was presented in order to simulate an infinite array of single metallic electrodes [1]. In this paper, a generalization of this model to elementary cells more complex than one electrode will be presented. Like in the 2011 publication, the Finite Element Method is used for the finite part of geometry (the electrodes, part of the dielectric, and part of the piezoelectric substrate), while the semi infinite part of the geometry (piezoelectric part, and dielectric part) are taken into account using a Boundary Integral Equation. The theoretical and the numerical parts of the model will be presented, as well as Coupling of Modes or P matrix parameters determination for Hanma-Hunsinger low loss cells with various substrate materials and orientations. Using the results of the FEM/BIE analysis, a simulation of a filter containing modified Hanma-Hunsinger electro acoustical cells is made and is compared with measurements.