Full analysis of the mixed matrix parameters for a SAW transducer having aperiodic multi electrode cells

Low loss SAW filters sometimes require a structure with a complex geometry in order to improve the electro-acoustic response (coupling coefficient, reflection coefficient, static capacitance). In the case of complex electro-acoustic cells (like Hanma-Hunsinger cells), it is necessary to obtain the mixed matrix parameters from the analysis of the entire multi-electrode electro-acoustic cell. Typically, the analysis is made for the cell in an infinite periodic array of identical cells using periodic boundary conditions. For each mixed matrix parameter, like the reflectivity or the piezoelectric coupling, the amplitude and the phase need to be known with a good accuracy in order to obtain good SAW filter designs. At the 2013 IEEE Ultrasonics Symposium, an original coupled Finite Element Model / Boundary Integral Equation (FEM/BIE) was presented which computes the harmonic admittance of an infinite array of aperiodic multi-electrode cells. In this paper, we will focus on extracting accurate mixed matrix parameters of Hanma-Hunsinger electro-acoustic cells. Numerical data will be given for an interesting six electrodes elementary cell structure called SFT and modified USFT. Applications in the design of SAW filters are shown.