International Symposium on Antennas and Propagation
Interdigital Transducer for Acoustic-Wave Filter
Chia-Cheng Ho, Jean-Fu Kiang,
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SAW filter on piezoelectric substrate can be designed by exciting and coupling several types of waves, including longitudinal (L), shear-vertical (SV), shear-horizontal (SH), and potential φ. Both L and SV waves are coupled with each other through boundary conditions to constitute an eigenmode called Rayleigh-type surface acoustic wave(SAW). Similarly, SH and φ are coupled by the boundary conditions to form a Bleustein-Gulyaev-Shimizu (BGS) wave. SAW filter designs based on Rayleightype, Bleustein-Gulyaev-Shimizu (BGS), Love and surface transverse wave (STW) have been presented in many literatures. Conventional transversal SAW filters composed of two interdigital transducers have high insertion loss due to triple transit echo (TTE). To eliminate mismatch of such regeneration and acoustic reflections, grating reflectors can be placed on both side of the interdigital transducer to construct a single-phase unidirectional transducers (SPUDT) which can serve as a resonator component to construct a more complex filter. Different types of resonator can be synthesized as an impedance element in conventional electrical networks, and SAW filter like ladder-type filter can be design to exhibit required passband and stopband characteristics. In this paper, the free software FEMSDA in  is used to obtain the dispersion relations of SAW propagation along interdigital metallic grating on 36?YX-LiTaO3 substrates. A resonator-type filter can thus be designed using several types of transducer which is basically a cascade of N interdigial elements. Instead of approximating the lumped electrical impedance, we simulate a two-port resonator filter based on the relation between the COM model and the dispersion properties calculated by applying the boundary element method upon anisotropic substrate and metal material.