6A-2 Design Modeling and Visualization of Low Transverse Modes R-SPUDT Devices

A problem when designing narrow band resonant SPUDT devices is the excitation of undesired transverse modes. These transverse modes result both in extra ripple in the passband and spurious response in the stop band. To avoid these issues, it was recently proposed by Mayer et al. [2005] to use an approach similar to the one used for BAW devices. The principle is to add a low velocity region at the edge of the transducer. If this region is adequately designed, the transducer supports a so-called "piston mode", i.e. a mode having a flat transverse amplitude across the aperture. A model of SPUDTs with transverse modes based on [Solal et al., 2004] is presented. The model is improved to account for the dependence of the velocity (and transverse modes) in the propagation direction and is used to analyze regular and piston mode devices. We investigate and compare different physical possibilities to implement the low velocity region. In particular, we found it important to design the transducer so that the acoustical sources and reflectors extend into the edge to minimize the coupling to higher order modes. From these considerations, a new implementation for piston modes devices is proposed and demonstrated on a GSM base station 199 MHz filter. Electrical measurements as well as acoustical fields measured with the optical interferometer [Knuuttila et al., 2000] are analyzed and compared to simulation