Minimizing the bulk scattering loss in CRF (DMS) devices

The losses arising from the scattering of SAW into bulk waves in the non-synchronous areas of SAW devices are studied numerically using the boundary element method combined with finite element method (FEM/BEM). We use a typical one-port hiccup resonator on 42°-LiTaO₃ as reference structure. Strong scattering into bulk wave occurs in the central gap due to an abrupt change in periodicity. To reduce the scattering, we replace the gap with electrodes with reduced pitches. We show that it is possible to increase the Q-factor of the resonator significantly while keeping the resonant frequency unchanged. Two types of structures are studied, the "distributed" gap and the "accordion" gap. To minimize the bulk-wave scattering in dual mode SAW filters, we replace the metallized gaps in the traditional filter with distributed gaps. We find an optimal combination of pitch and metallization ratio in the gaps, with insertion loss reduced by 0.3 dB.