Design considerations on wideband longitudinally-coupled double-mode SAW filters

This paper discusses the design of wide bandwidth SAW resonators when the internal reflection within IDTs is not negligible. In the discussion, the p-matrix expression is employed with a help of the coupling-of-modes theory. First, one-port SAW resonators are discussed. It is shown that the deformed IDT passband shape makes the gap between the IDT and reflector unnecessary. This is most preferable for the reduced scattering loss at the discontinuity. Secondly, two-port SAW resonators are discussed. It is shown that although the internal reflection limits the achievable passband width, it is effectively, used for controlling the location of the resonance frequencies in the conventional design. The result suggests that the proper handling of the internal reflection would be crucial in realising high performance DMS filters. Finally, design criteria are presented for DMS filters of wide bandwidth, and it is demonstrated how device performances are improved by proper design taking account of the criteria.