Resonant modes in coupled resonator filters and the unique equivalent circuit representation

An equivalent circuit representation, which has been successfully used in SAW transversely coupled resonator filter (TCF), is extended to longitudinally coupled resonator filters (CRF). In TCF case usually two fundamental modes are accurate enough to represent the passband but more modes (most of them are `quasimodes´ located outside of the stopband of the structure) are needed for CRF. A mode identification method is introduced in terms of equivalent motional parameters, R, L, and C and then applied to two experimental examples, a narrow bandfilter on quartz and a 3% bandwidth filter on 36° YX-LiTaO₃. The results confirm that it could be a useful tool for system designer as it is compatible property with commercial electronic circuit CAD software. The profiles of the main resonant modes detected by a laser probe are presented in this paper