Theoretical Investigation of Wideband Longitudinally Coupled Resonator Filter Using Lithium Niobate Thin Plates

This paper describes the possibility of wideband longitudinally coupled resonator filters (LCRFs) composed of LN-based thickness-shear BAW resonators (TSBARs). Not so small acoustic impedance is required for the coupling layer between stacked resonators because of extremely large electromechanical coupling factor $k^{2}$ of the structure. As the coupler, $\text{SiO}_{2}$ and $\mathbf{SiOC}$ with different C content are applied, and achievable performances are compared. When $\mathbf{SiO_{2}}$ is employed, although its acoustic impedance is slightly larger than optimal, flat and wide passband can be also achievable giving small inductors in series. It is also shown that flat and very wide passband can be realizable when SiOC with proper C content is used as the coupler.