Ultra-wideband T- And π-type ladder filters using a fundamental shear horizontal mode plate wave in a LiNbO₃ plate

A fundamental shear horizontal mode (SH₀) plate wave in a thin (0°, 115°-120°, 0°) LiNbO₃ plate has electromechanical coupling factor k ² larger than 50%. In this study, ultra-wideband π-type and T-type ladder filters composed of three SH₀ plate wave resonators were fabricated and characterized. The interdigital transducer (IDT) pitch (P) ratio of IDTs in the parallel and series arm resonators (PR: IDT pitch of parallel arm resonator (P _parallel)/that of series arm resonator (P _series)) was experimentally investigated. The 3 dB bandwidth is as large as 33%-61%, depending on the IDT pitch ratio. The insertion loss and shape factor in terms of the IDT pitch ratio are also reported.