Degenerated coupled resonator filters

A new class of filter is presented which works in a mode where there is a kind of mixture between Coupled Resonator Filters (CRFs) and ladder type SAW Impedance Element filters (IEFs). Long IDTs, working basically like resonators are used in a CRF type of operational mode. On substrates with high coupling coefficient and high finger reflectivity (as for example 36°-LiTaO₃) a resonance condition imposed on the gap between long transducers must be satisfied in order to provide sufficient transfer of energy from one (input) transducer to another (output) transducer. Further, the relation between the transduction and reflection properties on strong coupling materials are such that unwanted longitudinal modes are strongly suppressed, and simulation shows that a nice flat passband can be realized. The bandwidth for this kind of filter is typically narrower than for conventional CRFs, but this is not a big disadvantage, because there are a lot of potential applications where these filters can be used and they are easy to design