Transversely Coupled Resonator Filters Utilizing Reflection of Bleustein-Gulyaev-Shimizu Wave at Free Edges of Substrate

Smaller-sized and lower insertion loss surface acoustic wave (SAW) filters are urgently required for mobile telecommunications. We describe here that miniature, low-loss transversely coupled resonator filters without reflector electrodes could be realized and commercialized by utilizing the edge reflection of a Bleustein-Gulyaev-Shimizu (BGS) wave for the first time. The transversely coupled resonator filter has a narrower bandwidth than a longitudinally coupled one. A small, low-loss resonator filter for a first intermediate-frequency stage (first IF) of a spread spectrum (SS) codeless telephone system could be realized using a piezoelectric ceramic substrate with a coupling factor (k) of 0.20 and the transversely coupled resonator filter consisting of single finger electrodes. A filter for the first IF of the global system for mobile communication (GSM) requires a narrower bandwidth than SS codeless telephone, but a suitable substrate with a small coupling factor is not available. However, we could realize the small transversely coupled filter for GSM using split finger electrodes.