Transversely coupled resonator filters with 0.1% fractional bandwidth in quartz

This paper describes the realization of transversely coupled resonator filters (TCF) with 0.1% fractional passband width in quartz substrate. The coupling of modes analysis in the transverse direction is applied to the TCF design for the investigation of the passband width determination. As a result, it is found that the passband width can be widened by making the surface acoustic wave (SAW) velocity difference between the interdigital transducer (IDT) region and the resonator gap region smaller. We propose to apply the grating structure to the common ground bar to reduce the SAW velocity difference, instead of uniform metal. We fabricate TCFs using the grating type common ground bar in quartz for specific applications.