Passband widening of transversely coupled resonator filters using the fundamental symmetric and antisymmetric modes

This paper describes a method for widening the passband of transversely cou pled resonator filters (TCF) using only the fundamental symmetric and antis ymmetric modes. The coupling of modes analysis in the transverse direction is applied to the TCF design to investigate the passband width. As a result , it is found that the passband width can be increased by making the surfac e acoustic wave (SAW) velocity difference between the interdigital transduc er (IDT) region and the resonator gap region smaller. It is proposed that a grating structure be applied to the common ground bar, instead of the unif orm metal, to reduce the SAW velocity difference. Using the grating-type co mmon ground bar, filters are fabricated on ST-quartz substrate. The passban d of a single filter with a center frequency of 248 MHz is widened up to 41 0 kHz without any increase of the insertion loss. The effect of the impedance mismatch at the junction of two cascaded device s is investigated. It is shown that the filter performance is improved by r eduction of the small parasitic capacitance existing at the cascade point. Experimentally, the capacitance formed between the bus bar of the IDT and t he bottom surface of the ceramic package is reduced. The insertion loss is reduced by 0.6 dB, and 3-dB passband is widened by 8 kHz for a filter with a center frequency of 248 MHz. On the basis of these two improvements, cascaded TCFs are fabricated. For a filter with a center frequency of 248 MHz, an insertion loss of 5.5 dB and a 3-dB passband width of 270 kHz are obtained.