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

摘要

This paper describes a method for widening the passband ofntransversely coupled resonator filters (TCF) using only the fundamentalnsymmetric and antisymmetric modes. The coupling of modes analysis in thentransverse direction is applied to the TCF design to investigate thenpassband width. As a result, it is found that the passband width can benincreased by making the surface acoustic wave (SAW) velocity differencenbetween the interdigital transducer (IDT) region and the resonator gapnregion smaller. It is proposed that a grating structure be applied tonthe common ground bar, instead of the uniform metal, to reduce the SAWnvelocity difference. Using the grating-type common ground bar, filtersnare fabricated on ST-quartz substrate. The passband of a single filternwith a center frequency of 248 MHz is widened up to 410 kHz without anynincrease of the insertion loss. The effect of the impedance mismatch atnthe junction of two cascaded devices is investigated. It is shown thatnthe filter performance is improved by reduction of the small parasiticncapacitance existing at the cascade point. Experimentally, thencapacitance formed between the bus bar of the IDT and the bottom surfacenof the ceramic package is reduced. The insertion loss is reduced by 0.0ndB, and 3 dB passband is widened by 8 kHz for a filter with a centernfrequency of 248 MHz. On the basis of these two improvements, cascadednTCFs are fabricated. For a filter with a center frequency of 248 MHz, anninsertion loss of 5.5 dB and a 3-dB passband width of 270 kHz arenobtained