Development of low-loss band-pass filters using SAW resonators for portable telephones

A bandpass filter using surface acoustic wave (SAW) resonators in a ladder circuit structure for portable telephone systems is reported. For filter design, a simulation tool is used to consider the effects of electrodes (their apertures, the number of paired elements, thickness, and bulk wave radiation). Filter input and output impedance conditions are designed by the resonator capacitance to match the line impedance. The insertion loss and stopband rejection values are traded off against each other and are controlled by the ratio of capacitances between the parallel arm and series arm resonators, 800-MHz and 1.9-GHz band filters mounted in 3.8 mm×3.8-mm×1.5-mm ceramic packages were fabricated. They are suitable for surface mount technology (SMT). Typical frequency response characteristics are 2.5-dB maximum insertion loss in the 25-MHz passband and a stopband rejection of 25 dB