Yasuteru Asakawa

A Low-Profile Ladder Filter for Portable Telephones Employing Length Expander Mode

A low-profile piezoelectric ceramic ladder filter was developed. It is less than 2 mm high. Therefore it is suitable for super compact portable telephones. New rectangular bar resonators using the length expander mode were presented and a ladder filter was constructed using these new resonators. The optimization of capacitance ratio and resonator support structure in resonator design and a new piezoelectric ceramic material were discussed. In particular, the heat resistant properties of the material and the vibration transmission to the support structure were considered. These properties were improved by using the new ceramic material and a new support structure. Finally an experimental filter was constructed. It exhibited excellent characteristics which satisfied the specifications for the second IF filter of a digital cellular telephone.