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 porta ble telephones. New rectangular bar resonators using the length expand er mode were presented and a ladder filter was constructed using these new resonators. The optimization of capacitance ratio and resonator s upport structure in resonator design and a new piezoelectric ceramic m aterial were discussed. In particular, the heat resistant properties o f the material and the vibration transmission to the support structure were considered. These properties were improved by using the new cera mic material and a new support structure. Finally an experimental filt er was constructed. It exhibited excellent characteristics which satis fied the specifications for the second IF filter of a digital cellular telephone.