Filter device

摘要

In a communication system including a first bandpass filter having a relatively low passband or a second bandpass filter having a relatively high passband, a filter device is used as the first bandpass filter. Series-arm resonators are inserted in a series arm connecting an input terminal and an output terminal. Parallel-arm resonators are connected in parallel arms connecting the series arm and a reference potential. Inductances are connected in series with at least one of the parallel-arm resonators. The resonant frequency of a secondary resonance generated by insertion of the inductances is within or in the vicinity of the passband of the receiver or transmitter bandpass filter serving as a partner filter of the ladder filter.