Filter circuit employing surface acoustic wave device

摘要

A filter circuit comprises a surface acoustic wave device including input and output transducers. A tuning coil is interposed in series between an input terminal and the input transducer and/or between an output terminal and the output transducer. A piezoelectric resonator is connected in parallel with the tuning coil. The resonance frequency of the piezoelectric resonator is selected to be approximately equal to the frequency where an attenuation pole appears. The tuning coil and a capacitance component of the piezoelectric resonator and a clamp capacitance of the transducer constitute a tuning circuit, which is tuned to a given frequency in the passband.