Filter circuit utilizing a surface acoustic wave filter

Claim 1

A surface acoustic wave band-pass filter circuit comprising: a surface acoustic wave filter device including a piezoelectric substrate, and a uniform transducer and a weighted transducer which are disposed apart each other on a surface of the substrate, the uniform transducer comprising a first comb electrode and a second comb electrode, the first comb electrode having a first plurality of finger electrodes, and a first bus bar, bridging each end of said first plurality of electrodes, said second comb electrode having a second plurality of electrodes, said second plurality of electrodes being one less than the number of finger electrodes of a comb of a uniform transducer of a surface acoustic wave filter device designed to represent a frequency response which has a constant amoount of attenuation in a transmit band of the device, and a second bus bar bridging each end of said second plurality of finger electrodes, an inductance element connected to the weighted transducer of the surface acoustic wave device to compensate a capacitance existing in the transducer connected to the industance element; and means for adjusting Q-factor of the resonant circuit formed by said inductance element and the capacitance of the surface acoustic wave filter device and for optimizing a flatness of a frequency response in the pass band of the band pass filter circuit.