Transverse double mode saw filter

Abstract

A transversally coupled double-mode SAW filter using a surface acoustic wave has two SAW resonators which have curtain-like electrodes and a pair of reflectors on both side thereof, respectively, and which are disposed in parallel on a piezoelectric flat plate in the propagation direction of the surface acoustic wave. The curtain-like electrodes of the SAW resonators have a first bus bar of electrically positive electrode which extends in the propagation direction of SAW and second and third bus bars of electrically negative electrode which extend in parallel on both sides of the first bus bar. The first bus bar has multiple electrode fingers of electrically positive electrode which face the second and third bus bars on both sides thereof and which extend in a direction perpendicular to the propagation direction of SAW so as to suitably have a space of &lgr;/2, &lgr; being the wavelength of the surface acoustic wave, and a width of &lgr;/4. The second and third bus bars have multiple electrode fingers of electrically negative electrode which correspond to the electrode fingers of electrically positive electrode, face the first bus bar and extend in a direction perpendicular to the propagation direction of SAW so as to similarly have a space of &lgr;/2 and a width of &lgr;/4. Thereby, the curtain-like electrodes are constituted to invert the direction of driving field by 180° with the first bus bar therebetween, and to enable making the passband width sharply broader than before by use of the frequency difference between excited primary symmetric mode and primary anti-symmetric mode.