Patent ID: 7479853

Claim:
A balanced-type surface acoustic wave filter with a balanced-to-unbalanced conversion function including an unbalanced signal terminal and first and second balanced signal terminals, the balanced-type surface acoustic filter comprising: a piezoelectric substrate; a first longitudinally coupled resonator-type surface acoustic wave filter portion having first to third IDTs disposed along a propagation direction of a surface wave on the piezoelectric substrate, a middle second IDT of the first to third IDTs being connected to the unbalanced signal terminal; and a second longitudinally coupled resonator-type surface acoustic wave filter portion having fourth to sixth IDTs disposed along the propagation direction of the surface wave on the piezoelectric substrate, the fourth IDT connected to the first IDT, and the fifth IDT connected to the first and second balanced signal terminals; wherein an electric signal passing through a signal line connecting the first IDT and the fourth IDT is about 180 degrees different in phase from an electric signal passing through a signal line connecting the third IDT and the sixth IDT; and in the first longitudinally coupled resonator-type surface acoustic wave filter portion, in at least one of the portion where the first and second IDTs are adjacent to each other and the portion where the second and third IDTs are adjacent to each other, in at least one of the adjacent IDTs, weighting is provided on a plurality of electrode fingers including an outermost electrode finger which is the closest to the adjacent IDT.