Patent ID: 7479854

Claim:
A balanced-type surface acoustic wave filter including an unbalanced signal terminal and first and second balanced signal terminals, and having a balanced-to-unbalanced conversion function, the balanced-type surface acoustic wave filter comprising: a piezoelectric substrate; a first longitudinally coupled resonator surface acoustic wave filter unit including first, second, and third IDTs disposed along a propagation direction of a surface acoustic wave on the piezoelectric substrate, the second IDT being connected to the unbalanced signal terminal; and a second longitudinally coupled resonator surface acoustic wave filter unit including fourth, fifth, and sixth IDTs disposed along a propagation direction of a surface acoustic wave on the piezoelectric substrate, the second longitudinally coupled resonator surface acoustic wave filter unit being connected in a cascade arrangement to the first longitudinally coupled resonator surface acoustic wave filter unit, the fifth IDT being connected to the first and second balanced signal terminals; wherein the second IDT includes narrow pitch electrode finger portions extending from end portions adjacent to the first and third IDTs toward a center thereof in the propagation direction of the surface acoustic wave, and the fifth IDT includes narrow pitch electrode finger portions extending from end portions adjacent to the fourth and sixth IDTs toward a center thereof in the propagation direction of the surface acoustic wave, each of the narrow pitch electrode finger portions including some electrode fingers whose electrode periods are less than those of electrode fingers in a center portion of a corresponding IDT; and when the number of electrode fingers included in each of the narrow pitch electrode finger portions in the second IDT is defined as N 1 and the number of electrode fingers included in each of the narrow pitch electrode finger portions in the fifth IDT is defined as N 2 , the numbers N 1 and N 2 are set to N 1 >N 2 .