Patent ID: 8525621

Claim:
A boundary acoustic wave filter device comprising: a piezoelectric body; a dielectric body laminated on the piezoelectric body; and an electrode structure arranged at a boundary between the piezoelectric body and the dielectric body; wherein the boundary acoustic wave filter device utilizes an SH-type boundary acoustic wave that propagates along the boundary; the electrode structure includes a first IDT, and second and third IDTs that are provided on both sides of the first IDT in a direction in which a boundary acoustic wave propagates to define a longitudinally coupled resonator boundary acoustic wave filter portion; at portions of the first to third IDTs at which two IDTs are located adjacent to each other, a pitch of a portion of electrode fingers of one IDT located at an end adjacent to another IDT is less than a pitch of another portion of electrode fingers of the one IDT adjacent to that portion, so as to define a narrow pitch electrode finger portion; the pitch of the electrode fingers of at least one of the narrow pitch electrode finger portions provided in the first IDT and the second and third IDTs is different from the pitch of the electrode finger portions of each of the remaining narrow pitch electrode finger portions; the number of electrode fingers of each of the narrow pitch electrode finger portions of the first IDT is different from the number of electrode fingers of each of the narrow pitch electrode finger portions of the second and third IDTs; and the pitch of the electrode fingers of the at least one of the narrow pitch electrode finger portions provided in the first IDT and the second and third IDTs and the pitch of the electrode finger portions of each of the remaining narrow pitch electrode finger portions are selected so as to reduce a spurious response in a frequency band that is higher than a pass band of the boundary acoustic wave filter device.