Patent ID: 7378923
Filing Date: 2008-05-27
Classification: H03H

Abstract:
1. A balanced SAW filter including an unbalanced terminal and first and second balanced terminals and having a balanced-to-unbalanced conversion function, comprising: a piezoelectric substrate; and first to fifth IDTs arranged on the piezoelectric substrate along a surface-acoustic-wave propagation direction; wherein the phase of the second IDT differs from the phase of the fourth IDT by 180 degrees; the third IDT includes a first sub-IDT portion and a second sub-IDT portion that are divided and disposed in the surface-acoustic-wave propagation direction, the first sub-IDT portion and the first IDT are connected to the first balanced terminal, and the second sub-IDT portion and the fifth IDT are connected to the second balanced terminal; one of the first to fifth IDTs is adjacent to another one of the first to fifth IDTs in the surface-acoustic-wave propagation direction and the one of the first to fifth IDTs includes, on an end near the adjacent IDT, a narrow-pitched electrode finger portion including a plurality of electrode fingers whose pitch is narrower than the pitch of electrode fingers in an electrode finger portion other than the narrow-pitched electrode finger portion; the pitch of electrode fingers in one of a narrow-pitched electrode finger portion of each of the first and fifth IDTs that are connected to the balanced terminals and a narrow-pitched electrode finger portion of the third IDT that includes a larger number of electrode fingers is larger than the pitch of electrode fingers in the other one of the narrow-pitched electrode finger portion of each of the first and fifth IDTs and the narrow-pitched electrode finger portion of the third IDT that includes a smaller number of electrode fingers; and the pitch of electrode fingers in one of a narrow-pitched electrode finger portion provided in each of an area of the second IDT adjacent to the first IDT and an area of the fourth IDT adjacent to the fifth IDT and a narrow-pitched electrode finger portion provided in each of an area of the second IDT adjacent to the third IDT and an area of the fourth IDT adjacent to the third IDT that includes a larger number of electrode fingers is larger than the pitch of electrode fingers in the other one of the narrow-pitched electrode finger portion provided in each of the area of the second IDT adjacent to the first IDT and the area of the fourth IDT adjacent to the fifth IDT and the narrow-pitched electrode finger portion provided in each of the area of the second IDT adjacent to the third IDT and the area of the fourth IDT adjacent to the third IDT that includes a smaller number of electrode fingers.