Patent ID: 7425882
Filing Date: 2008-09-16
Classification: H03H

Abstract:
1. A balanced-type surface acoustic wave filter connected to an unbalanced terminal and first and second balanced terminals and provided with a balance-unbalance conversion function, comprising: a piezoelectric substrate; a first surface acoustic wave filter section including first to third IDTs arranged along a surface acoustic wave propagating direction on the piezoelectric substrate, one of the second IDT disposed in a center and the first and third IDTs disposed on both sides of the second IDT being connected to the unbalanced terminal, and the other of the first and third IDTs and the second IDT being connected to the first balanced terminal; and a second surface acoustic wave filter section including first to third IDTs arranged in the surface acoustic wave propagating direction on the piezoelectric substrate, one of the second IDT disposed in a center and the first and third IDTs disposed on both sides of the second IDT being connected to the unbalanced terminal, one of the first and third IDTs and the second IDT being connected to the second balanced terminal, and the second surface acoustic wave filter section having a phase of an output signal to an input signal different by about 180 degrees with respect to the first surface acoustic wave filter section; wherein in the first and second surface acoustic wave filter sections, in a pair of IDTs adjacent to each other with a gap interposed therebetween in the surface acoustic wave propagating direction, a section where a cycle of a portion of electrode fingers including an electrode finger facing the gap is less than a cycle of electrode fingers of a main portion of the IDT is set as a narrow pitch electrode finger section; and when an electrode finger pitch of the narrow pitch electrode finger section of the IDT connected to the unbalanced terminal of the first and second surface acoustic wave filter sections is P are satisfied.