Patent ID: 7034639
Filing Date: 2006-04-25
Classification: H03H

Abstract:
1. A longitudinally coupled resonator type surface acoustic wave filter having a balance-unbalance conversion function, the filter comprising: first-stage and second-stage longitudinally coupled resonator type surface acoustic wave filters longitudinally coupled to each other, each of the first-stage longitudinally coupled resonator type surface acoustic wave filter and the second-stage longitudinally coupled resonator type surface acoustic wave filter includes a piezoelectric substrate and first, second and third IDTs arranged on the piezoelectric substrate in a surface acoustic wave propagating direction, the second IDT of at least one of the first-stage and the second-stage longitudinally coupled resonator type surface acoustic wave filters is split into first and second parts; an unbalanced signal terminal connected to the second IDT of the first-stage longitudinally coupled resonator type surface acoustic wave filter; a first and a second balanced signal terminal connected to the second IDT of the second-stage longitudinally coupled resonator type surface acoustic wave filter; a first signal line connecting one end of the first IDT of the first-stage longitudinally coupled resonator type surface acoustic wave filter and one end of the first IDT of the second-stage longitudinally coupled resonator type surface acoustic wave filter; and a second signal line connecting one end of the third IDT of the first-stage longitudinally coupled resonator type surface acoustic wave filter and one end of the third IDT of the second-stage longitudinally coupled resonator type surface acoustic wave filter; wherein an electric signal propagating through the first signal line is 180° out of phase with an electric signal propagating through the second signal line; and in the second-stage longitudinally coupled resonator type surface acoustic wave filter, electrode fingers of the second IDT adjacent to the first and third IDTs have the same polarities, and electrode fingers of the first and third IDTs adjacent to the second IDT have the same polarities.