Patent ID: 7190242

Claim:
A surface acoustic wave filter comprising: a piezoelectric substrate; and a longitudinally-coupled-resonator surface acoustic wave filter portion provided on the piezoelectric substrate; wherein the longitudinally-coupled-resonator surface acoustic wave filter portion includes an odd number of at least three interdigital transducers arranged such that a plurality of comb electrodes having a plurality of electrode fingers are interdigitated, the interdigital transducers being disposed along a surface-acoustic-wave propagation direction, and first and second reflectors disposed along the surface-acoustic-wave propagation direction so that the at least three interdigital transducers are located between the first and second reflectors; the odd number of at least three interdigital transducers include a central interdigital transducer arranged in the approximate center, and first and second interdigital transducers disposed at two sides of the central interdigital transducer, an electrode finger of the first interdigital transducer which is adjacent to the central interdigital transducer is a ground electrode, and an electrode finger of the second interdigital transducer which is adjacent to the central interdigital transducer is a signal electrode; the comb electrodes disposed on one side of the central interdigital transducer include first and second bisected comb electrodes obtained by bisecting the one of the opposing comb electrodes along the surface-acoustic-wave propagation direction; the first and second bisected comb electrodes are respectively displaced toward the first and second interdigital transducers and are respectively connected to first and second balanced signal terminals; the first and second interdigital transducers which are adjacent to the central interdigital transducer are connected to an unbalanced signal terminal; when, in the central interdigital transducer, an imaginary central axis that is substantially perpendicular to the surface-acoustic-wave propagation direction is assumed, design parameters of at least one of the interdigital transducers and the reflectors, which are disposed on opposite sides of the imaginary central axis in the central interdigital transducer that is substantially perpendicular to the surface-acoustic-wave propagation direction, are set to be different from one another at the sides of the imaginary central axis; and the polarities of two outermost electrode fingers of the central interdigital transducer are substantially identical to that of a ground electrode or a float electrode, and an electrode-finger center-to-center distance between the first interdigital transducer and the first reflector adjacent to the first interdigital transducer is greater than an electrode-finger center-to-center distance between the second interdigital transducer and the second reflector adjacent to the second interdigital transducer.