Boundary acoustic wave filter device

A boundary acoustic wave filter device comprising:

• a piezoelectric substrate;
• a dielectric film disposed on the piezoelectric substrate;
• at least one longitudinally coupled resonator boundary acoustic wave filter including a plurality of IDTs disposed along a boundary between the piezoelectric substrate and the dielectric film;
• and
• at least two one-terminal-pair boundary acoustic wave resonators each having an IDT located at the boundary between the piezoelectric substrate and the dielectric film;
• wherein
• at least one of the at least two one-terminal-pair boundary acoustic wave resonators defines a serially connected boundary acoustic wave resonator connected in series to the longitudinally coupled resonator boundary acoustic wave filter;
• at least another of the at least two one-terminal-pair boundary acoustic wave resonators defines a parallel-connected boundary acoustic wave resonator connected in parallel to the longitudinally coupled resonator boundary acoustic wave filter;
• an anti-resonant frequency of the serially connected boundary acoustic wave resonator is located in a higher frequency range than a passband of the longitudinally coupled resonator boundary acoustic wave filter;
• a resonant frequency of the parallel-connected boundary acoustic wave resonator is equal to or higher than the anti-resonant frequency of the serially connected boundary acoustic wave resonator;
• the piezoelectric substrate is a single-crystal-LiNbO3 substrate, and φ and θ of the Euler angles of the piezoelectric substrate are in a range of −31°≦φ≦31° and in a range of 90°≦θ≦130°, respectively;
• and
• a terminal impedance of the at least one longitudinally coupled resonator boundary acoustic wave filter in the passband is inductive.