Boundary acoustic wave filter

A boundary acoustic wave filter device comprising:

• a piezoelectric body;
• a dielectric body laminated on the piezoelectric body;
• and
• an electrode structure arranged at a boundary between the piezoelectric body and the dielectric body;
• wherein
• the boundary acoustic wave filter device utilizes an SH-type boundary acoustic wave that propagates along the boundary;
• the electrode structure includes first and second longitudinally coupled resonator boundary acoustic wave filter portions, each of which includes a first IDT, and second and third IDTs that are arranged on both sides of the first IDT in a direction in which a boundary acoustic wave propagates;
• at portions of the first to third IDTs at which two IDTs are located adjacent to each other, a pitch of a portion of electrode fingers of one IDT located at an end adjacent to another IDT is less than a pitch of another portion of electrode fingers of the one IDT adjacent to that portion, so as to define a narrow pitch electrode finger portion;
• a polarity of each of the first to third IDTs of the first and second longitudinally coupled resonator boundary acoustic wave filter portions is set such that a phase of an output signal to an input signal of the second longitudinally coupled resonator boundary acoustic wave filter portion is different by about 180 degrees from a phase of an output signal to an input signal of the first longitudinally coupled resonator boundary acoustic wave filter portion;
• an unbalanced terminal and first and second balanced terminals are provided, input ends of the first and second longitudinally coupled resonator boundary acoustic wave filter portions are connected to the unbalanced terminal, and output ends of the first and second longitudinally coupled resonator boundary acoustic wave filter portions are respectively connected to the first and second balanced terminals;
• the pitch of the electrode fingers of at least one of the narrow pitch electrode finger portions of the first IDT and the second and third IDTs is different from the pitch of the electrode finger portions of each of the remaining narrow pitch electrode finger portions;
• for each of the first and second longitudinally coupled resonator boundary acoustic wave filter portions, the number of electrode fingers of each of the narrow pitch electrode finger portions of the first IDT is different from the number of electrode fingers of each of the narrow pitch electrode finger portions of the second and third IDTs;
• and
• the pitch of the electrode fingers of the at least one of the narrow pitch electrode finger portions provided in the first IDT and the second and third IDTs and the pitch of the electrode finger portions of each of the remaining narrow pitch electrode finger portions are selected so as to reduce a spurious response in a frequency band that is higher than a pass band of the boundary acoustic wave filter device.

A boundary acoustic wave filter device comprising:

• a piezoelectric body;
• a dielectric body laminated on the piezoelectric body;
• and
• an electrode structure arranged at a boundary between the piezoelectric body and the dielectric body;
• wherein
• the boundary acoustic wave filter device utilizes an SH-type boundary acoustic wave that propagates along the boundary;
• the electrode structure includes first and second longitudinally coupled resonator boundary acoustic wave filter portions, each of which includes a first IDT, and second and third IDTs that are arranged respectively on both sides of the first IDT in a direction in which a boundary acoustic wave propagates;
• at portions of the first to third IDTs at which two IDTs are located adjacent to each other, a pitch of a portion of electrode fingers of one IDT located at an end adjacent to the other IDT is less than a pitch of another portion of electrode fingers of the one IDT, adjacent to that portion, so as to define a narrow pitch electrode finger portion;
• a polarity of each of the first to third IDTs of the first and second longitudinally coupled resonator boundary acoustic wave filter portions is set such that a phase of an output signal to an input signal of the second longitudinally coupled resonator boundary acoustic wave filter portion is different by about 180 degrees from a phase of an output signal to an input signal of the first longitudinally coupled resonator boundary acoustic wave filter portion;
• an unbalanced terminal and first and second balanced terminals are provided, input ends of the first and second longitudinally coupled resonator boundary acoustic wave filter portions are connected to the unbalanced terminal, and output ends of the first and second longitudinally coupled resonator boundary acoustic wave filter portions are respectively connected to the first and second balanced terminals;
• the pitches of the electrode fingers of the narrow pitch electrode finger portions of the first to third IDTs in the first longitudinally coupled resonator filter portion are equal or substantially equal to one another;
• the pitches of the electrode fingers of the narrow pitch electrode finger portions of the first to third IDTs in the second longitudinally coupled resonator filter portion are equal or substantially equal to one another;
• the pitch of the electrode fingers of each of the narrow pitch electrode finger portions of the first to third IDTs in the first longitudinally coupled resonator filter portion is different from the pitch of the electrode fingers of each of the narrow pitch electrode finger portions of the first to third IDTs in the second longitudinally coupled resonator filter portion;
• and
• for each of the first and second longitudinally coupled resonator boundary acoustic wave filter portions, the number of electrode fingers of each of the narrow pitch electrode finger portions of the first IDT is different from the number of electrode fingers of each of the narrow pitch electrode finger portions of the second and third IDTs;
• and
• the pitch of the electrode fingers of each of the narrow pitch electrode finger portions of the first to third IDTs in the first longitudinally coupled resonator filter portion and the pitch of the electrode finger and the pitch of the electrode fingers of each of the narrow pitch electrode finger portions of the first to third IDTs in the second longitudinally coupled resonator filter portion are selected so as to reduce a spurious response in a frequency band that is higher than a pass band of the boundary acoustic wave filter device.