Acoustic wave filter and multiplexer

1. An acoustic wave filter comprising:

a piezoelectric substrate;

a first input-output terminal and a second input-output terminal provided on the substrate; and

a longitudinally coupled resonator unit provided on the substrate and in a path connecting the first input-output terminal and the second input-output terminal to each other; wherein

the longitudinally coupled resonator unit includes five or more interdigital transducer (IDT) electrodes that are aligned in a propagation direction in which a predetermined acoustic wave propagates on the substrate;

each of the five or more IDT electrodes includes a pair of substantially comb-shaped electrodes;

the five or more IDT electrodes include a center IDT electrode located at or substantially at a center in the propagation direction and a first IDT electrode and a second IDT electrode located at symmetric or substantially symmetric positions with respect to the center IDT electrode in the propagation direction;

each of the five or more IDT electrodes includes a main pitch portion, which includes an electrode finger provided at a position closest to a center in the propagation direction among a plurality of electrode fingers included in the IDT electrode and extending in a direction crossing the propagation direction;

each of the first IDT electrode and the second IDT electrode includes a pair of narrow-pitch portions each including, among the plurality of electrode fingers, electrode fingers provided between the main pitch portion and a corresponding one of both ends of the IDT electrode in the propagation direction and having a narrower pitch than the main pitch portion; and

the first IDT electrode and the second IDT electrode differ from each other in a number of electrode fingers included in the main pitch portion.

2. The acoustic wave filter according to claim 1 , further comprising:

one or more ground wires provided on the substrate; wherein

one of the pair of substantially comb-shaped electrodes is connected to the path; and

another one of the pair of substantially comb-shaped electrodes is connected to at least one of the one or more ground wires.

3. The acoustic wave filter according to claim 1 , wherein the substrate is made of LiNbO 3 .

4. The acoustic wave filter according to claim 1 , wherein a difference between the number of electrode fingers included in the main pitch portion of the first IDT electrode and that of the second IDT electrode is an even number.

5. The acoustic wave filter according to claim 1 , wherein parameters for the first IDT electrode are identical or substantially identical to those for the second IDT electrode except for the number of electrode fingers included in the main pitch portion.

6. The acoustic wave filter according to claim 1 , wherein the numbers of electrode fingers included in the main pitch portions of a pair of IDT electrodes are equal or substantially equal to each other, the IDT electrodes being provided as a pair at both ends of the five or more IDT electrodes in the propagation direction.

7. A multiplexer comprising:

the acoustic wave filter according to claim 1 ; and

a frequency filter having a pass band on the higher frequency side of a pass band of the acoustic wave filter.

8. The acoustic wave filter according to claim 1 , wherein each of the pair of substantially comb-shaped electrodes includes a busbar electrode that connects a side end of each of the plurality of electrode fingers to each other.

9. The acoustic wave filter according to claim 1 , wherein, in each pair of substantially comb-shaped electrodes, the plurality of electrode fingers of a first one of the substantially comb-shaped electrodes is interdigitated with the plurality of electrode fingers of a second one of the substantially comb-shaped electrodes.

10. The acoustic wave filter according to claim 1 , wherein, in each pair of substantially comb-shaped electrodes, a first one of the substantially comb-shaped electrodes is electrically connected to the first input-output terminal, and a second one of the substantially comb-shaped electrodes is electrically connected to the second input-output terminal.

11. The acoustic wave filter according to claim 1 , wherein the acoustic wave filter is an unbalanced filter.

12. The acoustic wave filter according to claim 1 , wherein at least one of the five or more IDT electrodes includes a multilayer structure including a plurality of metal layers.

13. The acoustic wave filter according to claim 12 , wherein at least one of the plurality of metal layers is a NiCr film, a PT film, a Ti film, or an AlCu film.

14. The acoustic wave filter according to claim 1 , wherein a protective layer covers each of the substantially comb-shaped electrodes.

15. The acoustic wave filter according to claim 14 , wherein the protective layer is a dielectric film including silicon dioxide.

16. The acoustic wave filter according to claim 1 , further comprising:

a first reflector; and

a second reflector, wherein

the five or more IDT electrodes are located between the first reflector and the second reflector in the propagation direction.

17. The acoustic wave filter according to claim 16 , wherein the first reflector and the second reflector each include a plurality of electrode fingers and a busbar electrode.