| 摘要 |
A 5-IDT surface acoustic wave filter that has a balanced-unbalanced conversion function and that can efficiently reduce a spurious component in the vicinity of the passband is provided. A surface acoustic wave filter (1) includes first to fifth IDTs (11 to 15) disposed on a piezoelectric substrate (2) in a surface acoustic wave propagation direction. The second IDT (12) and the fourth IDT (14) have a phase difference of 180° with respect to each other. The second and fourth IDTs are connected to an unbalanced terminal. The third IDT includes a first sub-IDT portion (13a) and a second sub-IDT portion (13b). The first IDT (11) and the first sub-IDT portion (13a) are connected to a first balanced terminal (4), and the second sub-IDT portion (13b) and the fifth IDT (15) are connected to a second balanced terminal (5). The following conditions: 1.55 ‰ N3/N1 ‰ 2.58 and N1 = N5 are satisfied, where N1 denotes the number of electrode fingers that determines the center frequency of the first IDT (11), N3 denotes the number of electrode fingers that determines the center frequency of the third IDT (13), and N5 denotes the number of electrode fingers that determines the center frequency of the fifth IDT (15), |
