Patent ID: 8339221

Claim:
An elastic wave filter device comprising: a piezoelectric substrate; first to fifth IDT electrodes sequentially arranged on the piezoelectric substrate in a direction in which elastic waves propagate; and first and second reflectors arranged on either side of an area in which the first to fifth IDT electrodes are provided in the direction in which elastic waves propagate; wherein the first to fifth IDT electrodes and the first and second reflectors define a longitudinally coupled resonator type elastic wave filter portion, and each of the first to fifth IDT electrodes includes a narrow-pitch electrode finger portion in which a period of electrode fingers is less than that in another portion at an end adjacent to a different of the first to fifth IDT electrodes; along the direction in which elastic waves propagate, an area including the first IDT electrode and a half of the second IDT electrode neighboring the first IDT electrode is defined as a first area, an area including a half of the second IDT electrode neighboring the third IDT electrode and a half of the third IDT electrode neighboring the second IDT electrode is defined as a second area, an area including a half of the third IDT electrode neighboring the fourth IDT electrode and a half of the fourth IDT electrode neighboring the third IDT electrode is defined as a third area, and an area including a half of the fourth IDT electrode neighboring the fifth IDT electrode and the fifth IDT electrode is defined as a fourth area; when each of total numbers of electrode fingers in the narrow-pitch electrode finger portions in the first area and in the fourth area is Nx and each of total numbers of electrode fingers in the narrow-pitch electrode finger portions in the second area and in the third area is Ny, the number of electrode fingers Nx is different from the number of electrode fingers Ny; an average period of electrode fingers in the narrow-pitch electrode finger portion located in an area including a larger number of electrode fingers between Nx and Ny is greater than an average period of electrode fingers in the narrow-pitch electrode finger portion located in an area including a smaller number of electrode fingers between Nx and Ny; and among the first, third, and fifth IDT electrodes, a period of electrode fingers in a portion other than the narrow-pitch electrode finger portion of the IDT electrode included in the area including the larger number of electrode fingers is less than that included in the area including the smaller number of electrode fingers.