Acoustic wave filter device
An acoustic wave filter device includes longitudinally coupled resonator-acoustic wave filters having a balanced-unbalanced conversion function, and increases the amount of attenuation in an attenuation band and improves the isolation characteristics when the acoustic wave filter device is used as a reception filter of a duplexer. The acoustic wave filter device includes an unbalanced terminal, first and second balanced terminals, first and second longitudinally coupled resonator-acoustic wave filters that are connected between the unbalanced terminal and the first balanced terminal and between the unbalanced terminal and the second balanced terminal, respectively, a first parallel resonator connected between the first balanced terminal and a ground potential, and a second parallel resonator connected between the second balanced terminal and the ground potential. The electrostatic capacitance of the first parallel resonator is different from the electrostatic capacitance of the second parallel resonator. Among the first parallel resonator and the second parallel resonator, the wavelength of the first parallel resonator having a relatively small electrostatic capacitance is shorter than the wavelength of the second parallel resonator having a relatively large electrostatic capacitance.
1. An acoustic wave filter device comprising:
an unbalanced terminal;
first and second balanced terminals;
longitudinally coupled resonator-acoustic wave filters that are connected between the unbalanced terminal and the first balanced terminal and between the unbalanced terminal and the second balanced terminal and that have a balanced-unbalanced conversion function;
a first parallel resonator connected between the first balanced terminal and a ground potential; and
a second parallel resonator connected between the second balanced terminal and the ground potential; wherein
the electrostatic capacitance of the first parallel resonator is different from that of the second parallel resonator; and
among the first and second parallel resonators, a wavelength of the parallel resonator having a relatively small electrostatic capacitance is shorter than that of the parallel resonator having a relatively large electrostatic capacitance.
2. The acoustic wave filter device according to claim 1 , wherein each of the longitudinally coupled resonator-acoustic wave filters is a one-stage longitudinally coupled resonator-acoustic wave filter.
3. The acoustic wave filter device according to claim 1 , wherein each of the first and second parallel resonators is defined by a one-port acoustic wave resonator including an IDT electrode.
4. The acoustic wave filter device according to claim 3 , wherein the IDT electrode in the first parallel resonator is differentiated from the IDT electrode in the second parallel resonator so that the electrostatic capacitance of the first parallel resonator is differentiated from the electrostatic capacitance of the second parallel resonator.
5. The acoustic wave filter device according to claim 4 , wherein the IDT electrode in the first parallel resonator is differentiated from the IDT electrode in the second parallel resonator in at least one of a number of pairs of electrode fingers, an opposing length, and a duty ratio of electrode fingers in the IDT electrode.
6. The acoustic wave filter device according to claim 1 , wherein each of the first and second parallel resonators is defined by a piezoelectric thin film resonator including a piezoelectric thin film and first and second resonant electrodes, the first resonant electrode being arranged so as to oppose the second resonant electrode with the piezoelectric thin film sandwiched therebetween.
7. The acoustic wave filter device according to claim 6 , wherein the first resonant electrode is differentiated from the second resonant electrode in the opposing area in the first and second parallel resonators so that the electrostatic capacitance of the first parallel resonator is differentiated from the electrostatic capacitance of the second parallel resonator.
8. The acoustic wave filter device according to claim 1 , wherein the acoustic wave is a surface acoustic wave and each of the longitudinally coupled resonator-acoustic wave filters is a longitudinally coupled resonator-surface acoustic wave filter.
9. The acoustic wave filter device according to claim 1 , wherein the acoustic wave is a boundary acoustic wave and each of the longitudinally coupled resonator-acoustic wave filters is a longitudinally coupled resonator-boundary acoustic wave filter.