Filters using acoustic wave resonators are used in wireless devices for example. Exemplary acoustic wave resonators are a surface acoustic wave resonator and a piezoelectric film resonator. A ladder-type filter using the acoustic wave resonator functions as a bandpass filter. An inductor may be connected in parallel to a series resonator in order to suppress a specific frequency band other than a passband in the ladder-type filter as disclosed in Japanese Patent Application Publication Nos. 9-167937 (Patent Document 1) and 2003-69382 (Patent Document 2) for example.
However, gentle attenuation characteristic is obtained when the methods disclosed in Patent Documents 1 and 2 are used. Thus, it is difficult to secure a certain amount of attenuation in a specific frequency band.
USPTO Backgrounds
USPTO Backgrounds