Patent ID: 8508316

Claim:
A bulk acoustic wave filter of ladder-type structure, comprising: an input terminal to carry an input signal into the filter; an output terminal to carry an output signal out of the filter; a reference terminal to carry a reference potential; a series arm coupled between the input terminal and the output terminal; at least one series arm resonator disposed in the series arm, such that the series arm is distributed in series arm sections between the input terminal and the output terminal by the at least one series arm resonator; a first parallel arm comprising a parallel arm resonator and an inductivity, the parallel arm resonator and the inductivity of the first parallel arm coupled in series between the series arm and the reference terminal; and at least one second parallel arm comprising a parallel arm resonator, the first parallel arm and the at least one second parallel arm being coupled in parallel between a common one of the series arm sections and the reference terminal; wherein a transmission parameter of the filter specifying a relationship between the input signal and the output signal of the filter has a first pole and a second pole, an insertion loss of the filter being lower in a frequency range between the first and second pole than in a frequency range below the first pole and above the second pole; and wherein the parallel arm resonator and the inductivity of the first parallel arm are arranged so that the transmission parameter of the filter has a third pole and a fourth pole, one of the third pole or the fourth pole being disposed above the first pole and the second pole and the other one of the third and fourth poles being disposed below the first pole and the second pole.