A method for the batch production of acoustic wave filters comprises: synthesizing N theoretical filters, each filter defined by a set of j theoretical resonator(s) having a triplet C.sub.0ij,eq, .sub.rij,eq and .sub.aij,eq, these parameters grouped into subsets; determining a reference resonator structure for each subset, naturally having a resonant frequency .sub.r,ref, where .sub.aij,eq<.sub.r,ref<.sub.rij,eq; determining, for each theoretical resonator, an elementary building block comprising an intermediate resonator R.sub.ij, a parallel reactance Xp.sub.ij and/or a series reactance Xs.sub.ij, the intermediate resonator R.sub.ij having a triplet C.sub.0ij, .sub.r,ref and .sub.a,ref, the parameters C.sub.0ij, Xpij and/or Xs.sub.ij defined so the elementary building block has a triplet: C.sub.0ij,eq, .sub.rij,eq and .sub.aij,eq; determining the geometrical dimensions of the actual resonators R.sub.ij of the filters so they have a capacitance C.sub.0ij; producing each actual resonator; associating series and/or parallel reactances with actual resonators in order to form the elementary building blocks.

A filter (10) includes a series arm circuit (11) and a parallel arm circuit (12). The parallel arm circuit (12) includes a parallel arm resonator (p1), a variable frequency circuit (12T), and a parallel arm resonator (p2) connected in parallel with a circuit in which the parallel arm resonator (p1) and the variable frequency circuit (12T) are connected in series. The variable frequency circuit (12T) includes a parallel arm resonator (p3) and a switch (SW) connected in parallel with the parallel arm resonator (p3), and is configured to change frequencies of a pass band and a frequency of an attenuation pole by switching between an on state and off state of the switch (SW). A resonant frequency and anti-resonant frequency of the parallel arm resonator (p1) are respectively different from a resonant frequency and anti-resonant frequency of the parallel arm resonator (p2).

An acoustic wave filter includes: a series-arm resonator disposed on a path that connects input/output terminals; and a parallel-arm circuit connected to a node on the path and a ground. The parallel-arm circuit includes a parallel-arm resonator and a capacitor connected in parallel to each other. The capacitor includes a comb-shaped electrode that includes electrode fingers. A frequency at which impedance of the capacitor has a local maximum value is located outside a passband of the acoustic wave filter. The comb-shaped electrode has at least two different electrode finger pitches or at least two different electrode finger duty ratios.

A filter includes a series-arm resonator connected on a path connecting input/output terminals, and first and second parallel-arm resonators connected between the same node on the path and ground. A resonant frequency of the second parallel-arm resonator is higher than a resonant frequency of the first parallel-arm resonator, and an anti-resonant frequency of the second parallel-arm resonator is higher than an anti-resonant frequency of the first parallel-arm resonator. Each of the first and second parallel-arm resonators includes an acoustic wave resonator including an IDT electrode. The IDT electrode in the second parallel-arm resonator has a higher duty ratio than the IDT electrode in the first parallel-arm resonator, where the duty ratio is the ratio of the width to the pitch of electrode fingers.

A multiplexer includes a plurality of filters, in which one input/output terminal of each of the plurality of filters is connected to a common terminal, a first filter included in the plurality of filters is a ladder filter and includes at least one series arm resonator connected on a path connecting the common terminal and another input/output terminal of the first filter, at least one parallel arm resonator connected between a connection node provided on the path and a ground, and a switch which is connected in series to a parallel arm resonator connected most nearby to the common terminal among at least one parallel arm resonator and switches between conduction and non-conduction of a node to which the parallel arm resonator is connected and the ground.

A filter device includes a filter (22A) connected to a common terminal (110) and having a first characteristic, a variable filter (22B) connected to the common terminal (110) and capable of changing a characteristic to one of a second characteristic and a third characteristic, and a switch (23). In the second characteristic, a second pass band including an overlapping band in which the second pass band and an attenuation band of the filter (22A) partially overlap in frequency is defined. Insertion loss within the overlapping band for the third characteristic is greater than insertion loss within the overlapping band for the second characteristic. When the filter (22A) is selected by the switch (23), the characteristic of the variable filter (22B) is set to the third characteristic.

A variable filter (10) includes a parallel arm resonance circuit and a serial arm resonance circuit, the parallel arm resonance circuit includes a parallel arm resonator (p1) having an IDT electrode (121) and a frequency variable circuit (11) connected to the parallel arm resonator (p1), the frequency variable circuit (11) includes a capacitance element (C1) and a switch (SW), the variable filter (10) further includes a capacitance element (C2) connected between a serial arm and ground, the IDT electrode (121), an IDT wiring connected to the IDT electrode (121), and a capacitance wiring connected to the capacitance element (C1) are formed on the same substrate, a wiring (141 or 144) of the capacitance wirings and a wiring (143) of the IDT wirings intersect with each other, and the capacitance element (C2) is configured of the capacitance wiring and the IDT wiring in the intersection region.

An acoustic wave filter device includes a first resonant circuit, a second resonant circuit, a first interconnect line, and a second interconnect line. The second resonant circuit includes a second parallel-arm resonator, and a frequency-tuning circuit that allows tuning of the resonant frequency of the second parallel-arm resonator. The frequency-tuning circuit includes a capacitor, and a switching element connected in parallel with the capacitor. The first interconnect line is an interconnect line connected to the portion of the series-arm resonator located adjacent to the first input/output terminal. The second interconnect line is an interconnect line connecting the parallel-arm resonator with the switching element, or an interconnect line connecting the parallel-arm resonator with the capacitor. A first transmission line formed by the first interconnect line, and a second transmission line formed by the second interconnect line are magnetically coupled with each other.

A radio frequency front-end circuit includes a first filter that is a frequency variable filter connected to a first select terminal of a switching circuit, and a second filter connected to a second select terminal of the switching circuit. The switching circuit includes a first switch that switches over conduction and non-conduction between a common terminal and the second select terminal. The first filter includes a serial arm resonance circuit connected to the first select terminal, a parallel arm resonator, and a frequency varying circuit. The frequency varying circuit includes a capacitor and a third switch connected in parallel to each other, and is connected in series to the parallel arm resonator. The frequency varying circuit shifts a frequency of the first filter depending on conduction and non-conduction of the third switch.

A filter device includes a first input/output terminal and a second input/output terminal, as well as a series-arm circuit disposed on a path that connects the first input/output terminal and the second input/output terminal, and a parallel-arm circuit connected to a node on the path and a ground. At least one of the series-arm circuit or the parallel-arm circuit includes a resonance circuit, and the resonance circuit includes a first acoustic wave resonator, and a first capacitor connected in parallel to the first acoustic wave resonator, and having a greater electrostatic capacitance per unit area than that for the first acoustic wave resonator.