A multiplexer includes a transmission-side filter electrically connected to a common terminal and a transmission input terminal, and a transmission-side filter electrically connected to the common terminal and a transmission input terminal. The transmission-side filter includes a plurality of series arm resonators and a plurality of parallel arm resonators. Capacitance elements are respectively electrically connected in parallel to the series arm resonator and the parallel arm resonator, which are connected most proximately to the common terminal. IDT electrodes of a series arm resonator and a parallel arm resonator connected most proximately to the common terminal do not include a thinning electrode, and others of the series arm resonators and the parallel arm resonators include thinning electrodes.

A duplexer includes a transmission filter with a ladder circuit configuration and a reception filter including a ladder filter portion connected to an antenna terminal and a longitudinally coupled resonator-type surface acoustic wave filter. In the transmission filter, a coupling capacitor is connected between an end portion of an inductor nearer to parallel arm resonators and a wiring line connecting the series arm resonators of the ladder filter portion to each other in the reception filter.

In accordance with an embodiment, an RF system includes a transmit path having a first tunable transmit band stop filter, and a power amplifier coupled to an output of the first tunable transmit band stop filter, where the first tunable transmit band stop filter is configured reject a receive frequency and pass a transmit frequency; a receive path comprising an LNA; and a duplex filter having a transmit path port coupled to an output of the power amplifier, a receive path port coupled to an input of the LNA, and an antenna port, where the duplex filter is configured to pass the transmit frequency and reject the receive frequency between the antenna port and the transmit path port, pass the receive frequency and reject the transmit frequency between the antenna port and the receive path port.

A duplexer includes a transmission filter with a ladder circuit configuration and a reception filter. The reception filter includes a series trap element connected to an antenna terminal and a longitudinally coupled resonator-type surface acoustic wave filter. In the transmission filter, a coupling capacitance is connected between an end portion at parallel arm resonators side of an inductance and a line connecting the series trap element to the longitudinally coupled resonator-type surface acoustic wave filter in the reception filter.

A SAW filter includes a substrate including a piezoelectric substrate, a transmission filter, and an additional resonator. The transmission filter is a ladder-type filter filtering signals from a transmission terminal and outputting the result to an antenna terminal. Further, the transmission filter includes one or more serial resonators and one or more parallel resonators which are connected in a ladder configuration on the piezoelectric substrate. An initial stage resonator is the serial resonator. The additional resonator includes an IDT electrode on the piezoelectric substrate. The IDT electrode is connected to the transmission terminal at a stage before the transmission filter and is connected to any of the one or more GND terminals. In the additional resonator, a resonance frequency and an antiresonance frequency are located outside of a passband of the transmission filter.

A filter includes: a series-arm circuit; a first parallel-arm circuit connected to a ground and a node; and a second parallel-arm circuit connected to the ground and a node. The first parallel-arm circuit includes a parallel-arm resonator, and a first switch circuit. The second parallel-arm circuit includes a parallel-arm resonator, and a second switch circuit. The first switch circuit includes a first switch. The second switch circuit includes a second switch. A voltage across the first switch is lower than a voltage across the second switch. A stack count of the first switch is lower than a stack count of the second switch.

A transmission filter includes a transmission filter circuit and an additional circuit. The transmission filter circuit is provided between a first terminal and a second terminal. Between the first terminal and the second terminal, the additional circuit is connected in parallel with at least a portion of the transmission filter circuit. The additional circuit includes an IDT electrode group including a plurality of IDT electrodes located next to each other in an acoustic wave propagation direction and two reflectors that sandwich the IDT electrode group and having different numbers of pairs of electrode fingers.

An acoustic wave filter device includes first and second terminals, a longitudinally coupled resonator coupled between the first terminal and the second terminal, and an inductor connected between a path and a ground potential, the path connecting the first terminal and the longitudinally coupled resonator to each other. The longitudinally coupled resonator includes at least one first IDT electrode coupled to the first terminal, and at least one second IDT electrode connected to the second terminal. A total capacitance value of the at least one first IDT electrode is smaller than a total capacitance value of the at least one second IDT electrode.

A resonator element for use in a filter is provided. The resonator element includes a first resonator acoustically coupled to a second or third resonator or both. The first resonator has terminals for incorporation in a filter structure. A tuning circuit is coupled to the second or third resonator or both to enable tuning of the resonator element. The tuning circuit includes a variable capacitor and an inductor.

A variable filter includes a serial arm resonator and a parallel arm resonance circuit, the parallel arm resonance circuit includes a parallel arm resonator and a capacitance element connected to the parallel arm resonator, the serial arm resonator and the parallel arm resonator respectively include IDT electrodes each formed of a plurality of electrode fingers formed on a substrate, and a film thickness of the plurality of electrode fingers of the parallel arm resonator is larger than a film thickness of the plurality of electrode fingers of the serial arm resonator.