The multiplexer includes a plurality of IDT electrodes on a substrate, an insulating cover located on the substrate so as to configure one or more spaces above the plurality of IDT electrodes, an antenna terminal, transmission terminal, and reception terminal which are all located on the substrate and pass through the cover, and a reinforcing layer which is located on the cover and is made of metal. By the plurality of IDT electrodes, a transmission filter located in a signal path connecting the antenna terminal and the transmission terminal and a receiving filter located in a signal path connecting the antenna terminal and the reception terminal. The reinforcing layer includes a first area part facing the transmission filter and a second area part which faces the receiving filter and is separated from the first area part.

A surface acoustic wave device includes a piezoelectric single crystal substrate and an electrode. The piezoelectric single crystal substrate is made of LiTaO.sub.3 or LiNbO.sub.3. The electrode includes a titanium film formed on the piezoelectric single crystal substrate and an aluminum film or a film containing aluminum as a main component. The aluminum film or the film is formed on the titanium film. The aluminum film or the film containing aluminum as the main component is a twin crystal film or a single crystal film, the aluminum film or the film has a (111) plane that is non-parallel to a surface of the piezoelectric single crystal substrate with an angle θ, and the aluminum film or the film has a [−1, 1, 0] direction parallel to an X-direction of a crystallographic axis of the piezoelectric single crystal substrate.

A acoustic wave resonator comprises a piezoelectric substrate and a plurality of interdigital transducer (IDT) electrodes disposed on the piezoelectric substrate, the plurality of IDT electrodes formed of a mixture of tungsten and chromium to provide for reduction in size and increase in quality factor of the acoustic wave resonator.

An acoustic wave filter includes a first signal terminal, an antenna terminal, a ladder-type filter connected between the first signal terminal and the antenna terminal and including one or more serial resonators and one or more parallel resonators connected in a ladder shape, and a capacitor part and an inductor part which are connected in series between the first signal terminal and a reference potential.

A multiplexer includes a transmission filter and a reception filter connected to a common terminal, a first inductor connected to the common terminal, and a multilayer substrate on which the transmission filter and the reception filter are mounted and which includes dielectric layers. The transmission filter includes a parallel-arm resonator connected to a path between the common terminal and a transmission terminal and a parallel-arm terminal, and a second inductor connected to the parallel-arm terminal and ground. The first inductor includes a first coil pattern on a first dielectric layer and a second coil pattern on a second dielectric layer. The second inductor includes a third coil pattern on the first dielectric layer and that is magnetically coupled to the first coil pattern. The inductance value of the second coil pattern is greater than that of the first coil pattern.

Provided is an acoustic wave device that uses a plate wave. The acoustic wave device includes a piezoelectric film and a first resonator and a second resonator each including an IDT electrode located on an upper surface of the piezoelectric film. The thickness of the piezoelectric film is smaller than twice the period of the electrode fingers of the IDT electrodes. The duty of the electrode fingers of the first resonator and the duty of the electrode fingers of the second resonator are different from each other.

Filter devices are disclosed. A filter device includes a piezoelectric plate comprising a supported portion, a first diaphragm, and a second diaphragm. The supported portion is attached to a substrate and the first and second diaphragms spans respective cavities in the substrate. A first interdigital transducer (IDT) has interleaved fingers on the first diaphragm. A second interdigital transducer (IDT) has interleaved fingers on the second diaphragm. A first dielectric layer is between the interleaved fingers of the first IDT, and a second dielectric layer is between the interleaved fingers of the second IDT. A thickness of the first dielectric layer is greater than a thickness of the second dielectric layer. The piezoelectric plate and the first and second IDTs are configured such that radio frequency signals applied to first and second IDTs excite primary shear acoustic modes in the respective diaphragms.

A transmission filter is arranged in a first filter region and has one or more acoustic wave resonators, a plurality of terminal electrodes, and a plurality of wires. A reception filter is arranged in a second filter region and has one or more acoustic wave resonators, a plurality of terminal electrodes, and a plurality of wires. The first filter region and the second filter region are arranged adjacently to each other and have at least sides constituting a pair and opposing to each other. At least either one of the first filter region and the second filter region has no wire extending along one side in a forbidden region that is defined by a width including a terminal electrode nearest to the one side, along the one side and over the one side opposing to the other filter region.

A transmit filter circuit includes an input terminal, an output terminal, plural series arm resonators, and a parallel arm resonator. The input terminal receives a transmit signal. The output terminal is electrically connected to an antenna. The plural series arm resonators are electrically connected in series with each other on a line between the input and output terminals. The plural series arm resonators include first and second series arm resonators. The first series arm resonator is closest to the output terminal. The second series arm resonator is second closest to the output terminal. A first end of the parallel arm resonator is electrically connected to a node between the first and second series arm resonators. A reference potential is provided to a second end of the parallel arm resonator. The resonant frequency of the first series arm resonator is higher than that of the second series arm resonator.

Aspects of this disclosure relate to a multiplexer that includes a switchable acoustic wave filter. The switchable acoustic wave filter can include one or more acoustic wave resonators, switchable acoustic wave resonators, and a switch configurable into at least a first state and a second state. The switch can select a different subset of the switchable acoustic wave resonators to filter a radio frequency signal together with at least the one or more acoustic wave resonators in the first state than in the second state. Related filters, radio frequency systems, wireless communication devices, and methods are also disclosed.