Multi-band filters and communications devices are disclosed. A multi-band filter has a lower pass-band and an upper pass-band separated by an intervening stop-band. The multi-band filter includes a first ladder network and a second ladder network coupled in series. The first ladder network is configured to provide transmission zeros at frequencies below a lower edge of the lower pass-band and transmission zeros at frequencies above an upper edge of the upper pass-band. The second ladder network is configured to provide transmission zeros at frequencies within the intervening stop-band.

Acoustic wave devices are disclosed. An acoustic wave device can include a first filter and a second filter coupled to a common node. The second filter includes acoustic wave resonators of a first type (e.g., bulk acoustic wave resonators) and a series acoustic wave resonator of the second type (e.g., a surface acoustic wave resonator) that is coupled between the acoustic wave resonators of the first type and the common node. The acoustic wave device can further include a loop circuit coupled to the first filter, in which the loop circuit is configured to generate an anti-phase signal to a target signal at a particular frequency. In certain embodiments, the first filter is a receive filter and the second filter is a transmit filter.

A filter device utilizing surface acoustic waves includes one or more series arm resonators connected to each other along a path between a first input/output terminal and a second input/output terminal, and three or more parallel arm resonators each connected between a connection node provided on the path and ground and defining the pass band of the filter device. Parallel arm resonators having different anti-resonant frequencies from each other are included. Parallel arm resonators among the three or more parallel arm resonators are connected to the same connection node provided along the path. Remaining parallel arm resonators are connected to different connection nodes from the same connection node. The anti-resonant frequencies of the parallel arm resonators connected to the same connection node are identical and are the lowest anti-resonant frequencies among the anti-resonant frequencies of the three or more parallel arm resonators.

A composite filter device includes a first filter and a plurality of second filters with different passbands. End portions of the first filter and the plurality of second filters are connected to a common connection. The first filter includes a piezoelectric substrate made of LiNbO.sub.3, an IDT electrode provided on the piezoelectric substrate, and a dielectric layer provided on the piezoelectric substrate so as to cover the IDT electrode. The first filter utilizes a fundamental wave of Rayleigh waves. The passband of the first filter is arranged in a frequency band that is lower than any of the passbands of the plurality of second filters.

A filter device includes first, second and third filter circuits that are connected to a common terminal. The first filter circuit includes a first inductor that is closest to the common terminal along a first signal line and a first capacitance element that is connected in parallel with the first inductor, the second filter circuit includes a series arm resonator, which is a second acoustic resonator, that is closest to the common terminal along a second signal line, and the third filter circuit includes a third acoustic resonator that is closest to the common terminal along a third signal line.

A filter includes series resonators in a signal path. An IDT electrode included in the series resonators includes at least either first electrode fingers including variant portions or second electrode fingers not including variant portions. In the IDT electrode included in one or more series resonators of the series resonators, a direction that connects the respective other ends of a plurality of electrode fingers intersects an acoustic wave propagation direction. In a first portion and a second portion of the IDT electrode, the first and second electrode fingers are arranged in a predetermined order.

An extractor includes a band pass filter and a band elimination filter. One filter of the band pass filter and the band elimination filter includes at least one serial arm resonator and at least one parallel arm resonator that are each defined by an acoustic wave resonator. Any one of the at least one serial arm resonator and the at least one parallel arm resonator includes, among a divided resonator group divided resonator including a plurality of divided resonators coupled in series to each other, a first divided resonator group that is a largest in number of the divided resonators coupled in series and a smallest in capacitance.

Aspects of this disclosure relate to a hybrid acoustic LC filter cascaded with a non-acoustic LC filter. The hybrid acoustic filter can filter a radio frequency signal. The hybrid acoustic LC filter can include acoustic resonators, an inductor, and a capacitor. The inductor and capacitor can be external to an acoustic resonator die. The non-acoustic LC filter includes an LC circuit. Related multiplexers, wireless communication devices, and methods are disclosed.

A variable filter circuit includes a serial arm connected between ports (P1-P2), a parallel arm having a resonator connected in series between ports (P1-P3), and another parallel arm having another resonator connected in series between ports (P2-P3). The serial arm includes a capacitor connected between the ports (P1-P2), and the parallel arms include variable capacitances connected in series to the resonators.

Provided is a variable filter circuit that can control the bandwidth and center frequency of a pass band, can realize steep attenuation characteristics in bands close to the pass band, and enables the total number of variable reactance units to be reduced. A variable filter circuit includes an inductor (Ls1) and a capacitor (Cs1), which are connected in series between a first input/output terminal (P1) and a second input/output terminal (P2), and resonators (Re_p1, Re_p2, Re_p3, Re_p4) and variable capacitors (Cc1, Cc2, Cc3, Cc4), which are connected in series between two ends of the inductor (Ls1) and the capacitor (Cs1) and ground connection terminals.