An acoustic device includes a foundation structure and a transducer provided over the foundation structure. The foundation structure includes a piezoelectric layer between a top electrode and a bottom electrode. The piezoelectric layer has an active portion within an active region of the transducer, and a bi-polar border portion within a border region of the transducer. The piezoelectric material in the active portion has a first polarization. The bi-polar border portion has a first sub-portion and a second sub-portion, which resides either above or below the first sub-portion. The piezoelectric material in the first sub-portion has the first polarization, and the piezoelectric material in the second sub-portion has a second polarization, which is opposite the first polarization.

A band-pass filter includes an unbalanced port, a first balanced port, a second balanced port, and first to third resonators provided between the unbalanced port and the first and second balanced ports. The second resonator and the third resonator each are a resonator with both ends open. The second resonator and the third resonator are adjacent to each other in a circuit configuration, and electromagnetically coupled by magnetic coupling as main coupling. The first resonator is provided closer to the second resonator than to the third resonator, and jump-coupled to the third resonator.

An interdigitated RF filter. The interdigitated RF filter includes input fingers connected to an input node and output fingers connected to an output node where at least one input finger is connected the output node or at least one output finger is connected to the input node. The described interdigitated RF filter can be implemented in various configurations such as series, shunt, ladder or a combination thereof.

An oscillator includes a forward stage including first and second terminals and a transformer-coupled band-pass filter (BPF) coupled between the first and second terminals and including a coupling device between the first and second terminals, and a transformer including first and second windings in a metal layer of an IC. The first winding includes a first conductive structure coupled to the first terminal and a second conductive structure coupled to a voltage node, a third conductive structure including first and second extending portions connected to the first and second conductive structures. The second winding includes a fourth conductive structure including a third extending portion coupled to the voltage node, and a fourth extending portion coupled to the second terminal. The third extending portion is between the second conductive structure and the first extending portion, and the fourth extending portion is between the first conductive structure and the second extending portion.

Disclosed is a radio frequency (RF) filter that vertically integrates an acoustic wave filter with an integrated passive device (IPD) filter. The acoustic wave filter provides selectivity at fundamental frequency band while the IPD filter provides rejection at harmonic frequency bands.

A band-pass filter includes a first input/output port, a second input/output port, a first high-pass filter, a first low-pass filter, and a first stub resonator. The first stub resonator includes a first distributed constant line. The first low-pass filter is provided between the first input/output port and the first high-pass filter in the circuit configuration. The first distributed constant line has a first end connected to a first path connecting the first input/output port and the first low-pass filter, and a second end closest to a ground in the circuit configuration.

Methods and system for using a multifunctional filter to minimize insertion loss in a multi-mode communications system are described. Specifically described is a multifunctional filter that is configurable to operate in a band-pass mode when a first type of signal is propagated through the multifunctional filter, and to operate in a low-pass mode when a second type of signal is propagated through the multifunctional filter. The multifunctional filter presents a lower insertion loss to the second type of signal when operating in the low-pass mode than in the band-pass mode.

A BAW resonator filter can include a BAW resonator pass-band filter ladder, the BAW resonator pass-band filter ladder can be configured to pass frequency components of an input signal in a pass-band of frequencies received at an input node of the BAW resonator pass-band filter ladder to an output node of the BAW resonator pass-band filter ladder. A first rejection-band series resonator can be coupled in series between an input port of the BAW resonator pass-band filter ladder and the input node, the first rejection-band series resonator can have a first anti-resonant frequency peak in a rejection-band of frequencies that is less than the pass-band of frequencies. A second rejection-band series resonator can be coupled in series between an output port of the BAW resonator filter and the output node, the second rejection-band series resonator can have a second anti-resonant frequency peak in the rejection-band of frequencies.

A filter circuit includes a pass band filter portion configured to pass signals in a first frequency spectrum and attenuate or block signals in a second frequency spectrum. The first frequency spectrum and the second frequency spectrum do not overlap. The pass band filter portion is configured to cause a return loss of more than 10 decibels (dB) in the first frequency spectrum.

An acoustic wave device includes a piezoelectric substrate made of LiNbO.sub.3, and a dielectric film provided on the piezoelectric substrate to cover first and second IDT electrodes on the piezoelectric substrate. The first and second IDT electrodes include main electrode layers. When wave lengths determined by electrode finger pitches of the first and second IDT electrodes are λ.sub.1 and λ.sub.2, respectively, the average value thereof is λ.sub.0, λ.sub.1/λ.sub.0=1+X, and λ.sub.2/λ.sub.0=1−X, a relationship of 0.05≤X≤0.65 is satisfied. The wavelength λ.sub.1 is the longest, and the wavelength λ.sub.2 is the shortest. In Euler angles (φ, θ, ψ) of the piezoelectric substrate, φ is 0°±5°, ψ is 0°±10°, and θ satisfies Expression 1, wherein a relationship of B.sub.1<T×r≤0.10λ.sub.0 and B.sub.2<T×r≤0.10λ.sub.0 are satisfied.