Acoustic resonators, such as bulk acoustic wave (BAW) resonators, are disclosed that include mode suppression structures. Acoustic resonators, including stacked crystal filters (SCFs), are disclosed that include spurious mode suppression by modifying a piezoelectric coupling profile within one or more layers of an SCF. Mode suppression configurations may include structures with one or more inverted polarity piezoelectric layers, one or more non-piezoelectric layers, one or more thicker electrodes of the SCF, and combinations thereof. Symmetric input and output electrical response for SCFs with mode suppression configurations may be exhibited by including piezoelectric materials with different electromechanical coupling values and/or by dividing stress profiles differently by configuring different thicknesses for input and output sides of SCFs.

A notch filter includes a first inductor coupled between an input node and an output node, a dual-resonator structure coupled between the input node and the output node, and a second inductor coupled between the dual-resonator structure and ground, and a bandpass filter includes a capacitor coupled between an input node and an output node, and a dual-resonator structure coupled between the input node, the output node, and ground.

Acoustic resonators, such as bulk acoustic wave (BAW) resonators and, in particular, acoustic resonators including stacked crystal filters (SCFs) are disclosed. SCF structures are disclosed with increased spurious free ranges by providing various arrangements of acoustically soft materials in one or more locations that correspond with high stress regions of one or more modes. For SCFs operating in first order modes, relative amounts of acoustically soft materials within shared electrodes may be increased. One or more additional layers of acoustically soft materials may also be added to SCF structures near shared electrodes. Accordingly, SCFs may be provided with increased frequency spreads between first order modes and second order modes.

In accordance with an embodiment, a method of operating an RF system includes filtering a wideband RF signal using an adjustable center frequency bandpass filter to produce a filtered RF signal; amplifying the filtered RF signal to produce an amplified RF signal; and band stop filtering the amplified RF signal to produce a band stopped RF signal.

An acoustic impedance transformation circuit and related apparatus are provided. In aspects discussed herein, the acoustic impedance transformation circuit can be configured to transform an input impedance into an output impedance higher than the input impedance. In this regard, the acoustic impedance transformation circuit can be provided in an apparatus to enable impedance matching between two electrical circuits. As a result, it may be possible to reduce signal reflection resulting from impedance mismatch between the two circuits, thus helping to improve performance of the apparatus.

An acoustic structure is provided. The acoustic structure includes an acoustic resonator structure configured to resonate in a series resonance frequency (e.g., passband frequency) to pass a signal, or cause a series capacitance to block the signal in a parallel resonance frequency (e.g., stopband frequency). The parallel resonance frequency may become higher than the series resonance frequency when the tunable capacitance is lesser than or equal to two times of the series capacitance (C.sub.Tune2C.sub.0), or lower than the series resonance frequency when the tunable capacitance is greater than two times of the series capacitance (C.sub.Tune>2C.sub.0). In this regard, the acoustic structure can be configured to include a tunable reactive circuit to generate the tunable capacitance (C.sub.Tune) to adjust the parallel resonance frequency. As such, it may be possible to flexibly configure the acoustic resonator structure to block the signal in desired stopband frequencies.

An acoustic impedance transformation circuit and related apparatus are provided. In aspects discussed herein, the acoustic impedance transformation circuit can be configured to transform an input impedance into an output impedance higher than the input impedance. In this regard, the acoustic impedance transformation circuit can be provided in an apparatus to enable impedance matching between two electrical circuits. As a result, it may be possible to reduce signal reflection resulting from impedance mismatch between the two circuits, thus helping to improve performance of the apparatus.

A coupled resonator filter includes a first resonator, a second resonator, one or more intervening layers, a first border ring, and a second border ring. The first resonator includes a first piezoelectric layer and a first electrode in contact with the first piezoelectric layer. The second resonator includes a second piezoelectric layer and a second electrode in contact with the second piezoelectric layer. The one or more intervening layers are between the first resonator and the second resonator and acoustically couple the first resonator and the second resonator. The first border ring is on the first electrode. The second border ring is on the second electrode. By providing both the first border ring and the second border ring, spurious modes in the coupled resonator filter may be suppressed, thereby improving the performance thereof.

An acoustic resonator device includes a bottom electrode disposed on a substrate over an air cavity, a first piezoelectric material layer disposed on the bottom electrode, an electrically-isolated layer of high-acoustic-impedance material disposed on the first piezoelectric material layer, a second piezoelectric material layer disposed on the electrically-isolated layer of high-acoustic impedance material, and a top electrode disposed on the second piezoelectric material layer, where an overlap among the top electrode, the first piezoelectric material layer, the electrically-isolated layer of high-acoustic-impedance material, the second piezoelectric material layer, and the bottom electrode over the air cavity defines a main membrane region.

A notch filter includes an inductor coupled between an input node and an output node, and a dual-resonator structure coupled between the input node, the output node, and ground.