A system for thermally protecting an amplifier driving a capacitive load may include a low-pass filter configured to filter, with a variable cutoff frequency, an input signal to generate a filtered input signal, wherein the amplifier is configured to receive the filtered input signal and amplify the filtered input signal to generate a driving signal to the capacitive load and a controller configured to receive a real-time estimate of a temperature associated with the amplifier and vary the variable cutoff frequency as a function of the temperature.

An adaptive logic board for a variable speed drive (VSD) of a heating, ventilation, air conditioning, and refrigeration (HVAC&R) system includes a signal sensing circuit configured to receive an input signal from a sensor of the VSD. The signal sensing circuit includes a filter configured to condition the input signal. The filter includes a variable resistance element configured to adjust a cutoff frequency of the filter. The filter is configured to attenuate waveforms in the input signal having frequencies that exceed the cutoff frequency to generate a conditioned signal. The adaptive logic board also includes a controller configured to receive the conditioned signal and to adjust the variable resistance element to adjust the cutoff frequency of the filter based on a parameter of the HVAC&R system.

A wireless transceiver including a receiver circuit coupled to an RF transceiver node, a tunable notch filter coupled between the RF transceiver node and a reference node, and a controller that programs the tunable notch filter with a selected blocker frequency and that selectively enables the tunable notch filter to attenuate at least one blocker signal. The tunable notch filter may include a variable capacitor and an inductor coupled in series between the RF transceiver node and ground. The inductor of the tunable notch filter may include a bondwire coupled between a semiconductor die and a semiconductor package. The inductance may include a physical inductor mounted on the package or a printed circuit board. The tunable notch filter may be enabled by a switch selectively coupling the filter to either the RF transceiver node or ground. The variable capacitor may be digitally programmed with digital values stored in a memory.

A first attenuation circuit is connected between a node and ground, and the node is located between a ladder resonance circuit and a transmitter-side terminal. A second attenuation circuit is connected between a first parallel arm resonator of the ladder resonance circuit and ground and is connected in series to the first parallel arm resonator. The first attenuation circuit includes a second parallel arm resonator and a first switch that switches between a first state in which the second parallel arm resonator is connected to the node and a second state in which the first switch is open. The second attenuation circuit includes a capacitor and a second switch that switches between a first state in which the capacitor is connected to the first parallel arm resonator and a second state in which the first parallel arm resonator is connected to ground.

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.

A variable filter circuit includes: a series arm connected in series between a signal input terminal and a signal output terminal; a parallel arm connected between the series arm and a ground terminal that has a resonator; and a variable reactance portion in the parallel arm, and the resonator of a parallel arm at an initial stage connected to the signal input terminal that has a resonant frequency fr and an anti-resonant frequency fa that satisfy 100(fnfr)/(fafr)23.9(%) for communication bands for each of which a stop band is set so as to be close to a high-frequency side of a pass band, among the plurality of communication bands, where a resonant frequency is fr, an anti-resonant frequency is fa, and a cutoff frequency at a high-frequency side of a pass band of each communication band is fn.

A high order filter circuit is integrated by a plurality of the low order filter circuits. Before correcting the high order filter circuit, switch units may restore the high order filter circuit to the low order filter circuits for correction, and then combine the corrected low order filter circuits to form the original high order filter circuit.

A reconfigurable microacoustic filter is specified which comprises a ladder-type-like filter topology and a suitably placed adjustable capacitive element.

A variable-frequency LC filter (1A) includes a multilayer circuit board, a series arm capacitor (11) formed in the multilayer circuit board and disposed in a series arm path that connects an input electrode to an output electrode, and a parallel arm inductor (21) formed in the multilayer circuit board and disposed in a parallel arm path that connects a ground electrode to a node (N1) in the series arm path. When the multilayer circuit board is viewed in plan, of the series arm capacitor (11) and the parallel arm inductor (21), only the parallel arm inductor (21) overlaps with the ground electrode.

An acoustic-wave device with active calibration mechanism is provided. The acoustic-wave device with active calibration mechanism includes at least one acoustic-wave duplexer, a voltage-controlled oscillator (VCO), a frequency discriminator and a control circuit. The acoustic-wave duplexer includes a TX filter and an RX filter. The voltage-controlled oscillator includes a calibration resonator and a tunable negative impedance circuit. The TX filter, the RX filter and the calibration resonator are disposed on the same piezoelectric substrate. The frequency discriminator generates a calibration signal according to a frequency deviation of the calibration resonator. The control circuit is connected to the acoustic-wave duplexer and the frequency discriminator. The control circuit adjusts an operating frequency of the TX filter or an operating frequency of the RX filter according to the calibration signal.