In some embodiments, a differential oscillator includes a differential circuit coupled between a first output node and a second output node and a transformer-coupled band-pass filter (BPF). The transformer-coupled BPF is coupled between the first output node and the second output node and includes a coupling device and a transformer. The coupling device is coupled between the first output node and the second output node. The transformer includes a first winding coupled between the first output node and a voltage node and a second winding coupled between the second output node and the voltage node.

A frontend module includes a first filter having a passband of a first frequency band, a second filter having a passband of a second frequency band, the second frequency band being higher than the first frequency band, a third filter having a passband of a third frequency band, the third frequency band being higher than the second frequency band, and a sub-filter, connected to the second filter, configured to provide attenuation characteristics for the first frequency band, wherein the second filter comprises a plurality of parallel LC resonance circuits arranged between a ground and different nodes, from among a plurality of nodes between a first terminal and a second terminal, wherein an inductor is connected to a portion of the plurality of parallel LC resonance circuits.

A filter circuit may include a transmission line, a quarter wave resonator, and an electrical component coupled in series with the quarter wave resonator at a first end and to the transmission line at a second end. The electrical component may be have a frequency dependent impedance. The electrical component may be an inductor, a capacitor, or an inductor in series with a capacitor. In another aspect, a filter circuit may include a transmission line, a first quarter wave resonator coupled to a first electrical component and a second quarter wave resonator coupled to a second electrical component. Each of the first and second electrical components may be coupled to the transmission line in parallel with each other. The first and the second electrical components may have a frequency dependent impedance. The first electrical component may be the same as or different from the second electrical component.

A filter assembly is disclosed that includes a monolithic filter having a surface and a heat sink coupled to the surface of the monolithic filter. The heat sink includes a layer of thermally conductive material that can have a thickness greater than about 0.02 mm. The heat sink may provide electrical shielding for the monolithic filter. In some embodiments, the filter assembly may include an organic dielectric material, such as liquid crystalline polymer or polyphenyl ether. In some embodiments, the filter assembly may include an additional monolithic filter.

A variable filter for an RF circuit has a signal loop comprising a signal input port and a signal output port, and a plurality of circuit elements connected within the signal loop. The plurality of circuit elements comprise a multi-pole resonator comprising a plurality of frequency tunable resonators and an adjustable scaling block that applies a gain factor. Adjacent frequency tunable resonators within the multi-pole resonator are reciprocally coupled. A controller is connected to tune the multi-pole resonator and to adjust the gain factor of the adjustable scaling block such that the signal loop generates a desired bandpass response.

Radio frequency (RF) systems with tunable filters are provided herein. In certain embodiments, an RF system includes a first RF processing circuit configured to process a first frequency band of a first communication standard and a second frequency band of a second communication standard. The first frequency band and the second frequency band are close in frequency and/or partially overlapping in frequency. The first RF processing circuit includes a tunable filter for changing the bandwidth of the first RF processing circuit to enhance the robustness of the first RF processing circuit to blocker or jammer signals of a third frequency band.

A band pass filter is provided with a first filter circuit which includes a first resonant circuit including a first inductor and a second resonant circuit including a second inductor, a second filter circuit which includes a third resonant circuit including a third inductor and a fourth resonant circuit including a fourth inductor, and a fifth resonant circuit including a fifth inductor. The fifth inductor is electromagnetically coupled to each of the first inductor, the second inductor, the third inductor, and the fourth inductor.

A method includes converting, by n analog to digital converter circuits, n analog signals into n first digital signals having a first data rate frequency; converting, by n digital decimation filtering circuits, the n first digital signals into n second digital signals having a second data rate frequency; and converting, by n digital bandpass filter (BPF) circuits, the n second digital signals into a plurality of outbound digital signals having a third data rate frequency. The coefficients for the taps of a digital BPF circuit is set to produce a bandpass region approximately centered at the oscillation frequency of the analog signal and having a bandwidth tuned for filtering a pure tone component of the analog signal. The first data rate frequency is a first integer multiple of the third data rate frequency. The second data rate frequency is a second integer multiple of the third data rate frequency.

A multiplexer includes a switch that individually connects or disconnects between a common terminal and first, second, third, and fourth selection terminals, first and second matching circuits, and first, second, and third filters having different pass bands. The first selection terminal is connected to one end of the first matching circuit, the second selection terminal is connected to one end of the second matching circuit, the other end of the first matching circuit and the other end of the second matching circuit are connected to each other and to one end of the first filter, the third selection terminal is connected to one end of the second filter, and the fourth selection terminal is connected to one end of the third filter.

A filter device includes a common connection terminal, a first bandpass filter connected to the common connection terminal and including an inductor, and a second bandpass filter connected to the common connection terminal and having a pass band lower in frequency than a pass band of the first bandpass filter. The filter device uses SH waves. The first bandpass filter is a ladder filter. Each of series arm resonators and parallel arm resonators includes an interdigital transducer electrode. Of the parallel arm resonators of the first bandpass filter, the inductor is connected in series to the parallel arm resonator with a shortest electrode finger pitch of the interdigital transducer electrode.