A signal transmission device includes a signal side electrode; a first signal line connected to one side of the signal side electrode; a second signal line connected to the other side of the signal side electrode; a power source side electrode that forms a pair with the signal side electrode and is connected to the signal side electrode via an electronic component including at least an inductor component; and a capacitive coupling part that capacitively couples the power source side electrode to a ground wiring or a power source wiring. The first signal line, the signal side electrode, and the second signal line form a transmission path for transmitting an electric signal. The first signal line and the second signal line transmit power via the signal side electrode, the electronic component, and the power source side electrode.

A wireless communication device and a filter are provided. The filter has an input end and an output end and includes a first energy storage element, a first series resonant circuit, a second series resonant circuit, a first parallel resonant circuit and a second parallel resonant circuit. The first and the second series resonant circuits respectively have a first capacitor and a first inductor connected in series. The first and the second parallel resonant circuits respectively have a second capacitor and a second inductor connected in parallel. The first series resonant circuit and the first parallel resonant circuit are electrically connected in cascade between a first end of the first energy storage element and a ground, and the second series resonant circuit and the second parallel resonant circuit are electrically connected in cascade between a second end of the first energy storage element and the ground.

A wireless communication device and a filter are provided. The filter has an input end and an output end and includes a first energy storage element, a first series resonant circuit, a second series resonant circuit, a first parallel resonant circuit and a second parallel resonant circuit. The first and the second series resonant circuits respectively have a first capacitor and a first inductor connected in series. The first and the second parallel resonant circuits respectively have a second capacitor and a second inductor connected in parallel. The first series resonant circuit and the first parallel resonant circuit are electrically connected in cascade between a first end of the first energy storage element and a ground, and the second series resonant circuit and the second parallel resonant circuit are electrically connected in cascade between a second end of the first energy storage element and the ground.

The present disclosure provides a variable filter circuit capable of controlling a band width and a center frequency of a pass band, and also capable of suppressing the total number of pieces of variable reactance. That is, a variable filter circuit includes a serial arm in which a plurality of circuit elements are connected in series with respect to a signal path and a parallel arm in which a plurality of circuit elements are connected in parallel with respect to the signal path, wherein the serial arm and the parallel arm each includes a variable reactance element, a series reactance element that is connected in series to the variable reactance element and resonates therewith, and a parallel reactance element that is connected in parallel to the variable reactance element and resonates therewith.

The present disclosure provides a variable filter circuit capable of controlling a band width and a center frequency of a pass band, and also capable of suppressing the total number of pieces of variable reactance. That is, a variable filter circuit includes a serial arm in which a plurality of circuit elements are connected in series with respect to a signal path and a parallel arm in which a plurality of circuit elements are connected in parallel with respect to the signal path, wherein the serial arm and the parallel arm each includes a variable reactance element, a series reactance element that is connected in series to the variable reactance element and resonates therewith, and a parallel reactance element that is connected in parallel to the variable reactance element and resonates therewith.

A front-end circuit includes an antenna terminal, front-end terminals (Pfe1, Pfe2), a circulator, and frequency-variable filters. The circulator sends transmission signals of communication bands of a frequency division duplex system and a time division duplex system from the front-end terminal (Pfe1) to the antenna terminal. The circulator sends a reception signal from the antenna terminal to the front-end terminal (Pfe2). The frequency-variable filter is connected between the front-end terminal (Pfe1) and the circulator, passes transmission signals of the frequency division duplex system and the time division duplex system, and attenuates signals other than the above transmission signals. The frequency-variable filter is connected between the circulator and the front-end terminal (Pfe2), passes a reception signal of the frequency division duplex system, and attenuates signals other than the above reception signal.

A front-end circuit includes an antenna terminal, front-end terminals (Pfe1, Pfe2), a circulator, and frequency-variable filters. The circulator sends transmission signals of communication bands of a frequency division duplex system and a time division duplex system from the front-end terminal (Pfe1) to the antenna terminal. The circulator sends a reception signal from the antenna terminal to the front-end terminal (Pfe2). The frequency-variable filter is connected between the front-end terminal (Pfe1) and the circulator, passes transmission signals of the frequency division duplex system and the time division duplex system, and attenuates signals other than the above transmission signals. The frequency-variable filter is connected between the circulator and the front-end terminal (Pfe2), passes a reception signal of the frequency division duplex system, and attenuates signals other than the above reception signal.

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.

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.

In order to pass a signal having a wide pass bandwidth with respect to a center frequency of a pass band, a surface acoustic wave device includes a first surface acoustic wave element provided with a first pass band; and a second surface acoustic wave element having a second pass band in a high frequency band compared with the first pass band of the first surface acoustic wave element, in which the first surface acoustic wave element and the second surface acoustic wave element have a common input terminal and a common output terminal, and a frequency of a high frequency side of the first pass band of the first surface acoustic wave element is partially overlapped with a frequency of a low frequency side of the second pass band of the second surface acoustic wave element.