Ladder-type frequency-variable filter, multiplexer, radio-frequency front end circuit, and communication terminal
Even when frequency characteristics are changed in association with multiple communication bands, an attenuation required for a specific frequency band outside a pass band is obtained. A frequency-variable filter ( 10 ) includes multiple series-arm resonators ( 111, 112, 113 ), multiple parallel-arm resonators ( 121, 122, 123 ), a variable capacitor ( 21 ), and an inductor ( 31 ) having a fixed inductance. The multiple series-arm resonators ( 111, 112, 113 ) and the multiple parallel-arm resonators ( 121, 122, 123 ) are connected in a ladder shape. The variable capacitor ( 21 ) is connected in series with the parallel-arm resonator ( 121 ). The fixed inductor ( 31 ) is connected in series with the parallel-arm resonator ( 123 ).
1. A ladder-type frequency-variable filter comprising:
a series-arm resonator;
a first parallel-arm resonator in a first parallel arm,
a second parallel-arm resonator in a second parallel arm,
a variable impedance element connected in series with the first parallel-arm resonator,
a first fixed inductor having a fixed inductance, wherein:
the series-arm resonator, the first parallel-arm resonator, and the second parallel arm resonator are connected according to a ladder configuration, no variable impedance element is connected in series with the second parallel-arm resonator, and
a third parallel-arm resonator in a third parallel arm,
wherein the first fixed inductor is connected to the second parallel-arm resonator and the third parallel-arm resonator.
2. A ladder-type frequency-variable filter comprising,
a series-arm resonator,
a first parallel-arm resonator in a first parallel arm,
a second parallel-arm resonator in a second parallel arm,
a variable impedance element connected in series with the first parallel-arm resonator; and
a first fixed inductor having a fixed inductance,
wherein:
the series-arm resonator, the first parallel-arm resonator, and the second parallel-arm resonator are connected according to a ladder configuration,
no variable impedance element is connected in series with the second parallel-arm resonator, and
the first fixed inductor is connected in series with the second parallel-arm resonator, and
wherein a frequency of an attenuation pole formed by a sub-resonance of the second parallel-arm resonator formed by the first fixed inductor overlaps a frequency of an attenuation pole formed by a sub-resonance point of the first parallel-arm resonator.
3. A ladder-type frequency-variable filter comprising,
a series-arm resonator,
a first parallel-arm resonator in a first parallel arm,
a second parallel-arm resonator in a second parallel arm,
a variable impedance element connected in series with the first parallel-arm resonator, and
a first fixed inductor having a fixed inductance,
wherein:
the series-arm resonator, the first parallel-arm resonator, and the second parallel-arm resonator are connected according to a ladder configuration,
no variable impedance element is connected in series with the second parallel-arm resonator, and
the first fixed inductor is connected in series with the second parallel-arm resonator, and
wherein a frequency of an attenuation pole formed by the first fixed inductor overlaps a frequency of a harmonic of a communication signal passing through the ladder-type frequency-variable filter.
4. The ladder-type frequency-variable filter according to claim 3 , further comprising:
a third parallel-arm resonator in a third parallel arm,
wherein no variable impedance element is connected in series with the third parallel-arm resonator, and a second fixed inductor having a fixed inductance is connected in series with the third parallel-arm resonator.
5. A multiplexer comprising:
the ladder-type frequency-variable filter according to claim 3 ; and
a second filter,
wherein the ladder-type frequency-variable filter and the second filter are connected to a common node.
6. The multiplexer according to claim 5 , wherein the second filter is a longitudinally-coupled resonator filter.
7. The multiplexer according to claim 5 ,
wherein the second filter includes a resonator, and
wherein the multiplexer further comprises a third fixed inductor having a fixed inductance, the third fixed inductor being connected between the common node and the second filter.
8. The multiplexer according to claim 7 , wherein an attenuation pole of an LC series resonance circuit including the second filter and the third fixed inductor overlaps a frequency of an attenuation pole formed by a sub-resonance point of the first parallel-arm resonator of the ladder-type frequency-variable filter.
9. The multiplexer according to claim 7 , wherein the second filter is a longitudinally-coupled resonator filter.
10. The multiplexer according to claim 5 ,
wherein the ladder-type frequency-variable filter is a transmission filter, and
wherein the second filter is a reception filter.
11. A radio-frequency front end circuit comprising:
the multiplexer according to claim 10 ;
a transmission-side amplification circuit that is connected to the transmission filter;
a reception-side amplification circuit that is connected to the reception filter; and
a radio frequency integrated circuit (RFIC) that is connected to the transmission-side amplification circuit and to the reception-side amplification circuit.
12. A communication terminal comprising:
the radio-frequency front end circuit according to claim 11 ; and
a baseband integrated circuit (IC) that is connected to the RFIC.
13. The multiplexer according to claim 5 ,
wherein the ladder-type frequency-variable filter is a reception filter, and
wherein the second filter is a transmission filter.
14. A radio-frequency front end circuit comprising:
the multiplexer according to claim 13 ;
a transmission-side amplification circuit that is connected to the transmission filter;
a reception-side amplification circuit that is connected to the reception filter; and
a radio frequency integrated circuit (RFIC) that is connected to the transmission-side amplification circuit and to the reception-side amplification circuit.
15. A communication terminal comprising:
the radio-frequency front end circuit according to claim 14 ; and
a baseband integrated circuit (IC) that is connected to the RFIC.