Filter device and multiplexer
A filter device having a pass band and a stop band on a lower frequency side than the pass band includes a filter having a pass band including the pass band, a series arm resonator connected in series to the filter, a first inductor directly connected in series to the series arm resonator, and a parallel arm resonator connected between a node on a path connecting the filter and the series arm resonator and the ground. The parallel arm resonator constitutes a resonance circuit having a resonant frequency at which an attenuation pole corresponding to a high frequency end of the first stop band, and the series arm resonator and the inductor constitute a resonance circuit having an anti-resonant frequency on a lower frequency side than the pass band and having a sub-resonant frequency higher than a resonant frequency of the resonance circuit.
1. A filter device that has a first pass band and a first stop band on a lower frequency side than the first pass band, comprising:
a filter having a pass band including the first pass band;
a series arm resonator connected in series to the filter;
a first inductor directly connected in series to the series arm resonator; and
a first parallel arm resonator connected between a first node on a path connecting the filter and the series arm resonator, and the ground,
wherein the first parallel arm resonator constitutes a first resonance circuit having a resonant frequency at which an attenuation pole corresponding to the first stop band is formed, and
the series arm resonator and the first inductor constitute a second resonance circuit having an anti-resonant frequency on a lower frequency side than the first pass band and a sub-resonant frequency higher than a resonant frequency of the first resonance circuit.
2. The filter device according to claim 1 ,
wherein the sub-resonant frequency of the second resonance circuit is included in a range of 100% to 110% of the resonant frequency of the first resonance circuit.
3. The filter device according to claim 2 ,
wherein the sub-resonant frequency of the second resonance circuit is included in a range of 100% to 105% of the resonant frequency of the first resonance circuit.
4. The filter device according to claim 1 ,
wherein the filter, the series arm resonator and the first inductor are disposed on a path connecting a first input/output terminal and a second input/output terminal, and
the first inductor is directly connected to the first input/output terminal or the second input/output terminal.
5. The filter device according to claim 1 , further comprising a second parallel arm resonator connected between a second node on a path connecting a first input/output terminal and a second input/output terminal,
wherein:
the filter, the series arm resonator, and the first inductor are disposed on the path connecting the first input/output terminal and the second input/output terminal,
the second parallel arm resonator constitutes a third resonance circuit having a resonant frequency on a lower frequency side than the first pass band, and
a sub-resonant frequency of the second resonance circuit is higher than the resonant frequency of the third resonance circuit.
6. The filter device according to claim 5 ,
wherein the filter is disposed on a path connecting the first node and the second node.
7. The filter device according to claim 6 ,
further comprising a second inductor connected in parallel to the second parallel arm resonator, wherein:
the third resonance circuit comprises the second inductor and the second parallel arm resonator, and
an anti-resonant frequency of the third resonance circuit is higher than a resonant frequency of the first resonance circuit.
8. The filter device according to claim 5 ,
further comprising a second inductor connected in parallel to the second parallel arm resonator, wherein:
the third resonance circuit comprises the second inductor and the second parallel arm resonator, and
an anti-resonant frequency of the third resonance circuit is higher than a resonant frequency of the first resonance circuit.
9. The filter device according to claim 1 ,
wherein the filter is an acoustic wave resonator.
10. The filter device according to claim 1 ,
wherein the filter is an LC filter.
11. The filter device according to claim 1 ,
wherein the filter device has a second pass band on a lower frequency side than the first stop band,
the second pass band corresponds to a communication band of equal to or higher than 1710 MHz and equal to or lower than 2370 MHz,
the first stop band corresponds to a communication band of equal to or higher than 2402 MHz and equal to or lower than 2481.5 MHz, and
the first pass band corresponds to a communication band of equal to or higher than 2510 MHz and equal to or lower than 2690 MHz.
12. A multiplexer comprising:
the filter device according to claim 1 ; and
at least one other filter device, wherein the filter device and the at least one other filter device are connected to a common terminal.
13. The multiplexer according to claim 12 ,
wherein the filter device further has a second pass band, a second stop band, and a third stop band,
the second stop band corresponds to a communication band of equal to or higher than 1428 MHz and equal to or less than 1511 MHz,
the third stop band corresponds to a communication band of equal to or higher than 1559 MHz and equal to or lower than 1608 MHz,
the second pass band corresponds to a communication band of equal to or higher than 1710 MHz and equal to or lower than 2370 MHz,
the first stop band corresponds to a communication band of equal to or higher than 2402 MHz and equal to or lower than 2481.5 MHz,
the first pass band corresponds to a communication band of equal to or higher than 2510 MHz and equal to or lower than 2690 MHz, and
the at least one other filter devices includes:
a first filter device having a pass band corresponding to a communication band of equal to or higher than 690 MHz and equal to or lower than 960 MHz,
a second filter device having a pass band corresponding to the communication band of equal to or higher than 1428 MHz and equal to or lower than 1511 MHz,
a third filter device having a pass band corresponding to the communication band of equal to or higher than 1559 MHz and equal to or lower than 1608 MHz, and
a fourth filter device having a pass band corresponding to the communication band of equal to or higher than 2402 MHz and equal to or lower than 2481.5 MHz.