Filter and multiplexer
A filter includes: one or more series resonators connected in series between an input terminal and an output terminal, the one or more series resonators including a series resonator located closest to the output terminal, the series resonator located closest to the output terminal having a resonant frequency that is 99.6% or less of or 102.2% or greater of a center frequency of a passband; one or more parallel resonators connected in parallel between the input terminal and the output terminal; and an inductor connected in parallel to the series resonator located closest to the output terminal.
1. A filter comprising:
one or more series resonators connected in series between an input terminal and an output terminal, the one or more series resonators including a series resonator located closest to the output terminal, the series resonator located closest to the output terminal having a resonant frequency that is 102.2% or greater of a center frequency of a passband;
one or more parallel resonators connected in parallel between the input terminal and the output terminal; and
an inductor connected in parallel to the series resonator located closest to the output terminal,
wherein the series resonator located closest to the output terminal and the inductor connected in parallel thereto form an attenuation pole within a frequency band equal to twice the passband.
2. The filter according to claim 1 , wherein the series resonator located closest to the output terminal and the inductor connected in parallel thereto generate a second harmonic having a peak not located within the frequency band equal to twice the passband.
3. The filter according to claim 1 , wherein
the series resonator located closest to the output terminal has a resonant frequency that is 103% or greater of the center frequency of the passband.
4. The filter according to claim 1 , wherein
the one or more series resonators are a plurality of series resonators, and
the resonant frequency of the series resonator located closest to the output terminal is a highest among resonant frequencies of the plurality of series resonators.
5. The filter according to claim 1 , wherein the one or more series resonators are a plurality of series resonators, and
an inductor is not connected in parallel to at least one series resonator of the plurality of series resonators other than the series resonator closest to the output terminal.
6. A multiplexer comprising:
the filter according to claim 1 .
7. The filter according to claim 1 , wherein at least one of the one or more series resonators and the one or more parallel resonators are divided in reverse series or in reverse parallel.
8. The filter according to claim 1 , wherein
the one or more series resonators and the one or more parallel resonators are piezoelectric thin film resonators.
9. The filter according to claim 1 , wherein
the inductor is located on a substrate on which a chip including the one or more series resonators and the one or more parallel resonators is to be mounted.
10. The filter according to claim 1 , wherein
the series resonator located closest to the output terminal is located closest to the output terminal in a pathway connecting the input terminal and the output terminal.
11. A filter comprising:
series resonators connected in series between an input terminal and an output terminal, the series resonators including a series resonator located closest to the output terminal, the series resonator located closest to the output terminal having a highest resonant frequency among the series resonators;
one or more parallel resonators connected in parallel between the input terminal and the output terminal; and
an inductor connected in parallel to the series resonator located closest to the output terminal, the inductor and the series resonator located closest to the output terminal forming an attenuation pole located within a frequency band equal to twice a passband.
12. The filter according to claim 11 , wherein the series resonator located closest to the output terminal is located closest to the output terminal in a pathway connecting the input terminal and the output terminal.