Ladder filter and duplexer
A ladder filter includes a plurality of series-arm resonators and parallel-arm resonators including surface acoustic wave resonators. A metallization ratio of the series-arm resonator having the smallest electrostatic capacity among the plurality of series-arm resonators is the smallest among the plurality of series-arm resonators and an electrode finger pitch of the series-arm resonator having the smallest electrostatic capacity is the largest among electrode finger pitches of the plurality of series-arm resonators.
1. A ladder filter comprising:
a plurality of series-arm resonators including a surface acoustic wave resonator; and
a parallel-arm resonator including a surface acoustic wave resonator; wherein
a metallization ratio of a series-arm resonator with a smallest electrostatic capacity among the plurality of series-arm resonators is smallest among the plurality of series-arm resonators and an electrode finger pitch of the series-arm resonator having the smallest electrostatic capacity is largest among electrode finger pitches of the plurality of series-arm resonators.
2. The ladder filter according to claim 1 , wherein the plurality of series-arm resonator includes at least three series-arm resonators.
3. The ladder filter according to claim 1 , wherein the series-arm resonator having the smallest electrostatic capacity is serially divided to include a first division resonator and a second division resonator connected in series with the first division resonator.
4. The ladder filter according to claim 1 , further comprising:
an input terminal and an output terminal; wherein
the plurality of series-arm resonators are provided in a series arm that links the input terminal and the output terminal to each other, and the series-arm resonator having the smallest electrostatic capacity is closest to the input terminal among the plurality of series-arm resonators.
5. The ladder filter according to claim 1 , wherein the ladder filter is a transmission filter.
6. The ladder filter according to claim 1 , wherein an inductance is connected between the parallel-arm resonator and a ground potential.
7. The ladder filter according to claim 1 , wherein a plurality of the parallel-arm resonators is provided and connected between the plurality of series-arm resonators and a ground potential.
8. The ladder filter according to claim 1 , wherein the plurality of series-arm resonator includes at least six series-arm resonators.
9. A duplexer comprising:
a transmission filter including the ladder filter according to claim 1 ; and
a reception filter including one end that defines a common connection with one end of the transmission filter.
10. The duplexer according to claim 9 , further comprising an antenna terminal connected to the common connection.
11. The duplexer according to claim 9 , further comprising an inductance between the common connection and a ground.
12. The duplexer according to claim 9 , wherein the reception filter includes a resonator and a longitudinally-coupled-resonator surface acoustic wave filter.
13. The duplexer according to claim 12 , wherein the longitudinally-coupled-resonator surface acoustic wave filter includes at least two 3-IDT longitudinally-coupled-resonator surface acoustic wave filters cascade connected to each other.
14. The duplexer according to claim 9 , wherein the plurality of series-arm resonator includes at least three series-arm resonators.
15. The duplexer according to claim 9 , wherein the series-arm resonator having the smallest electrostatic capacity is serially divided to include a first division resonator and a second division resonator connected in series with the first division resonator.
16. The duplexer according to claim 9 , wherein the transmission filter further comprises:
an input terminal and an output terminal; wherein
the plurality of series-arm resonators are provided in a series arm that links the input terminal and the output terminal to each other, and the series-arm resonator having the smallest electrostatic capacity is closest to the input terminal among the plurality of series-arm resonators.
17. The duplexer according to claim 9 , wherein the ladder filter is a transmission filter.
18. The duplexer according to claim 9 , wherein an inductance is connected between the parallel-arm resonator and a ground potential.
19. The duplexer according to claim 9 , wherein a plurality of the parallel-arm resonators is provided and connected between the plurality of series-arm resonators and a ground potential.
20. The duplexer according to claim 9 , wherein the plurality of series-arm resonator includes at least six series-arm resonators.