Filter device with balanced signal input and output terminals
View Patent ↗A filter device has a novel structure, satisfying requirements to have balanced input and output, with a small number of configuration elements. The above filter device includes first and second balanced signal terminals on the input side to be connected to an external balance circuit; first and second balanced signal, terminals on the output side to be connected to an external balance circuit; at least one first resonator connected in series between the first balanced signal terminal on the input side and the first balanced signal terminal on the output side; at least one second resonator connected in series between the second balanced signal terminal on the input side and the second balanced signal terminal on the output side; and at least one impedance element connected between the first balanced signal terminal on the output side and the second balanced signal terminal on the input side.
1. A filter device including a filter circuit consisting of:
first and second balanced signal input terminals to be connected to an external balance circuit;
first and second balanced signal output terminals to be connected to an external balance circuit; and
a series circuit of first through third branches, wherein
a first terminal of the first branch is the first balanced signal input terminal,
a first terminal of the second branch is the second balanced signal output terminal,
a connection point of a second terminal of the first branch and a first terminal of the third branch is the first balanced signal output terminal, and
a connection point of a second terminal of the second branch and a second terminal of the third branch is the second balanced signal input terminal, wherein
the first branch comprises a first resonator, the second branch comprises a second resonator, and the third branch comprises a first impedance element.
2. The filter device according to claim 1 , wherein at least one of the first and second branches comprise a second impedance element is connected in series with at least one of the first resonator and the second resonator.
3. The filter device according to claim 1 , wherein at least one of the first and second branches comprise a second impedance element connected in parallel with at least one of the first resonator and the second resonator.
4. The filter device according to claim 1 , wherein the first and the second resonators have a virtually equal frequency characteristic.
5. The filter device according to claim 1 , wherein the first and the second resonators and the impedance element are housed in an identical package.
6. The filter device according to claims 1 , wherein the first impedance element is an externally connected element disposed outside a package.
7. The filter device according to claim 1 , wherein the first and the second resonators are acoustic wave elements.
8. The filter device according to claim 7 , wherein the first and the second resonators are surface acoustic wave (SAW) elements.
9. The filter device according to claim 7 , wherein the first and the second resonators are bulk acoustic wave (BAW) elements.
10. The filter device according to claim 7 , wherein the first and the second resonators are piezoelectric thin film resonator (FBAR) elements.
11. The filter device according to claim 1 , wherein the first and the second resonators are formed on an identical piezoelectric substrate.
12. The filter device according to claim 11 , wherein the first impedance element is formed on the piezoelectric substrate.
13. The filter device according to claim 1 , wherein the first impedance element is an inductive element.
14. The filter device according to claim 1 , wherein the first impedance element is a capacitive element.
15. The filter device according to claim 1 , wherein the first impedance element is configured of a lumped constant element.
16. The filter device according to claim 1 , wherein the first impedance element is configured of a distributed constant element.
17. The filter device according to claim 1 , further comprising: a balance-unbalance conversion means on at least one of the input side and the output side.
18. The filter device according to claim 1 , wherein the filter circuit functions as a band elimination filter.
19. A filter device including a plurality of filter circuits connected in cascade, each of the plurality of filter circuits consisting of:
first and second balanced signal input terminals to be connected to an external balance circuit;
first and second balanced signal output terminals to be connected to an external balance circuit; and
a series circuit of a first resonator, an impedance element, and a second resonator, wherein
a first terminal of the first resonator is the first balanced signal input terminal,
a first terminal of the second resonator is the second balanced signal output terminal,
a connection point of a second terminal of the first resonator and a first terminal of the impedance element is the first balanced signal output terminal, and
a connection point of a second terminal of the impedance element and a second terminal of the second resonator is the second balance signal input terminal.
20. A filter device including a filter circuit comprising:
first and second balanced signal input terminals to be connected to an external balance circuit;
first and second balanced signal output terminals to be connected to an external balance circuit; and
a series circuit of a first resonator, an impedance element, and a second resonator, wherein
a first terminal of the first resonator is the first balanced signal input terminal,
a first terminal of the second resonator is the second balanced signal output terminal,
a connection point of a second terminal of the first resonator and a first terminal of the impedance element is the first balanced signal output terminal, and
a connection point of a second terminal of the impedance element and a second terminal of the second resonator is the second balanced signal input terminal, and
wherein the first and the second resonators have a virtually different frequency characteristics.
21. A filter device including a filter circuit comprising:
first and second balanced signal input terminals to be connected to an external balance circuit;
first and second balanced signal output terminals to be connected to an external balance circuit; and
a series circuit of a first resonator, an impedance element, and a second resonator, wherein
a first terminal of the first resonator is the first balanced signal input terminal,
a first terminal of the second resonator is the second balanced signal output terminal,
a connection point of a second terminal of the first resonator and a first terminal of the impedance element is the first balanced signal output terminal, and
a connection point of a second terminal of the impedance element and a second terminal of the second resonator is the second balanced signal input terminal, and
wherein the antiresonant frequency of one resonator of the first and the second resonators is a frequency between the resonant frequency and the antiresonant frequency of the other resonator.