Balanced high-frequency filter, antenna duplexer, balanced high-frequency circuit and communication apparatus
View Patent ↗With an antenna duplexer having balanced terminals, there has been a problem that the amount of leakage of common-mode signal components in the balanced terminals is large. A balanced high-frequency filter designed to solve this problem comprises a balanced high-frequency element and a phase-shifting circuit which includes a transmission line and placed between output terminals, and which is series resonance circuit having a length set to λ T /2 (λ T : the wavelength at a frequency in a transmission frequency band) and capable of resonating with common-mode signal components at a predetermined frequency.
1. A balanced high-frequency filter comprising:
a balanced high-frequency element having at least one balanced terminal; and
a phase-shifting circuit,
wherein the phase-shifting circuit is a series resonance circuit which is electrically connected between the balanced terminals and which resonates with common-mode signal components at a predetermined frequency;
a resonance frequency of the series resonance circuit is set in a second frequency band;
a first frequency band is a pass band of the balanced high-frequency element; and
the second frequency band is an attenuation band of the balanced high-frequency element.
2. The balanced high-frequency filter according to claim 1 , wherein the first frequency band is a reception frequency band, and the second frequency band is a transmission frequency band.
3. The balanced high-frequency filter according to claim 1 or 2 , wherein the phase-shifting circuit has a transmission line which has a length equal to about ½ of a wavelength in the second frequency band; and
the phase-shifting circuit is connected between the balanced terminals.
4. The balanced high-frequency filter according to claim 3 , wherein the phase-shifting circuit has at least two transmission lines;
one of the transmission lines has a length equal to about ½ of a wavelength in the second frequency band;
the other of the transmission lines differs in length from said one of the transmission lines; and
the phase-shifting circuit is connected between the balanced terminals.
5. The balanced high-frequency filter according to claim 1 or 2 , wherein the phase-shifting circuit has at least first, second and third impedance elements;
the first impedance element and the second impedance element are connected in series between the balanced terminals;
a connection point between the first impedance element and the second impedance element is grounded through the third impedance element;
the first impedance element and the third impedance element form a series resonance circuit; and
the second impedance element and the third impedance element form a series resonance circuit.
6. The balanced high-frequency filter according to claim 5 , wherein each of the first and second impedance elements is a capacitor, and the third impedance element is an inductor.
7. The balanced high-frequency filter according to claim 5 , wherein each of the first and second impedance elements is an inductor, and the third impedance element is a capacitor.
8. The balanced high-frequency filter according to claim 6 , wherein the impedance of each of the first and second impedance elements in the first frequency band is set so that a value of the first or second impedance element normalized on a characteristic impedance value of one of the balanced terminals is equal to or larger than 3.
9. The balanced high-frequency filter according to claim 1 or 2 , wherein the balanced high-frequency element is constituted by a surface acoustic wave filter.
10. The balanced high-frequency filter according to claim 1 or 2 , wherein the balanced high-frequency element is constituted by a filter using an FBAR.
11. The balanced high-frequency filter according to claim 1 or 2 , wherein the balanced high-frequency filter is connected to an input side of a low-noise amplifier having balanced terminals.
12. The balanced high-frequency filter according to claim 1 or 2 , wherein the balanced high-frequency filter is connected to an input side of a mixer having balanced terminals.
13. An antenna duplexer comprising the balanced high-frequency filter according to claim 1 or 2 .
14. An antenna duplexer according to claim 13 , wherein the balanced high-frequency filter is a receiving filter in the antenna duplexer;
the first frequency band is a reception frequency band in the antenna duplexer; and
the second frequency band is a transmission frequency band in the antenna duplexer.
15. An antenna duplexer according to claim 14 , wherein the antenna duplexer is connected to an input side of a low-noise amplifier having balanced terminals.
16. A balanced high-frequency circuit comprising:
a low-noise amplifier having balanced terminals;
a mixer having balanced terminals; and
a phase-shifting circuit,
wherein the phase-shifting circuit is a series resonance circuit which is electrically connected between the balanced terminals connecting the low-noise amplifier and the mixer to each other, and which resonates with common-mode signal components at a predetermined frequency;
a resonance frequency of the series resonance circuit is set in a second frequency band;
a first frequency band is the frequency band of desired waves; and
the second frequency band is the frequency band of interference waves.
17. The balanced high-frequency circuit according to claim 16 , wherein the first frequency band is a reception frequency band, and the second frequency band is a transmission frequency band.
18. The balanced high-frequency circuit according to claim 16 or 17 , wherein the phase-shifting circuit has a transmission line which has a length equal to about ½ of a wavelength in the second frequency band; and
the phase-shifting circuit is connected between the balanced terminals.
19. The balanced high-frequency circuit according to claim 16 or 17 , wherein the phase-shifting circuit has at least first, second and third impedance elements;
the first impedance element and the second impedance element are connected in series between the balanced terminals;
a connection point between the first impedance element and the second impedance element is grounded through the third impedance element;
the first impedance element and the third impedance element form a series resonance circuit; and
the second impedance element and the third impedance element form a series resonance circuit.
20. The balanced high-frequency circuit according to claim 19 , wherein each of the first and second impedance elements is a capacitor, and the third impedance element is an inductor.
21. The balanced high-frequency circuit according to claim 19 , wherein each of the first and second impedance elements is an inductor, and the third impedance element is a capacitor.
22. The balanced high-frequency circuit according to claim 20 , wherein the impedance of each of the first and second impedance elements in the first frequency band is set so that a value of the first or second impedance element normalized on a characteristic impedance value of one of the balanced terminals is equal to or larger than 3.
23. A balanced high-frequency circuit comprising:
a circuit board having balanced lines; and
a phase-shifting circuit,
wherein the phase-shifting circuit is mounted on the circuit board;
the phase-shifting circuit is a series resonance circuit which is electrically connected between the balanced terminals, and which resonates with common-mode signal components at a predetermined frequency;
a resonance frequency of the series resonance circuit is set in a second frequency band;
a first frequency band is the frequency band of desired waves; and
the second frequency band is the frequency band of interference waves.
24. The balanced high-frequency circuit according to claim 23 , wherein the first frequency band is a reception frequency band, and the second frequency band is a transmission frequency band.
25. The balanced high-frequency circuit according to claim 23 or 24 , wherein the phase-shifting circuit has a transmission line, the transmission line has a length equal to about ½ of a wavelength in the first frequency band; and
the phase-shifting circuit is connected between the balanced terminals.
26. The balanced high-frequency circuit according to claim 23 or 24 , wherein the phase-shifting circuit has at least first, second and third impedance elements;
the first impedance element and the second impedance element are connected in series between the balanced terminals;
a connection point between the first impedance element and the second impedance element is grounded through the third impedance element;
the first impedance element and the third impedance element form a series resonance circuit; and
the second impedance element and the third impedance element form a series resonance circuit.
27. The balanced high-frequency circuit according to claim 26 , wherein each of the first and second impedance elements is a capacitor, and the third impedance element is an inductor.
28. The balanced high-frequency circuit according to claim 26 , wherein each of the first and second impedance elements is an inductor, and the third impedance element is a capacitor.
29. The balanced high-frequency circuit according to claim 27 , wherein the impedance of each of the first and second impedance elements in the first frequency band is set so that a value of the first or second impedance element normalized on a characteristic impedance value of one of the balanced terminals is equal to or larger than 3.
30. A communication apparatus using the balanced high-frequency filter according to claim 1 or 2 .
31. A communication apparatus using the antenna duplexer according to claim 13 .
32. A communication apparatus using the antenna duplexer according to claim 14 .
33. A communication apparatus using the balanced high-frequency circuit according to claim 16 or 17 .
34. A communication apparatus using the balanced high-frequency circuit according to claim 23 or 24 .