Directional coupler and wireless communication device

1. A directional coupler comprising:

a primary line for transmitting a transmission signal;

an input port provided at one end of said primary line for inputting the transmission signal to said primary line;

an output port provided at the other end of said primary line for outputting the transmission signal from said primary line;

a secondary line for electromagnetically coupling with said primary line to extract part of the transmission signal;

a coupling port provided at one end of said secondary line;

an isolation port provided at the other end of said secondary line; and

a low pass filter unit having a function of low pass filter, disposed between said secondary line and said coupling port, wherein:

said low pass filter unit is configured to form a resonant point on a higher side of a used frequency band to provide a flat degree of coupling in the used frequency band.

2. The directional coupler according to claim 1 , wherein:

the difference between a resonant frequency Fr and a center frequency Fc is normalized by the bandwidth BW and the center frequency to derive a value (Fr-Fc)/Fc/BW, said value being within a range of 0.6 to 1.2 inclusive, where:

said used frequency band is defined by a lower limit frequency F 1 , a higher limit frequency F 2 , the center frequency Fc, and the bandwidth BW, and

said resonant frequency Fr is the frequency at said resonant point.

3. The directional coupler according to claim 2 , wherein:

said value (Fr-Fc)/Fc/BW is within a range of 0.65 to 0.9 inclusive.

4. The directional coupler according to claim 1 , wherein:

said resonant point is formed such that the degree of coupling at the lower limit frequency of the used frequency band is substantially equal to the degree of coupling at the higher limit frequency of the used frequency band.

5. The directional coupler according to claim 1 , wherein said low pass filter unit includes:

an inductor connected in series between said secondary line and said coupling port; and

a capacitor having one end connected between said secondary line and said coupling port and the other end connected to the ground.

6. The directional coupler according to claim 1 , further comprising:

a capacitor having one end connected between said primary line and said input port and the other end connected between said secondary line and said coupling port.

7. The directional coupler according to claim 1 , further comprising:

a capacitor having one end connected between said primary line and said input port, and the other end connected to the ground.

8. The directional coupler according to claim 1 , further comprising in addition to said low pass filter unit:

another low pass filter unit having a function of low pass filter, disposed between said secondary line and said isolation port.

9. The directional coupler according to claim 8 , wherein:

said low pass filter disposed between said secondary line and said coupling port includes:

an inductor connected in series between said secondary line and said coupling port; and

a capacitor having one end connected between said secondary line and said coupling port and the other end connected to the ground, and

said low pass filter unit disposed between said secondary line and said isolation port includes:

an inductor connected in series between said secondary line and said isolation port; and

a capacitor having one end connected between said secondary line and said isolation port and the other end connected to the ground.

10. The directional coupler according to claim 8 , further comprising:

a capacitor having one end connected between said secondary line and said coupling port, and the other end connected between said secondary line and said isolation port.

11. The directional coupler according to claim 1 , further comprising:

an attenuator connected one or both of between said secondary line and said coupling port and between said secondary line and said isolation port.

12. The directional coupler according to claim 8 , further comprising:

an attenuator connected one or both of between said secondary line and said coupling port and between said secondary line and said isolation port.

13. The directional coupler according to claim 11 , further comprising:

a reactance unit having a reactive element, connected within said attenuator or between said coupling port and said isolation port.

14. The directional coupler according to claim 12 , further comprising:

a reactance unit having a reactive element, connected within said attenuator or between said coupling port and said isolation port.

15. A radio communication device comprising:

an antenna capable of transmitting a signal;

a directional coupler for detecting a level of a transmission signal transmitted to said antenna; and

a transmitter circuit capable of generating at least a transmission signal in a

first communication frequency band and a transmission signal in a second communication

frequency band different from the first conummication frequency band,

wherein said directional coupler is connected between said antenna and said

transmission circuit such that said radio communication device can receive both the

transmission signal in the first communication frequency band and the transmission signal in

the second communication frequency band, and

said radio communication device comprises the directional coupler according to claim 1 as said directional coupler.

16. A radio communication device comprising:

an antenna capable of transmitting a signal;

a directional coupler for detecting a level of a transmission signal transmitted to said antenna; and

a transmitter circuit capable of generating at least a transmission signal in a first communication frequency band and a transmission signal in a second communication frequency band different from the first communication frequency band,

wherein said directional coupler is connected between said antenna and said transmission circuit such that said radio communication device can receive both the transmission signal in the first communication frequency band and the transmission signal in the second communication frequency band, and

said radio communication device comprises the directional coupler according to claim 8 as said directional coupler.

17. A radio communication device comprising:

an antenna capable of transmitting a signal;

a directional coupler for detecting a level of a transmission signal transmitted to said antenna; and

a transmitter circuit capable of generating at least a transmission signal in a first communication frequency band and a transmission signal in a second communication

frequency band different from the first communication frequency band, wherein said directional coupler is connected between said antenna and said transmission circuit such that said radio communication device can receive both the transmission signal in the first communication frequency band and the transmission signal in the second communication frequency band, and

said radio communication device comprises the directional coupler according to claim 11 as said directional coupler.