Variable attenuator

1. A variable attenuator for a frequency band of a range with a lower end of at least 10 GHz comprising:

an input port and an output port;

a first transmission line connecting the input port with the output port;

an attenuating unit provided between the first transmission line and ground, the attenuating unit including at least one transistor having two current terminals coupled with the first transmission line and the ground, respectively, and other transistors; and

a second transmission line connected between the two current terminals of the at least one transistor, the second transmission line operating as an inductor in the frequency band,

wherein the at least one transistor and the second transmission line cause a resonance frequency formed by a capacitor between the two current terminals of the at least one transistor and the inductor attributed to the second transmission line, the resonance frequency being within the frequency band,

wherein the other transistors are connected in series between the first transmission line and the ground, and

wherein the at least one transistor with the second transmission line between the two current terminals thereof is disposed closest to the first transmission line.

2. The variable attenuator according to claim 1 ,

further including other attenuating units provided in parallel between the first transmission line and the ground and between the input port and the output port, the other attenuating units each providing at least one transistor,

wherein the attenuating unit that provides the at least one transistor where the second transmission line is connected between the two current terminals thereof is disposed closet to the input port.

3. The variable attenuator according to claim 2 ,

wherein the first transmission line provides transmission elements between the attenuating units.

4. The variable attenuator according to claim 2 ,

wherein one of the other attenuating units is disposed closest to the output port and has a transistor with a size greater than a size of the at least one transistor in the attenuating unit disposed closest to the input port.

5. The variable attenuator according to claim 1 ,

further including other attenuating units each provided in parallel between the first transmission line and the ground and between the input port and the output port, the other attenuating units each providing at least one transistor,

wherein one of the other attenuating units is disposed closest to the output port and provides a transistor where another second transmission line is connected between the two current terminals thereof.

6. The variable attenuator according to claim 5 ,

wherein the first transmission line provides transmission elements between the attenuating units.

7. The variable attenuator according to claim 5 ,

wherein the transistor in the one attenuating unit disposed closest to the output port has a size greater than a size of the at least one transistor in the attenuating unit disposed closest to the input port.

8. The variable attenuator according to claim 1 ,

further including one or more other attenuating units each provided in parallel between the first transmission line and the ground and between the input port and the output port, the other attenuating units each providing at least one transistor,

wherein the attenuating unit that has the at least one transistor is disposed closest to the input port and one of the other attenuating units that is disposed closest to the output port each provide transistors with second transmission lines between the two current terminals thereof.

9. The variable attenuator according to claim 8 ,

wherein the first transmission line provides transmission elements between the attenuating units.

10. The variable attenuator according to claim 8 ,

wherein the transistor in the attenuating unit disposed closest to the output port has a size greater than a size of the at least one transistor in the attenuating unit disposed closest to the input port.

11. The variable attenuator according to claim 10 ,

wherein, in the attenuating unit disposed closest to the output port, the transistor and the second transmission line cause a second resonance frequency formed by a capacitor between the two current terminals of the transistor and the inductance of the second transmission line connecting the two current terminals,

wherein, in the attenuating unit disposed closest to the input port, the resonance frequency formed by the capacitor between the two current terminals of the transistor and the inductance of the second transmission line connecting the two current terminals is a first resonance frequency, and

wherein the first resonance frequency and the second resonance frequency are within the frequency band.

12. A variable attenuator for a frequency band of a range with a lower end of at least 10 GHz, comprising:

an input port and an output port;

a first transmission line connecting the input port with the output port;

a first attenuating unit provided between the first transmission line and ground, the attenuating unit included at least one transistor having two current terminals coupled with the first transmission line and ground, respectively;

a second transmission line connected between the two current terminals of the transistor of the first attenuating unit, the second transmission line operating as an inductor in the frequency band; and

one or more other attenuating units provided in parallel between the first transmission line and the ground and between the input port and the output port, the other attenuating units each providing at least one transistor,

wherein the transistor of the first attenuating unit and the second transmission line cause a resonance frequency formed by a capacitor between the two current terminals of the transistor of the first attenuating unit and the inductor attributed to the second transmission line, the resonance frequency being within the frequency band,

wherein the first attenuating unit is disposed closest to the input port, and

wherein one of the other attenuating units is disposed closest to the output port and the transistor in the attenuating unit closest to the output port has a size greater than a size of the transistor in the first attenuating unit.

13. A variable attenuator for a frequency band of a range with a lower end of at least 10 GHz, comprising:

an input port and an output port;

a first transmission line connecting the input port with the output port;

a first attenuating unit provided between the first transmission line and ground, the attenuating unit including at least one transistor having two current terminals coupled with the first transmission line and the ground, respectively;

a second transmission line connected between the two current terminals of the transistor of the first attenuating unit, the second transmission line operating as an inductor in the frequency band; and

one or more other attenuating units each provided in parallel between the first transmission line and the ground and between the input port and the output port, the other attenuating units each providing at least one transistor,

wherein the transistor of the first attenuating unit and the second transmission line cause a resonance frequency formed by a capacitor between the two current terminals of the transistor of the first attenuating unit and the inductor attributed to the second transmission line, the resonance frequency being within the frequency band,

wherein one of the other attenuating units is disposed closest to the output port and provides a transistor where another second transmission line is connected between the two current terminals thereof, and

wherein the transistor in the attenuating unit disposed closest to the output port has a size greater than a size of the transistor m the first attenuating unit.