1. Field of the Invention
The present invention relates to variable attenuators and mobile communication apparatuses.
2. Description of the Related Art
In general, a mobile communication apparatus such as a portable telephone is provided with a variable attenuating unit in which a plurality of attenuators having different attenuations are switched into use in 1order to variably attenuate a high-frequency signal.
FIG. 8 shows a conventional variable attenuating unit used in the microwave band. A variable attenuating unit 70 includes an input terminal 71, an output terminal 72, field-effect transistors (hereinafter called FETs) 731 to 733 and 741 to 743 for permitting connection and disconnection from the input to output, and T-shaped resistor attenuators 751 to 753 having attenuations of A dB, B dB, and C dB, respectively. The drain electrodes D of the input-side FETs 731 to 733, are connected to the input terminal 71 through a capacitor C71, and the drain electrodes D of the output-side FETs 741 to 743, are connected to the output terminal 72 through a capacitor C72. The sources S of the FETs 731 to 733 are connected to ends of resistors R71 to R73 of the T-shaped resistor attenuators 751 to 753 through capacitors C73 to C75, respectively, and the sources S of the FETs 741 to 743 are connected to ends of resistors R74 to R76 of the T-shaped resistor attenuators 751 to 753 through capacitors C76 to C78, respectively. The other ends of the resistors R71 to R73 are connected to the other ends of the resistors R74 to R76, respectively, in the T-shaped resistor attenuators, and the connection points are grounded through resistors R77 to R79. The gate electrodes G of the FETs 731 to 733 and 741 to 743 are grounded through capacitors C79 to C84, respectively, and connected to control terminals Vc71 to Vc76 through high-frequency blocking inductors L71 to L76, respectively.
A negative voltage about equal to the pinch-off voltage of an FET to be controlled (e.g., ØV) is selectively applied to the control terminals Vc71 to Vc76. When 0 V is applied to the control terminals Vc71 and Vc74 of FETs 731 and 741, which are included in a first path, the channel resistances between the drains and sources of the FETs 731 and 741 become sufficiently lower than the characteristic impedance of the T-shaped resistor attenuator 751. When negative voltages about equal to the pinch-off voltages of the FETs 732, 742, 733, and 743 (which are included in second and third paths, respectively) are applied to the control terminals Vc72, Vc75, Vc73, and Vc76, the channel resistances between the drains and sources of the FETs 732, 742, 733, and 743 become extremely high because the depletion layers extend in the channels. As a result, a microwave input into the input terminal 71 passes through only the first path, including the T-shaped resistor attenuator 751. The second and third paths, including the T-shaped resistor attenuators 752 and 753, respectively, are in cut-off states. Therefore, attenuation between the input terminal 71 and the output terminal 72 is A dB.
To switch the attenuation between the input terminal 71 and the output terminal 72 to B dB, 0 V is applied to the control terminals Vc72 and Vc75 of FETs 732, 742 of the second path, and negative voltages about equal to the pinch-off voltages of the FETs 731, 741, 733, and 743 are applied to the control terminals Vc71, Vc74, Vc73, and Vc76 such that only the second path, including the T-shaped resistor attenuator 752, is in a pass condition. The attenuation can be switched to C dB by a similar operation. With these operations, a plurality of attenuations can be variably controlled in a discontinuous manner.
In the above conventional variable attenuating unit, however, since a plurality of attenuators having different attenuations are switched using switches, the attenuation cannot be variably controlled in a continuous manner.
In addition, since the number of FETs required for the switches is double that of the attenuations to be variably controlled, the total number of components becomes large, and the structures of the switches become complicated. Thus, the structure of the variable attenuating unit itself becomes complicated and large, and its manufacturing cost increases.
To overcome the above described problems, preferred embodiments of the present invention provide a compact variable attenuator and a compact mobile communication apparatus which allow attenuation to be variably controlled in a continuous manner.
One preferred embodiment of the present invention provides a variable attenuator comprising: a comb line comprising a first line and a second line electromagnetically coupled to each other; and a plurality of diodes connected to the first and second lines constituting the comb line; one end of each of the first and second lines being grounded; and the diodes being connected between the ground and the other ends of the first and second lines such that the anodes of the diodes are connected to the other ends of the first and second lines.
According to the above described variable attenuator, since the diodes are connected between the ground and the other ends of the first and second lines constituting the comb line, when a voltage applied to the diodes is variably controlled, the resistances of the diodes are variably controlled. As a result, the coupling degree of the first and second lines constituting the comb line is variably controlled. Therefore, the level of a high-frequency signal sent from the input port of the comb line to the output port is variably controlled. This means that the attenuation of the variable attenuator is variably controlled. In addition, the reflection loss is made to xe2x88x9213 dB or less when the voltage standing-wave ratio (VSWR) is 1.5 or less.
Since the diodes are connected between the ground and the other ends of the first and second lines constituting the comb line, the input terminal, the output terminal, and the diodes are connected to different ends of the first and second lines. Therefore, while the diodes are on or off, the impedance of the first line viewed from the input terminal and the impedance of the second line viewed from the output terminal can be made identical to the characteristic impedance of the high-frequency circuit section of a mobile communication apparatus on which the variable attenuator is mounted.
In addition, since the variable attenuator comprises the comb line and the diodes, its structure is simple. As a result, the variable attenuator can be made compact and its manufacturing cost can be reduced.
The above described variable attenuator may comprise a plurality of the comb lines. The plurality of the comb lines are connected to each other in cascade such that one end of a first line of the comb line is connected to one end of a second line of the adjacent comb line among the plurality of the comb lines.
According to the variable attenuator, since a plurality of comb lines are connected in cascade, the attenuation can be variably controlled in an extended range. Therefore, the number of components used in a mobile communication apparatus on which such a variable attenuator is mounted can be reduced. As a result, the mobile communication apparatus can be made compact.
The above described variable attenuator may further comprise a ceramic substrate formed by laminating a plurality of sheet layers made of ceramic, strip electrodes constituting the comb line being built in said ceramic substrate, and the diodes being mounted on said ceramic substrate.
According to the above described variable attenuator, since the ceramic substrate formed by laminating a plurality of sheet layers made of ceramic is provided and the ceramic substrate includes the strip electrodes constituting the comb line, a high-frequency band of 1 GHz or more can be handled due to a wavelength reduction effect of the ceramic substrate.
The foregoing object is achieved in another aspect of the present invention through the provision of a mobile communication apparatus including the above variable attenuator. According to the mobile communication apparatus, since a compact variable attenuator is used, a compact mobile communication apparatus can be implemented while a constant receiving balance of receiving sections are maintained.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.