JP2000068776A - Variable attenuator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress occurrence of inter-modulation despite the increase of amplitude of an applied RF signal by providing a capacitance dividing circuit using a capacitance variable diode to which an input signal is applied in the opposite direction and an impedance converter which is connected to the capacitance dividing circuit and by taking out the amplitude of the input signal attenuated. SOLUTION: The capacitance variable diodes D1 and D2 of the same structure show the same junction capacitance and they have the output voltage equal to 1/2 input voltage when the control voltage Cv is set at 1/2 fixed battery voltage Ev by the variable resistor Rv. Thereby, the attenuation value is decreased or increased when the relation between the voltage Cv and the voltage Ev is defined by the resistance Rv as Cv<Ev/2 or Cv>Ev/2 respectively. As the output is taken out as voltage, the load of a low impedance can not be directly connected. Therefore, an impedance converter I-CONV employing an emitter or source follower is used. As a result, a variable attenuator supresses inter-modulation generation more effectively compared with a diode attenuator and can cope with a signal of a large amplitude.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in the configuration of a variable attenuator which is suitable for use in an antenna input circuit of a receiver and has little intermodulation or intermodulation.

[0002]

2. Description of the Related Art In a receiver for a short wave band or a very short wave band, a signal having a frequency relatively close to a target reception signal and relatively stronger than the target signal exists in many cases. In such a case, AGC (Automatic Gain Control) that controls only a normal amplifier
In the method, the AGC operates with the weaker target signal even though the stronger signal is not sufficiently suppressed by the RF (Radio Frequency) circuit alone. Since it operates, cross-modulation and inter-modulation occur. To remedy this, a resistance attenuator is inserted into the antenna input circuit, or a variable attenuator using a diode is controlled by the AGC of only the RF unit. Many diodes suitable for the variable attenuator have been developed, and the change in the internal resistance of a PIN diode or the like may be as high as 10 8 times.

[0003]

However, since the conventional variable attenuator using the diode utilizes the forward characteristics of the diode, the range of the voltage applied to the diode, that is, the range of the forward voltage is large. Since the resistance change characteristic within this voltage range exhibits an exponential change at most around 0.5 volts, it can be used only in the range where the amplitude of the applied RF signal is extremely small, otherwise the diode attenuation The device itself causes cross modulation. In view of such a current situation, an object of the present invention is to provide a variable attenuator in which cross-modulation hardly occurs even when the amplitude of an applied RF signal is increased to about 10 times the current level.

[0004]

In order to achieve the above object, a variable attenuator according to the present invention comprises a capacitance dividing circuit using a variable capacitance diode to which an input signal is applied in a reverse direction, and connecting to the capacitance dividing circuit. And an impedance converter configured to attenuate and extract an input signal amplitude.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, a conventional variable attenuator utilizes the forward characteristics of a diode. Therefore, the variable attenuator must be used with a reduced amplitude of an RF signal applied to the diode. Was. However, if the diode is used in the reverse direction and its junction capacitance is used, that is, if a variable capacitance diode is used, the applicable voltage range, that is, the reverse bias voltage range is the voltage when using in the forward direction. Since it is significantly larger than the range and can be applied up to about 10 volts, it is possible to increase the amplitude of the applicable RF signal to about 10 times. Therefore, the variable attenuator using the diode in the forward direction uses the diode as a resistance division circuit, whereas in this case, the variable capacitance diode is used as a capacitance division circuit. The variable attenuator of the present invention has a configuration in which an input signal is attenuated by a capacitance dividing circuit using a variable capacitance diode in a reverse direction and is taken out via an impedance converter.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of a variable attenuator according to an embodiment of the present invention. In FIG. 1, D1 and D2 are variable capacitance diodes, I-CONV is an impedance converter, Cv is a control voltage, Ev is a fixed battery voltage, and L1 to L4.
Is a capacitor, R1 and R2 are resistors, and Rv is a variable resistor. In the configuration shown in FIG. 1, the same variable capacitance diodes are used as D1 and D2, and the control voltage Cv is changed to Cv by Rv.
= Ev / 2, D1 and D2 exhibit the same junction capacitance, so that the output voltage is 1/2 of the input voltage and R
If Cv <Ev / 2 by v, the amount of attenuation decreases,
If Cv> Ev / 2, the amount of attenuation increases. As described above, since the output of this capacitance dividing circuit is obtained by voltage, a low-impedance load cannot be directly connected. The impedance converter I-CONV shown in FIG. 1 is used for this purpose, and an emitter follower is used.
A lower or source follower may be used, through which the load is connected to the circuit. The configuration of FIG. 1 can be expanded in various ways, and as shown in FIG. 2, the number of stages can be increased to increase the amount of attenuation. Here, D3 to D6 are variable capacitance diodes, R3 to R5 are resistors, and C5 to C6 are capacitors. As described above, the present invention requires the impedance converter I-CONV in the output stage.
Since no current flows through the element that provides attenuation, power is not consumed for a multi-stage configuration.

[0007]

As described above in detail, according to the present invention, a variable capacitance diode used with a reverse bias is used as a variable attenuating element, so that it is more mixed than a normal diode attenuator using a diode in a forward direction. Modulation hardly occurs, and a signal with a much larger amplitude can be applied.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram for explaining an embodiment of the present invention.

FIG. 2 is a circuit configuration diagram showing a modified example in which the circuit of FIG. 1 is extended to a multi-stage configuration.

[Explanation of symbols]

 D1 Variable capacitance diode D2 Variable capacitance diode Cv Control voltage Ev Fixed battery voltage I-CONV Impedance converter

Claims (1)

[Claims] 1. A capacitance dividing circuit using a variable capacitance diode to which an input signal is applied in a reverse direction, and an impedance converter connected to the capacitance dividing circuit, wherein the amplitude of the input signal is attenuated and extracted. A variable attenuator characterized in that: