JPH02285807A - Variable attenuator - Google Patents

Abstract

PURPOSE:To control the attenuation with high accuracy by providing a polygonal line function approximation circuit and a constant current receiving an output of the polygonal line function approximation circuit and using an output of the constant current circuit as a bias signal supplied to a PIN diode. CONSTITUTION:When three PIN diodes D1, D2, D3 of pi type constitution are used, the high frequency resistance of the diodes D1-D3 of constant current circuits 1, 2 acts like being stable against temperature. N sets of unit circuits UC1-UCN each provided with an operational amplifier OPA3 receiving a variable voltage in the range of +V--V to its input are connected in parallel with polygonal line function approximation circuits 3, 4. Then an output of each unit circuit connects to an output operational amplifier OPA4 via a resistor. That is, a control signal from an attenuation variable terminal is supplied via a polygonal line function approximation circuit and a constant current circuit, then it is possible to keep the constant impedance characteristic constant as the principle.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a variable attenuator, and particularly to a variable attenuator having good constant impedance characteristics.

(Jourai's technology) As a conventional constant impedance type variable attenuator, a circuit as shown in Fig. 4 is used.
O and Lll are high-frequency cutoff coils, CIO to C13 are DC current cutoff capacitors, and the current from the input terminal IN is supplied to PIN diodes DIO, Dll, and DI2. By changing the voltage applied to the control voltage terminal 100, the PIN data 41r-FDIO, Dll, DI2
4. : The flowing current changes, resistor RIO and capacitor C
The output obtained from the output terminal OUT via I2 is attenuated.

(Problem to be Solved by the Invention) However, the above-described conventional attenuation circuit has a drawback that constant impedance characteristics are not good.

In the circuit shown in FIG. 4, in order to keep the characteristic impedance constant, it is necessary to satisfy the following relationship (1).

R8=2XZO' XRx+/ (Rx+' Zo
)...(1) However, Rx+ is the high frequency resistance of the diodes Dll and D12, Rx2 is the high frequency resistance of the diode DIO, and Zo is the characteristic impedance of the circuit.

Further, the amount of attenuation when the equation (1) is satisfied is given by the following equation (2).

20XIOg+o (<Rx+10Zo), / (RXI
-Zo)) ...(2) In the conventional circuit described above, it is difficult to satisfy the relationship in equation (1) due to temperature characteristics, etc., and the problem is that highly accurate attenuation control cannot be performed. There is.

(Means for Solving the Problem) A variable attenuator according to the present invention includes a PIN diode and a bias circuit that supplies a bias signal to the PIN diode. and a constant current circuit that receives the output of the function approximation circuit, and the output of this constant current circuit is used as a bias signal to be supplied to the PIN diode.

(Example) First, the present invention will be explained with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a variable attenuator according to the present invention.

In this embodiment, three PIN diodes DI, D2 . D3 is used. Similar to the circuit in Figure 4, L1 to L4 are high frequency cutoff coils, 01 to C5 are DC current cutoff capacitors, and PIN diodes D.
If the input and output impedances of the π type attenuator consisting of 1 to D3 are the same, the current flowing through the PIN diodes installed in parallel is the same, so it is possible to use a common bias circuit to flow current through the PIN diodes. be. In the example of FIG. 1, diodes D2-D3 are driven by one common bias circuit, and the same result as when each diode is driven individually is obtained.

The constant flow passages 1 and 2 function so that the high frequency resistance of the diodes D1 to D3 becomes stable with respect to temperature. The constant current diagram Ill, 2 shows two operational amplifiers 0P as shown in FIG.
It consists of AL and 0PA2.

Function approximation diagrams #13 and 4 are constructed as shown in Figure 3,
N unit circuits UC each including an operational amplifier 0PA3 to which a variable voltage in the range of +V to -■ is input to the input side
I to UCN are connected in parallel to the inputs, and the output of each unit circuit is connected to the output operational amplifier 0PA4 via a resistor.

Generally, the high frequency resistance of a PIN diode is R=I
Q (^1..f-11, , (3) where A and B are constants specific to the diode, and ■ indicates the current flowing through the diode.

Now, in equation (3), RX=U(I), in equation (1), RX2=V (Rx+), and in equation (2), A
When T=ω(Rx1), the function approximation circuit 4 is 1 (through), and the function approximation circuit 3 is Rx2=V (Rx
+) = V (u (1) ), then VSWRl
, 0, a variable attenuator with an attenuation amount AT=ω(Rx1)=ω(U (4)) is obtained.

Also, the function approximation circuit 4 is I/ω~(U(I)), and the function approximation circuit 3 is (I/ω(U(I)))・V(U(I))
, the attenuation changes linearly with respect to the current or voltage applied to the variable attenuation terminal 100, and VSWRl
, 0 is obtained. The accuracy of the approximation of the function approximation circuit shown in FIG. 3 depends on the number N of unit circuits, and as N increases, VSWR1 approaches 0.

(Effects of the Invention) As explained above, the present invention supplies the control mid signal from the variable attenuation terminal via the function approximation circuit and the constant current circuit, so that the constant impedance characteristic is kept constant in principle. becomes possible. Further, not only can the amount of attenuation be controlled linearly with respect to the voltage or current applied to the variable terminal 6, but also there is an advantage that temperature fluctuations are small because the PIN diode is driven with a constant current.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an example of a variable attenuator according to the present invention, FIG. 2 is a diagram showing an example of the constant current circuit in FIG. 1, and FIG. 3 is an example of the polygonal line approximation circuit in FIG. The figure shown in FIG. 4 is a circuit diagram of a conventional variable attenuator. 1.2... constant current circuit, 3.4... function approximation circuit,
100...attenuation variable terminal, IN...input terminal, O
UT...output terminal, D1 to D3. DIO~D12...
・PIN diode, L1 to L4. LIO, Lll...
・High frequency cutoff coil, 01~C5, CI 1~C13
...DC current cutoff capacitor, 0PAL~0PA4
...Operation amplifier.

Claims (1)

[Claims] A variable attenuator comprising a PIN diode and a bias circuit that supplies a bias signal to the PIN diode, the variable attenuator includes a linear function approximation circuit and a constant current circuit that receives the output of the linear function approximation circuit. A variable attenuator characterized in that the output of the current circuit is used as a bias signal to be supplied to the PIN diode.