JPH088675A - Variable attenuator - Google Patents

Abstract

(57) [Summary] [Objective] To provide a variable attenuator that reduces the nonlinearity of the input / output characteristics. [Configuration] A π-type variable attenuator is applied, and a PIN diode (3, 3) is provided in a series path between its input / output terminals (P ₁ , P ₂ ).
4) are connected in series with mutually opposite polarities, and a plurality of PIN diodes (D _{a1 to} D _a3 ) and (D _{b1 to} D _b3 ) are connected in series in a parallel path to the PIN diode (3, 4), and a choke coil is connected. By applying the control voltage (V _CNT ) via (5), the forward current of the PIN diode (3, 4) and the PIN diodes (D _{a1 to} D _a3 ) and (D _{b1 to} D _b3 ) can be changed. Attenuation is variably controlled by controlling their associated dynamic impedance.

Description

Detailed Description of the Invention

The present invention relates to a variable attenuator to which a PIN diode is applied as a variable resistance element, and more particularly to a variable attenuator in which the nonlinearity of input / output characteristics due to changes in attenuation is reduced.

[0002]

2. Description of the Related Art Conventionally, a variable attenuator using such a PIN diode utilizes the variable resistance characteristic of the PIN diode, in which the dynamic impedance changes according to the forward current, to attenuate the high frequency signal amplitude. The amount is variably controlled, and is disclosed in JP-A-54-31251. This variable attenuator uses two PIN diodes connected so as to have opposite polarities at high frequencies as a variable resistance element for variable signal attenuation, and cancels the non-linearity of each PIN diode to each other. The purpose is to reduce the non-linearity of the input / output characteristics due to the change in the attenuation amount.

[0003]

However, even with such a conventional variable attenuator, the second-order intermodulation distortion near the maximum attenuation and the second-order intermodulation distortion near the minimum attenuation are still not sufficiently reduced. There was a problem. This means that when the maximum attenuation amount and the minimum attenuation amount are set, the forward current of either one of the two PIN diodes connected to the opposite polarity has a small current value, so that PI
This is because the N diode has a non-linear characteristic and the non-linearity is not sufficiently canceled out.

The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to provide a variable attenuator capable of further reducing non-linearity.

[0005]

In order to achieve such an object, the present invention applies a π type circuit configuration, and
By connecting a set of PIN diodes in series with mutually opposite polarities in a series path between the input and output terminals, and by cascade-connecting a plurality of PIN diodes in a parallel path to the PIN diode, and applying a control voltage, The PIN in cascade connection with the set of PIN diodes
The attenuation is variably controlled by controlling the dynamic impedance of the diodes according to the change in the forward current.

[0006]

First, the PIN diode has the characteristic that the dynamic impedance value becomes smaller (larger) as the forward current value increases (decreases), and in particular, the forward current becomes smaller than a certain current value. In this region, the PIN diode has a characteristic that the nonlinearity becomes remarkable.

In the variable attenuator of the present invention having the above-mentioned configuration, a desired variable resistance value is not obtained by using one PIN diode, but a variable resistance value (serial Resistance value). In other words, the dynamic impedance value of one PIN diode at a certain forward current is r
_{If D} and the number of cascaded stages are k, the total dynamic impedance value (series resistance value) r _T of a plurality of PIN diodes connected in series is k × r _D. Therefore, P
As a result of using the variable resistance element formed by connecting a plurality of IN diodes in series, as a whole, the forward current of a small current is not set so that the dynamic impedance of each PIN diode becomes remarkable non-linearity. Since a desired resistance value can be obtained, it is possible to provide a variable attenuator that further reduces nonlinearity.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the variable attenuator according to the present invention will be described below with reference to the drawings. In this embodiment, several k
It utilizes the dynamic impedance of a PIN diode (sometimes referred to as high-frequency resistance or internal resistance depending on the technical literature) for AC signals of Hz or higher. For example, a suitable one applied to a high-frequency transmission system or the like is used. It will be disclosed. Therefore, in this embodiment, the dynamic impedance of the PIN diode is called a high frequency resistance.

First, the configuration of the embodiment will be described with reference to FIG. Between the signal input / output terminals P ₁ and P ₂ , the capacitance elements 1 and 2 for blocking direct current and the PINs connected to each other in opposite polarities.
The diodes 3 and 4 are connected in series. For the sake of explanation, the signal path consisting of the capacitive elements 1 and 2 and the PIN diodes 3 and 4 connected between the signal input / output terminals P ₁ and P ₂ will be called a serial path.

The anode contacts of the PIN diodes 3 and 4 commonly connected to each other are connected to the control voltage applying terminal P _c via the choke coil 5, and the control voltage applying terminal P _c is grounded via the capacitive element 6. It is connected to the contact. Further, the cathode contact of the PIN diode 3 is connected to the earth contact via the resistor 7, and the cathode contact of the PIN diode 4 is connected to the earth contact via the resistor 8.

Further, the cathode contacts of the PIN diode 3 are connected to each other in series (three in this embodiment).
Number) of PIN diodes D _a1 , D _a2 , D _a3 and a resistor 9 and a capacitive element 10 and connected to the ground contact, and
The cathode contact of the PIN diode 4 is connected to a ground contact via a plurality (three in this embodiment) of PIN diodes D _b1 , D _b2 , D _b3 connected in series with each other and a resistor 11 and a capacitive element 12. ing. As shown in the drawing, the respective PIN diodes D _a1 , D _a2 , D _a3 and D _b1 , D _b2 ,
D _b3 is connected in series so that the anode contact side is connected to the resistors 9 and 11 and the cathode contact side is connected to the cathode contacts of the PIN diodes 3 and 4.

The anode of the PIN diode D _a3 and the resistor 9
Bias setting resistors 13 and 14 are connected in series between the common connection contact with the resistor and the common connection contact with the anode of the PIN diode D _b3 and the resistor 11.
The common connection contacts of 3, 14 are the specified DC power supply voltage terminal +
It is connected to V _cc and is also connected to the ground contact via the capacitive element 15.

The capacitance values of the capacitive elements 1 and 2 are equal,
The capacitance values of the capacitive elements 10 and 12 are equal, the resistance values of the resistors 7 and 8 are equal, and the resistance values of the resistors 9 and 11 are equal,
The resistance values of the resistors 13 and 14 are set to be equal. Furthermore,
PIN diodes D _a1 , D _a2 , D _a3 and D _b1 , D _b2 , D _b3
And PIN diodes 3 and 4 have the same characteristics.

Next, the operation of the variable attenuator having such a structure will be described. Forward current I _Ba flowing through the PIN diodes D _a1 , D _a2 , D _a3 and the PIN diodes D _b1 , D _b2 , D
_The forward current I _Bb flowing through _b3 is set by the bias resistors 13 and 14; V _CNT and V _CC , both of which have the same value, and therefore have the same current value. As a result, the total high frequency resistance value (series resistance value) r _Ta generated at both ends of the PIN diodes D _a1 , D _a2 , D _a3 and the total high frequency resistance value generated at both ends of the PIN diodes D _b1 , D _b2 , D _b3 ( The series resistance value r _Tb becomes equal. On the other hand, the forward current I ₃ flowing through the PIN diode 3 and the forward current I ₃ flowing through the PIN diode 4
_{Since 4} is set by the resistors 7 and 8 and the resistors 13 and 14 and the voltages V _CNT and V _CC , which have the same value, ₄ has the same current value. As described above, this variable attenuator has the input / output terminal P
_It is a π-type variable attenuator having a symmetrical circuit configuration on the _1st side and the input / output terminal P ₂ side.

Next, the change characteristic of the attenuation amount with the voltage change of the control voltage V _CNT applied to the control voltage applying terminal P _c will be described. First, as shown as a general characteristic in FIG.
N diodes 3, 4, D _a1 , D _a2 , D _a3 , D _b1 , D _b2 ,
Each high frequency resistance value of D _b3 has a characteristic that it decreases as the forward current value increases, and conversely increases as the forward current value decreases. (See FIG. 2) In FIG. 1, the control voltage V _CNT is 0 ≦ V _CNT ≦ + V _cc
Variable control shall be performed within the voltage range of. Control voltage V _CNT
Is set to the voltage V _cc , the PIN diode D _a1 ,
Since D _a2 , D _a3 and D _b1 , D _b2 , D _b3 are in the reverse bias state, the forward currents I _Ba and I _{Bb stop} flowing, and the maximum forward current I ₃ flows to the PIN diodes 3 and 4. I ₄ flows. In this bias state, as is clear from FIG. 2, the high frequency resistance values r ₃ and r ₄ of the PIN diodes 3 and 4 are high.
Becomes smaller, PIN diodes D _a1 , D _a2 , D _a3 and D
The total high-frequency resistance values r _Ta and r _{Tb of b1} , D _b2 , and D _b3 increase. Therefore, the attenuation amount of the input / output variable attenuator becomes the minimum (minimum attenuation amount).

When the control voltage V _CNT is gradually lowered from V _CC volt, the high frequency resistance values r ₃ , r are gradually increased as the forward currents I ₃ , I ₄ of the PIN diodes ₃ , ₄ are reduced.
₄ increases, and conversely, PIN diodes D _a1 , D _a2 ,
Forward currents I _Ba and I _{Bb of} D _a3 and D _b1 , D _b2 and D _{b3 respectively.}
The total high-frequency resistance values r _Ta and r _Tb gradually decrease with the increase of. Therefore, as the control voltage V _CNT decreases,
The amount of attenuation increases.

When the AC signal is supplied to the input / output terminal P ₁ (or P ₂ ), the high frequency signal attenuated by the amount of attenuation set by the control voltage V _CNT is input / output terminal P ₂
(Or P ₁ ) occurs. For high frequency signals,
Since the capacitors ₁ , ₂ , 10, 12 are substantially short-circuited, the resistors 7, 8, 9, 11 and the PIN diodes 3, 4, D _a1 , D _a2 , D _a3 and D _b1 , D _b2 , D _b3. The amount of attenuation is determined by the voltage division of the respective resistance values. The capacitors 6 and 15 remove noise from the control voltage V _CNT and the power supply voltage + V _CC , respectively.

In this way, when the control voltage V _CNT is changed, the PIN diodes D _a1 , D _a2 , D _a3 and D _b1 , from the power supply voltage terminal + V _cc via the bias resistors 13 and 14,
The forward currents I _Ba and I _Bb supplied to D _b2 and D _b3 and the forward currents I ₃ and I ₄ supplied from the control voltage V _CNT to the PIN diodes 3 and 4 via the choke coil 5 are: Since the high frequency resistance values r ₃ and r ₄ and the total high frequency resistance values r _Ta and r _Tb also increase and decrease in the opposite relationship, the variable control of the attenuation amount is achieved.

Next, the effect of the embodiment having such a structure will be described. A plurality of PIN diodes D _a1 , D _a2 , which are connected in series, are connected to a series path composed of the capacitive elements 1 and 2 and the PIN diodes 3 and 4 connected between the signal input / output terminals P ₁ and P ₂ . Since D _a3 and D _b1 , D _b2 , D _b3 are connected so as to form a π type (hereinafter, such a connection path is referred to as a parallel path with respect to a series path), one PIN diode of such a parallel path is used. A resistance value (total high frequency resistance value) that is multiple times (three times in this embodiment) compared to the case where it is provided is obtained.

Therefore, even if the PIN diode does not use the forward current (small current) in the region that causes significant non-linearity, the forward current (relatively large current) in the region with good linearity is used. And by generating a desired high resistance by connecting this low high frequency resistance value in series,
A variable resistor having good linearity can be realized.

As shown in FIG. 3, a multi-channel oscillator 16 for supplying an input signal to be measured to the variable attenuator 17 of this embodiment and an unnecessary frequency component from the output signal of the variable attenuator 17 are used. Bandpass filter 18 for removing
The effect of this embodiment was confirmed by using an evaluation system equipped with an amplifier 19 for amplitude-amplifying the signal passed through the bandpass filter 18 and a spectrum analyzer 20 for frequency-analyzing the output signal of the amplifier 19. Here, the output signal level of the multi-channel oscillator is 90 dBμV (3
0 dBmV) and the frequency range is 55.25 MHz to 54
80 channels are arrayed at 7.25 MHz.

FIG. 4 shows the characteristics of the second-order intermodulation distortion (CSO) with respect to the amount of attenuation measured by such an evaluation system, and the signal level of the input signal is 90 dBμV (30
dBmV) and the measurement frequency is 547.25 MHz,
Furthermore, the PIN diodes D _a1 to
The number of cascade connections k of D _a3 and D _{b1 to} D _b3 is shown as a parameter.

First, the PIN diodes D _{a1 to} D _a3 and D _b1.
When the number of serially connected stages k of Db3 to _Db3 is set to one, in other words, when one PIN diode _Da1 and one PIN diode _Db1 are used, in the region where the attenuation amount is relatively small,
There is a drawback that the second intermodulation distortion is significantly deteriorated. On the other hand, when the number of serially connected stages k is set to two (2 for each)
In the case of using individual PIN diodes D _a1 , D _a2 and D _b1 , D _b2 ), such a second-order intermodulation distortion is reduced as a whole and the second-order intermodulation distortion is greatly deteriorated. Is also resolved. Further, as shown in FIG. 1, when the number of cascade connection stages k is set to 3 and three PIN diodes D _{a1 to} D _a3 and D _{b1 to} D _b3 are used, respectively, 2
Second-order intermodulation distortion is further reduced overall. Then, almost the same result could be obtained even when the input signal of the remaining frequency was used.

As described above, according to this embodiment, in the π-type variable attenuator, a plurality of PIN diodes connected in parallel are connected in cascade,
It is possible to further reduce the nonlinearity of the input / output characteristics.
By increasing the number of stages k in the cascade connection, the forward current of each PIN diode can be relatively increased, and as a result, the non-linearity can be improved. However, in order to increase the number k of the cascade connections. If this forward current is excessively increased, other technical inconveniences, such as temperature fluctuations and deterioration of the PIN diode, may occur. Therefore, the optimum number k of cascade stages should be set while considering other electrical characteristics. It is desirable to set.

Further, in this embodiment, the variable attenuator which handles a high frequency transmission signal has been described, but the present invention can be applied to the case where the variable attenuation control is applied to a signal of several kHz or more.

[0026]

As described above, in the π-type variable attenuator using the PIN diode, since a plurality of PIN diodes are cascade-connected in the parallel path,
It is possible to obtain the optimum variable high frequency resistance by setting the optimum forward current while avoiding the nonlinearity of the IN diode.
Therefore, it is possible to provide a variable attenuator in which the nonlinearity of the input / output characteristics is reduced. For example, such a variable attenuator is used in an optical transmitter, an optical receiver, a main amplifier, a branch amplifier, etc. used in a CATV system. When applied, it exhibits excellent effects.

[Brief description of drawings]

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

FIG. 2 is a characteristic diagram showing general characteristics of a PIN diode.

FIG. 3 is a block diagram showing the configuration of an evaluation system for measuring the characteristics of the variable attenuator of the example.

FIG. 4 is a characteristic diagram showing the characteristics of the variable attenuator of the example obtained by a measurement experiment.

[Explanation of symbols]

P ₁ , P ₂ ... Input / output terminals, ₁ , ₂ , ₆ , ₁₀ , ₁₂ , ₁
5 ... Capacitor, 5 ... Choke coil, 7, 8, 9, 1
1, 13, 14 ... Resistance, 3, 4, D _{a1 to} D _a3 , D _{b1 to} D
_b3 ... PIN diode.

Claims (1)

[Claims] 1. A first variable resistance element is provided in a series path between input / output terminals, and a second variable resistance element is provided in a parallel path to the series path, and the first and second variable resistance elements are provided. In a variable attenuator having a π-type circuit configuration for controlling an attenuation amount by changing a voltage division ratio of a resistance element, a PIN diode whose dynamic impedance changes with a change in forward current is used as the first resistance element. A variable attenuator, wherein a plurality of cascade-connected PIN diodes whose dynamic impedance changes with changes in other forward currents are provided as the second variable resistance element.