JP2001188074A - Impedance measuring method and device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an impedance measuring method and device capable of inexpensive, stable, and highly accurate impedance measurements. SOLUTION: A voltage is impressed from a signal line 1 on the series circuit of a sample impedance 2 and known impedance 4 for detecting a current flowing through the sample impedance 4. At the time of obtaining a corresponding voltage corresponding to the impressing voltage and a corresponding current corresponding to a current flowing through the sample impedance 2, a current flowing through the sample impedance 2 is changed by an impedance 3, switch 5, etc. A vector ratio between the corresponding voltage and the corresponding current obtained before and after the current change is obtained by a vector ratio measuring device 8. The value of the sample impedance 2 is computed on the basis of the obtained vector ratio. Here, the current flowing through the sample impedance 2 is changed by inserting/not inserting a known impedance 4 with a predetermined impedance value into the series circuit or parallelly inserting/not inserting it into the sample impedance 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to impedance measurement, and more particularly to an impedance measurement method and apparatus using a vector ratio measuring device.

[0002]

2. Description of the Related Art As one method for measuring impedance, a measuring method using a vector ratio measuring device has been conventionally used. FIG. 3 shows an example of a circuit configuration of a measuring apparatus that measures a test impedance Z using such a vector ratio measuring device. Here, the vector ratio measuring device calculates the ratio of two input signals including amplitude and phase information, and obtains the ratio of the input signal as an output, for example, in the form of (Acos θ + jAsin θ) or Aexp (jθ). . Here, A is an absolute value, and θ is a phase.

[0003] With reference to FIG.
The principle and operation of measuring the impedance Z of the first embodiment will be described. A signal of voltage V is applied from an AC signal source 201 to a series circuit of a test impedance 202 and a known impedance 204 having an impedance value r, and a current I flows through the series circuit. Here, the known impedance 20
4 is an impedance for current detection, usually | Z | >>
r.

A series circuit of the co-test impedance 202 and the known impedance 204 includes, for example, resistors R1 and R2.
And a signal conditioner 206 having a transfer coefficient K1 is connected in parallel. This is because the output voltage V of the AC signal source 201 is, for example, 200V or 3V.
If the voltage is a high voltage such as 00 V, if this voltage is applied to the vector ratio measuring device as it is, the measurement range will be exceeded, and a signal conditioner is required here.

The voltage V applied to the series circuit of the test impedance 202 and the known impedance 204 is adjusted to an appropriate level by the signal conditioner 206 and input to one input terminal A of the vector ratio measuring device 208. . As described above, the signal conditioner 206 functions as a voltage regulator for appropriately adjusting the input voltage to the vector ratio measuring device 208.

On the other hand, a voltage generated at both ends of the known impedance 204 is input to the other input terminal B of the vector ratio measuring device 208 through a signal conditioner 207 having a transfer coefficient K2 with the same configuration as the signal conditioner 206. You.

Now, assuming that the transfer coefficient on the input A side of the vector ratio measuring device 208 is K3 and the transfer coefficient on the input B side is K4,
The voltages Ea and Eb applied to the input terminals A and B are expressed by the following equations, respectively. Ea = V ・ K1 ・ K3 Eb = r ・ I ・ K2 ・ K4

In the vector ratio measuring device 208, the voltage Ea
And Eb are calculated and output, and this ratio is expressed as A · exp (jθ).
Then, the following equation is obtained. (VK1K3 / rIK2K4) = Aexp (j
θ) Here, as described above, A indicates the amplitude value indicated by the vector ratio measuring instrument, and θ indicates the phase value. In addition, A ·
The same applies when exp (jθ) is expressed as (Acosθ + jAsinθ).

Now, when the above equation is modified by paying attention to V / I on the left side, the following equation is obtained. V / I = A · (r · K2 · K4 / K1 · K3) exp (jθ) The above voltage / current ratio V / I is the impedance (= Z +
After all, the test impedance Z is calculated according to the following equation. Z = A · (r · K2 · K4 / K1 · K3) exp (jθ) −r In this way, the test impedance Z can be calculated by using the vector ratio measuring device.

[0010]

The conventional test impedance measuring apparatus and method as shown in FIG.
The transfer coefficients K1, K2, K3, and K4 of the signal conditioners 206 and 207 and the vector ratio measuring device 208 are included in the equation for obtaining the test impedance Z. When the input of the vector measuring device 208 exceeds an allowable range or when the signal voltage is at a level that is too small, these transfer coefficients are appropriately divided (attenuated) or amplified by the input signal. It is for adjusting to the level.

As described above, the transfer coefficients K1 and K2 of the signal conditioners 206 and 207 are included in the equation for obtaining the test impedance Z. These transfer coefficients are problematic because the frequency characteristics change in various environments. Occurs. For example, when the signal conditioner functions as a voltage divider, the frequency characteristics, that is, the transfer coefficient K1 deteriorates due to the influence of the stray capacitance. In addition, the signal conditioner 207
Functions as an amplifier, the transfer characteristic K2 deteriorates due to the characteristics of the amplifier used.

As described above, since the transfer coefficients of the two signal conditioners have errors such as frequency characteristics,
If the ratio between the input signals of the two input terminals A and B is determined, an error occurs in the ratio. This error can be compensated for the transfer coefficient K1 by connecting a capacitor in parallel to each of the resistors R1 and R2, but in this case, the number of components and the number of adjustment steps increase.

Although an amplifier having good frequency characteristics may be used for the transfer coefficient K2, such a problem arises that the cost is increased.

Further, similarly, the accuracy of the measurement is determined by the accuracy of the amplitude and frequency characteristics of the vector ratio measuring device.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and eliminates the influence on the frequency characteristics of the signal conditioner and the influence of the vector ratio measurement. It is an object of the present invention to provide an impedance measuring method and an impedance measuring apparatus capable of measuring impedance.

[0016]

In order to solve the above-mentioned problems, an impedance measuring method and apparatus according to the present invention are provided.
The following characteristic configuration is adopted.

(1) A signal is applied from a signal source to a series circuit including at least a circuit including a test impedance and a detection circuit connected in series with the test impedance and detecting a current flowing through the test impedance. An impedance measuring method for determining a corresponding voltage corresponding to a voltage and a corresponding current corresponding to a current flowing through the test impedance, and measuring a value of the test impedance based on a vector ratio of the obtained corresponding voltage and the corresponding current. And an impedance measuring method for changing a current flowing through the test impedance and calculating a value of the test impedance based on the vector ratio obtained before and after the current change.

(2) The impedance measuring method according to (1), wherein the current flowing in the test impedance is changed by inserting / non-inserting a known impedance having a predetermined impedance value into the series circuit.

(3) The impedance measuring method according to (1), wherein the insertion / non-insertion of the known impedance into / from the series circuit is performed by turning on / off a switch connected in parallel with the known impedance.

(4) The impedance measuring method according to (1), wherein the current flowing through the test impedance is changed by inserting / non-inserting a known impedance having a predetermined impedance value in parallel with the test impedance. .

(5) A method of inserting / non-inserting the known impedance in parallel with the test impedance is as follows.
The impedance measuring method according to the above (1), which is performed by turning on / off a switch connected in series to the known impedance.

(6) The impedance measuring method according to (1), wherein the corresponding voltage and the corresponding current are adjusted to predetermined levels by amplifying or attenuating the applied voltage and the current flowing through the impedance.

(7) The impedance measuring method according to (1), wherein the applied voltage is changed in conjunction with the current change to compensate for the change in the corresponding voltage caused by the internal impedance of the signal source.

(8) A signal is applied from a signal source to a series circuit including at least a circuit including a test impedance and a detection circuit connected in series with the test impedance and detecting a current flowing through the test impedance, and a voltage applied to the circuit. Means for obtaining a corresponding voltage corresponding to the current and a corresponding current corresponding to the current flowing through the test impedance, and means for obtaining a vector ratio of the obtained corresponding voltage and the corresponding current, based on the obtained vector ratio. In the impedance measuring device for calculating the value of the test impedance, means for changing a current flowing through the test impedance, and the value of the test impedance based on the vector ratio obtained before and after the current change. An impedance measuring device comprising an impedance calculating means for calculating.

(9) The impedance measuring device according to (8), wherein the means for changing the current flowing through the test impedance is means for inserting / non-inserting a known impedance having a predetermined impedance value into the series circuit. .

(10) A switch is connected in parallel with the known impedance, and the switch is turned on / off to insert the known impedance into the series circuit.
/ The impedance measuring device according to the above (8), which performs non-insertion.

(11) The means for changing the current flowing through the test impedance is means for inserting / non-inserting a known impedance having a predetermined impedance value in parallel with the test impedance. Impedance measurement device.

(12) The switch according to (8), further comprising a switch connected in series to the known impedance, and inserting / non-inserting the known impedance in parallel with the test impedance by turning on / off the switch. Impedance measurement device.

(13) The impedance measuring apparatus according to (8), further including a signal conditioner for amplifying or attenuating the applied voltage and the current flowing through the impedance to adjust the corresponding voltage and the corresponding current to predetermined levels.

(14) The impedance measuring apparatus according to the above (8), further comprising means for changing the applied voltage in conjunction with the current change to compensate for the change in the corresponding voltage caused by the internal impedance of the signal source.

(15) The impedance measuring device according to (8), wherein the current detecting means is a resistor or a current transformer.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of an impedance measuring method and apparatus according to the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a circuit diagram showing a first embodiment of the impedance measuring device according to the present invention.

In FIG. 1, the voltage V from the signal source 1 is:
It is applied to a series circuit of a test impedance 2 having an impedance Z, an impedance 3 having a known impedance value Ro, and an impedance 4 having an impedance value r, and a current I flows through this series circuit. A switch 5 for short-circuiting the impedance 3 is connected in parallel to the impedance 3.

The voltage V across the series circuit is adjusted to a predetermined voltage level by a signal conditioner 6 having the same function as that of the signal conditioner 206 in FIG. Table 8
Is input to the input terminal A.

On the other hand, the voltage (= r · I) generated at the connection point between the impedances 3 and 4 is applied to the input B of the vector ratio measuring device 8 via the signal conditioner 7 having the transfer coefficient K2. As described above, the signal conditioner 7 not only attenuates the signal, but also amplifies the signal as necessary, thereby reducing the SN ratio when the voltage (= rI) generated at the connection point is small. Can be prevented. As in FIG. 3, the transfer coefficients of the input terminals A and B of the vector ratio measuring device 8 are K3 and K3, respectively.
4.

In this embodiment, the impedance 3
Switch 5 is connected in parallel with
A major feature is that the current flowing through the test impedance Z is changed by turning off the power, and the test impedance Z is calculated by obtaining the measurement value of the vector ratio measuring device 8 corresponding thereto. Hereinafter, the operation and principle will be described in detail.

Now, considering a series circuit of the test impedance 2, impedance 3 and impedance 4, the following equation (1) is obtained using the voltage V and the current I. V / I = Z + Ro + r (1)

If the voltages applied to the input terminals A and B are Ea and Eb, respectively, the following equations (2) and (3) are obtained. Ea = V · K1 · K3 (2) Eb = r · I · K2 · K4 (3) The vector ratio measuring device 8 measures the ratio between Ea and Eb, and
Output (display) in the form of exp (jθ), the following equation is obtained. (The right side may be expressed as (Acosθ + jAsinθ).) (VK1K3 / rIK2K4) = Aexp (jθ) (4) Here, A is the vector ratio The amplitude value indicated by the measuring instrument and θ also indicate the phase value.

Now, when equation (1) is substituted for V / I on the left side, the following relational equations (5) and (6) are obtained according to whether the switch 5 is turned on or off.

(A) When the switch 5 is off (that is,
(When the impedance of the impedance 3 is Ro): (Z + Ro + r) (K1 · K3 / r · K2 · K4) = A1 · exp (jθ1) (5) where A1 and θ1 are vector ratios in this case. It is amplitude and phase which a measuring instrument shows.

(B) When the switch 5 is turned on (that is,
(When the impedance of impedance 3 is 0): (Z + r) (K1 · K3 / r · K2 · K4) = A2 · exp (jθ2) (6) where A2 and θ2 are the amplitudes indicated by the vector ratio measuring device in this case. And the phase. By dividing equation (5) by equation (6), the following equation (7) is obtained. (Z + r + Ro) / (Z + r) = (A1 / A2) ・ exp (j (θ1−θ2)) (7) When this equation (7) is solved for Z, Z = Ro / ｛(A1 / A2) · exp (j (θ1−θ2))-1｝ −r (8) is obtained.

In the equation (8), Ro, r, A1, A2,
Since θ1 and θ2 are known, the test impedance Z can be easily calculated according to the equation (8). Furthermore, since the coefficients K1 to K4 are not included in the equation (8), errors between the measurement ranges of the signal conditioners 6 and 7 and the vector ratio measuring device 8 and errors when the measurement frequency is changed are ignored. become able to.

In the present invention, the measurement is performed by switching the known impedance value. Since the known impedance value is small, the change of the measurement voltage and the current is small. No need. Therefore, since the measurement can be performed within the same range, there is also an advantage that an error between the ranges of the measuring instrument does not occur.

In the above-described embodiment, various modifications and extensions of the present invention can be considered. For example, the test impedance Z
Although the switch 5 is used to change the current flowing through the switch, any other means other than the switch may be used as long as the means enables the attachment / detachment of the impedance 3 or the insertion / short-circuiting of the series circuit. Obviously you can. Also, not limited to insertion or non-insertion of impedance, any means can be used as long as it can change the current flowing through the series circuit including the test impedance, and the means is not limited. It will be clear to those skilled in the art.

FIG. 2 is a circuit diagram showing a second embodiment of the impedance measuring device according to the present invention.

The first embodiment shown in FIG. 1 can cancel the influence of the transfer characteristic of the measuring system (signal conditioner and the like) compared with the conventional test impedance measuring apparatus and method shown in FIG. Stable and highly accurate impedance measurement is possible. To realize this, the value of the known impedance Ro in FIG.
It is necessary to make it sufficiently smaller than. Therefore, when the test impedance Z is about 1 ohm or less, the known impedance Ro needs to be about 1/100, that is, about 10 milliohm or less. However, it is practically difficult to insert / non-insert a resistor having such a low impedance value in series in consideration of the influence of the impedance of the wire. Also, as the measurement frequency increases, the inductance component of the resistor itself becomes a problem, causing an error in the measurement.

Therefore, in the second embodiment described below, an impedance measuring method and apparatus capable of performing stable and accurate impedance measurement even when the test impedance is a low impedance value of 1 ohm or less is shown.

In FIG. 2, a series circuit of a test impedance 102 having an impedance Z (a series circuit of a known impedance 103 having a value of Ro and a switch 105 is connected in parallel) and an impedance 104 having an impedance value r is formed. The voltage Vs is applied from the signal source 101, and a current I as shown flows through this circuit. The test impedance 102 is connected in parallel with the known impedance 103 by turning on the switch 105.

The voltage Vs across the test impedance 102
Is adjusted via the amplifying or attenuating function to a predetermined voltage level by the signal conditioner 106 having the same function as the signal conditioner 206 of FIG.
It is input to the input terminal A of the vector ratio measuring device 8. For convenience of explanation, the impedance value of the signal conditioner 106 is set to a value that is much larger than the impedance value of the test impedance 102. When the impedance value of the test impedance 102 is small, the signal conditioner 106 is substantially unnecessary because the voltage generated there is also low.

On the other hand, the voltage (= r · I) generated at both ends of the impedance 104 is applied to the input B of the vector ratio measuring device 108 via the signal conditioner 7 having the transfer coefficient K2. This signal conditioner 107
As described above, the voltage (= r) generated at the connection point by amplifying the signal as needed as well as attenuating the signal
-When I) is small, it is possible to prevent the SN ratio from deteriorating. As in FIG. 3, the vector ratio measuring device 108
Of the input terminals A and B are K3 and K4, respectively.

In this embodiment, the impedance 1
03 and a switch 105 in series with
05 by turning on / off
The characteristic feature is that the test impedance Z is calculated by changing the current flowing through the circuit and obtaining the measurement value of the vector ratio measuring device 108 corresponding thereto. Hereinafter, the operation and principle will be described in detail.

(A) When the switch 105 is on:
Consider the case where the switch 105 is turned on. As described above, it is assumed that the value of the signal conditioner 106 is very large as compared with the impedance under test 102. For this reason, since the impedance becomes a circuit in which the impedance 104 is connected in series to the parallel circuit of the test impedance 102 and the known impedance 103, the following equation (11) is obtained using the voltage Vs and the current I. Vs / I = 1 / {(1 / Z) + (1 / Ro)} (11) Here, the voltages applied to the input terminals A and B are Ea and Eb, respectively.
Then, the following equations (12) and (13) are obtained. Ea = VsK1K3 (12) Eb = rIK2K4 (13)

The vector ratio measuring unit 108 measures the ratio between Ea and Eb (= Ea / Eb) and calculates the ratio of the vector A.ex
Outputting (displaying) with p (jθ), the following equation is obtained. Here, A indicates the amplitude value indicated by the vector ratio measuring instrument, and θ indicates the same phase value. (The right side may be expressed as (Acosθ + jAsinθ).) Ea / Eb = (Vs / I) · (1 / r) ｛(K1 · K3) / (K2 · K4)｝ = [1 / {(1/1 / Z) + (1 / Ro)}] · (1 / r) · {(K1 · K3) / (K2 · K4)} = A1 · exp (jθ1) (14)

(B) When the switch 105 is off: Next, when the switch 105 is turned off, the following equation (15) is obtained.
Is obtained. Ea ′ / Eb ′ = (Vs ′ / I ′) · (1 / r) ｛(K1 · K3) / (K2 · K4)｝ = Z · (1 / r) · ｛(K1 · K3) / (K2) · K4)｝ = A2 · exp (jθ2) (15) By dividing equation (14) by equation (15), the following equation (16) is obtained.
Is obtained. 1 / {1+ (Z / Ro)} = (A1 / A2) ・ expj (θ2−θ1) (16) When this equation (16) is solved for Z, Z = [｛1− (A1 / A2) expj ( θ1−θ2)｝ / (A1 / A2) expj (θ1−θ2)] · Ro (17)

In the equation (17), A1, A2, θ1, θ
2. Since Ro is known, the test impedance Z can be easily calculated according to the equation (17). Equation (1)
7) does not include the coefficients K1 to K4, so that errors between the measurement ranges of the signal conditioners 106 and 107 and the vector ratio measuring device 108 and errors when the measurement frequency is changed can be ignored. . Further, equation (17)
There is no impedance r for current detection. Therefore, there is an advantage that even if there is a change in the frequency characteristic or the phase characteristic in r itself, the measurement result is not adversely affected.

The present invention also relates to a test impedance 102.
Is measured by inserting / non-inserting the known impedance 103 in parallel using the switch 105, and the known impedance value is sufficiently large with respect to the test impedance value. For this reason, since the change in the voltage between both ends of the test impedance due to ON / OFF of the switch 105 is small, there is no need to switch the range of the measured voltage and current. Therefore, since the measurement can be performed within the same range, there is no need to switch the range of the measuring instrument, and there is an advantage that no error occurs between the ranges during measurement.

Although the embodiments of the present invention have been described above, the present invention is not limited to such embodiments, and various changes and extensions can be considered.

For example, as in the first embodiment, the switch 105 is used to change the current flowing through the test impedance Z.
It is apparent that any other means can be used as long as it can be removed or inserted / short-circuited into the parallel circuit, not limited to the switch. In addition, any means can be used as long as it can change the current flowing through the circuit including the test impedance, not limited to insertion or non-insertion of impedance, and the means is not limited. It is clear to a trader.

In the above-described embodiment, an impedance having an impedance value r is used for current detection. However, it goes without saying that current detection means such as a CT (current transformer) may be used. .

Further, in the present invention, the following characteristic configuration is adopted. That is, in the embodiment shown in FIG. 1, when the switch 5 is turned on / off, the voltage between both ends of the series circuit of the test impedances 2, 3 and 4 (= terminal of the signal source) is caused by the internal impedance of the signal source 1. Voltage), it is necessary to switch the measurement range when measuring near the upper or lower limit of the measurement range.

Therefore, according to the present invention, the switch 5 is turned on / off.
The output of the signal source 1 can be changed in conjunction with turning off. Or, more generally, a means for compensating for a change in the voltage between both ends of the series circuit in conjunction with the on / off of the switch 5 may be provided. That is, it is natural that the output of the signal source 1 can be changed or a means for compensating the change in the voltage across the series circuit can be provided according to the timing of changing the current flowing through the test impedance Z. .

It should be understood that this can be similarly applied to the second embodiment shown in FIG.

[0063]

As described above, in the impedance measuring method and apparatus according to the present invention, in order to change the current flowing through the test impedance, for example, a known impedance is turned on / off by a switch and inserted / not inserted. Or equivalently, but the vector ratio when a known impedance is inserted / not inserted is measured, and the test impedance value is measured by calculation, so that the input level to the vector ratio measuring device is appropriately adjusted. The transfer coefficient of the signal conditioner does not change under the influence of the stray capacitance, and the transfer characteristic during the amplification function does not deteriorate, so that inexpensive, stable and accurate impedance measurement can be performed.

More specifically, in the first embodiment,
Since the value of the known impedance is usually small, the change in the measured value during measurement can be suppressed to a small range. Furthermore, since the range does not need to be switched at the time of impedance measurement, errors in the signal conditioner and the measuring instrument can be ignored.

In the second embodiment, since the value of the known impedance is usually sufficiently large, the change in the measured value during measurement can be suppressed to a small range. Furthermore, since the range does not need to be switched at the time of impedance measurement, errors in the signal conditioner and the measuring instrument can be ignored.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of an impedance measuring method and apparatus according to the present invention.

FIG. 2 is a circuit diagram showing a second embodiment of the impedance measuring method / apparatus according to the present invention.

FIG. 3 is a circuit diagram of a conventional impedance measuring device.

[Explanation of symbols]

 1, 101, 201 Signal source 2, 102, 202 Test impedance 3, 103, 203 Known impedance 4, 104, 204 Impedance 5, 105, 205 Switch 6, 7 Signal conditioner 106, 107 Signal conditioner 206, 207 Signal Conditioner 8, 108, 208 Vector ratio measuring instrument

Claims (15)

[Claims] A signal is applied from a signal source to a series circuit including at least a circuit including a test impedance and a detection circuit connected in series with the test impedance and detecting a current flowing through the test impedance. An impedance measuring method for determining an applied voltage corresponding to and a corresponding current corresponding to a current flowing through the test impedance, and measuring the value of the test impedance based on a vector ratio of the obtained corresponding voltage and the corresponding current, An impedance measuring method, wherein a current flowing through the test impedance is changed, and a value of the test impedance is calculated based on the vector ratio obtained before and after the current change. 2. The current flowing through the test impedance is:
The impedance measuring method according to claim 1, wherein the known impedance having a predetermined impedance value is changed by inserting / non-inserting the impedance into the series circuit. 3. The impedance measuring method according to claim 1, wherein the insertion / non-insertion of the known impedance into the series circuit is performed by turning on / off a switch connected in parallel with the known impedance. . 4. The current flowing through the test impedance is:
The impedance measuring method according to claim 1, wherein the known impedance having a predetermined impedance value is changed by inserting / non-inserting in parallel with the test impedance. 5. A method for inserting / non-inserting said known impedance in parallel with said test impedance, comprising: turning on / off a switch connected in series with said known impedance.
2. The impedance measuring method according to claim 1, wherein the measuring is performed by turning off. 6. The impedance measuring method according to claim 1, wherein the corresponding voltage and the corresponding current are adjusted to a predetermined level by amplifying or attenuating the applied voltage and the current flowing through the impedance. 7. The apparatus according to claim 1, wherein said applied voltage is changed in conjunction with said current change to compensate for a change in said corresponding voltage caused by an internal impedance of said signal source.
The impedance measurement method described in 1. 8. A signal is applied from a signal source to a series circuit including at least a circuit including a test impedance and a detection circuit connected in series with the test impedance and detecting a current flowing through the test impedance. Means for obtaining a corresponding current corresponding to the applied voltage and the current flowing through the test impedance, and means for obtaining a vector ratio between the obtained corresponding voltage and the corresponding current, based on the obtained vector ratio. In the impedance measuring apparatus for calculating the value of the test impedance, means for changing a current flowing through the test impedance, and the value of the test impedance based on the vector ratio obtained before and after the current change. An impedance measuring device comprising an impedance calculating means for calculating. 9. The apparatus according to claim 8, wherein said means for changing the current flowing through the test impedance is means for inserting / non-inserting a known impedance having a predetermined impedance value into the series circuit. The impedance measuring device according to any one of the preceding claims. 10. A system according to claim 8, further comprising a switch connected in parallel to said known impedance, and inserting / non-inserting said known impedance into said series circuit by turning on / off said switch. The impedance measuring device according to any one of the preceding claims. 11. The apparatus according to claim 11, wherein said means for changing a current flowing through said test impedance is means for inserting / non-inserting a known impedance having a predetermined impedance value in parallel with said test impedance. Item 10. The impedance measuring device according to item 8. 12. A switch connected in series with the known impedance, wherein the known impedance is inserted / not inserted in parallel with the test impedance by turning on / off the switch. Item 8
3. The impedance measuring device according to claim 1. 13. The impedance measurement according to claim 8, further comprising a signal conditioner for amplifying or attenuating the applied voltage and the current flowing through the impedance to adjust the corresponding voltage and the corresponding current to predetermined levels. apparatus. 14. An impedance according to claim 8, further comprising means for changing said applied voltage in conjunction with said current change to compensate for a change in said corresponding voltage caused by an internal impedance of a signal source. measuring device. 15. The impedance measuring apparatus according to claim 8, wherein said current detecting means is a resistor or a current transformer.