JPH1010170A - Impedance measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a highly accurate measurement with high reproducibility at high frequency by eliminating a measurement error due to the capacitance of a cable being loaded on the contact resistance between the voltage measuring terminal of an impedance measuring apparatus of four terminal pair measuring method and an object to be measured and the contact resistance between the current measuring terminal and the object to be measured. SOLUTION: An Hp measuring terminal 20 is connected with a voltage measuring section through a triaxial cable 24. The intermediate conductor of the cable 24 is held at the same potential as the central conductor thereof through action of an amplifier circuit 23 in order to eliminate the effect of capacitance between the central conductor and the intermediate conductor. Similarly in the case of a Lc measuring terminal 40, the outer conductors of coaxial cables 44, 45 are held at the same potential as the central conductor thereof through action of an amplifier circuit 32 in order to eliminate the effect of capacitance between the cables 44, 45.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an impedance measuring apparatus for measuring circuit constants such as impedance and characteristics of materials in a high frequency band.

[0002]

2. Description of the Related Art FIG. 3 shows a basic configuration of a conventional four-terminal impedance measuring apparatus. A measurement current of the measurement signal source 13 is supplied from the Hc measurement terminal 10 to one terminal 61 of the measurement target 60 via the resistor 12 and the coaxial cable 11. The measurement current flows from the other terminal 62 of the measurement target 60 to the Lc measurement terminal 40, and is measured by the ammeter 42 via the coaxial cable 41.

On the other hand, the potential of the terminal 62 to be measured is applied from the Lp measuring terminal 30 to the inverting input of the Lp amplifier circuit 32 via the coaxial cable 31. The non-inverting input of the Lp amplification circuit 32 is connected to the outer conductor of the coaxial cable. The output of the Lp amplification circuit 32 controls the variable current source 43 to make the potential of the terminal 62 to be measured equal to the potential of the non-inverting input of the amplification circuit 32. That is, the potential of the terminal 62 is maintained at the potential of the outer conductor of the coaxial cable. The outer conductors of the coaxial cable are connected to each other, and the potential is the ground potential of the impedance measuring device. The ground potential is output from the ground terminal 63 to the outside.

The potential of the terminal 61 of the measuring object 60 is measured by the voltmeter 22 from the Hp measuring terminal 20 via the coaxial cable 21. Since the potential of the terminal 62 is maintained at the ground potential as described above, the voltmeter 22 measures the voltage applied to both ends of the measurement target 60. Therefore, a desired impedance measurement value can be obtained from the ratio between the voltage measurement value and the measurement value of the ammeter 42.

The four-terminal pair measurement method is an excellent measurement method that is not affected by the contact resistance of the four measurement terminals. However, when the measurement frequency increases, the capacitance and contact resistance between the central conductor and the external conductor of the coaxial cable 21 of the Hp measuring terminal 20 and the coaxial cable 41 of the Lc measuring terminal 40 cause a measurement error.

FIG. 4 shows an equivalent circuit formed by the contact resistance 27 between the terminal 61 to be measured and the Hp measuring terminal 20 and the capacitance 28 of the coaxial cable 21. Will be described. Hereinafter, the impedance of the capacitance will be referred to as “cable shunt impedance”.

When the frequency is low, the shunt impedance 28 of the coaxial cable is sufficiently high, and the reading of the voltage 22 does not drop due to the contact resistance 27 between the Hp measuring terminal 20 and the terminal 61 to be measured. However, as the frequency increases, the shunt impedance 28 of the cable decreases, and a voltage division occurs between the shunt impedance 28 and the contact resistance 27. This voltage division causes an error in the voltage measurement of the voltmeter 22.

FIG. 5 shows an equivalent circuit composed of a contact resistance 53 between the terminal 62 to be measured and the Lc measuring terminal 40 and a capacitance (shunt impedance) 54 of the coaxial cable 41, which causes an error in the current measurement. What it gives is described below. Since the contact resistance 53 generates a potential between the terminal 62 to be measured and the Lc terminal 40, a current obtained by dividing this potential difference by the shunt impedance of the cable flows through the shunt impedance 54. Since this current is superimposed on the current to be measured and flows through the ammeter 42, a measurement error occurs.

Since the shunt impedance due to the capacitance of the coaxial cable decreases as the measurement frequency increases, the influence of the contact resistance increases as the frequency increases in the conventional four-terminal pair method, and the measurement error increases. Moreover, since the contact resistance has no reproducibility, this error cannot be removed by correction.

[0010]

The capacitance of the measuring cable acts as a load on the contact resistance between the measuring terminal of the four-terminal pair of the impedance measuring device and the object to be measured, causing a measurement error. The aim of the present case is to eliminate this measurement error and to realize high-precision measurement with high reproducibility at high frequency.

[0011]

The Hp measuring terminal and the voltage measuring section are connected by a cable having a double shield structure. The operation of the amplifier circuit keeps the middle conductor of the cable at the same potential as the center conductor, and eliminates the effect of the capacitance between the center conductor and the middle conductor. The Lc measurement terminal and the current measurement unit are connected by a double shielded cable having a shield conductor outside two parallel coaxial cables. The effect of the amplifier circuit keeps the middle conductor of the cable (the outer conductor of the coaxial cable) at the same potential as the center conductor, thereby eliminating the effect of the capacitance of the cable.

[0012]

FIG. 1 shows a first embodiment of the present invention. Components having the same functions as those in FIG. 3 of the prior art are denoted by the same reference symbols. The Hp measurement terminal 20 and the voltage measurement unit are connected by a triaxial cable 24. The triaxial cable 24 is a type of a double shielded cable, and has a structure in which a mesh-like intermediate conductor is provided between a center conductor and an outer conductor of a coaxial cable.

The center conductor of the triaxial cable 24 is connected to the non-inverting input of the amplifier circuit 23, and the intermediate conductor is connected to the inverting input. Since the output is fed back to the inverting input, the inverting input and the intermediate conductor are maintained at the same potential as the center conductor. Therefore, no current flows through the capacitance between the center conductor and the intermediate conductor of the triaxial cable, and the shunt impedance between the Hp measurement terminal 20 and the ground becomes infinite in principle. The voltage measurement of the measurement target
The voltage between the intermediate conductor and the outer conductor of the triaxial cable 24 may be measured.

Between the Lc measuring terminal 40 and the current measuring unit,
They are connected by a double shielded cable composed of two coaxial cables 44 and 45 and an outer shield braid 46 surrounding the coaxial cables. The center conductors of the coaxial cables 44 and 45 are connected to the Lc measurement terminal 40. The center conductor of the coaxial cable 44 on the side of the current measuring unit is connected to the inverting input of the amplifier circuit 47, and the outer conductor is connected to the non-inverting input. The output of the amplifier circuit 47 controls the variable current source 43.
An ammeter 4 is provided between the center conductor and the outer conductor of the coaxial cable 45.
2 and the variable current source 43 are connected in series. The outer conductor of the coaxial cable 45 is connected to the Lp measurement terminal 30.
Is controlled by the amplifier circuit 32 that amplifies the voltage of

The output of the amplifier circuit 32 is a coaxial cable 45
And the variable current source 43. The potential of the Lp measurement terminal is determined by the amplification circuit 32, the outer conductor of the coaxial cable 45, the outer conductor of the coaxial cable 44, and the amplification circuit 47.
Is used to control the current source 43 via the. As a result, the center conductor and the outer conductor of the coaxial cable 31 are controlled to the same potential, and the center conductor and the outer conductor of the coaxial cables 44 and 45 are controlled to the same potential. Since there is no potential difference between the center conductor and the outer conductor of the coaxial cable 45, no current flows through the capacitance. Regardless of the potential drop due to contact resistance, coaxial cable 4
Since no current flows through the capacitance of No. 5, no extra current flows into the ammeter 42.

As described above, a means for preventing a current from flowing through the shunt impedance at the Hp measurement terminal 20 is devised, the voltage at both ends of the measurement target 60 is faithfully detected, and
c Means have been devised that can faithfully detect the current passing through the measurement target 60 irrespective of the voltage generated by the contact resistance at the measurement terminal 40.

As a second embodiment of the present invention, FIG. 2 shows an example in which impedance matching is applied to a cable. Each cable is trimmed without affecting the basic operation and effects shown in the first embodiment. The purpose of trimming is to eliminate the problem of singularities in cable resonance that occur when a quarter wavelength of the measurement frequency approaches or exceeds the cable length. The effectiveness of normal four-terminal pair trimming is described in Japanese Patent Application No. 63-167061. An example of trimming a similar triaxial cable is
See Japanese Patent Application No. 5-352012.

FIG. 2 shows an example in which the characteristic impedances of the coaxial cables are all Ro. In the triaxial cable, the characteristic impedance between the center conductor and the intermediate conductor and the characteristic impedance between the intermediate conductor and the outer conductor are each Ro. Hp measurement terminal 2
Triaxial cable 2 between 0 and voltage measuring unit
4 and a coaxial cable 21. The center conductor of the triaxial cable 24 is the Hp measuring terminal 20
Is applied to the non-inverting input of the amplifier circuit 23. The inverting input of the amplifier circuit 23 is connected to the output of the amplifier circuit 23 and the intermediate conductor of the triaxial cable.
Therefore, the potential of the middle conductor of the triaxial cable is equal to the potential of the center conductor. A resistor 25 is provided between the center conductor and the intermediate conductor of the triaxial cable 24.
Are connected for impedance matching.

As in the first embodiment, the ground of the amplifier circuit 23 is connected to the outer conductor of the triaxial cable 24, and the voltage between the intermediate conductor and the outer conductor of the triaxial cable 24 is measured by the Hp measuring terminal 20. Equal to the voltage of

The intermediate conductor on the Hp terminal side of the triaxial cable 24 is connected to the center conductor of the coaxial cable 21, and the outer conductor of the coaxial cable 21 is connected to the triaxial cable.
It is connected to the outer conductor of the cable 24. Since the resistor 26 for matching is connected to the voltage measuring unit side of the coaxial cable 21, the coaxial cable 21 is connected to the triaxial cable 24 so as to match, and the Hp measuring terminal 2 is connected.
A voltage equal to zero appears across resistor 26. This is read by the voltmeter 22.

Between the Lc measuring terminal 40 and the current measuring section,
They are connected by two triaxial cables 48 and 49. The outer shield braid 46 of the cable shown in FIG. 1 is divided into outer shields for the coaxial cables 44 and 45, respectively, to form two triaxial cables. Triaxial cable 48
And 49 are connected with resistors 51, 50 and 52 for matching. The coaxial cables 11 and 31 can be connected to the resistors 14 and 33, respectively, to obtain matching.
In FIG. 2 of the second embodiment, a resistor 2 for trimming is used.
5, 26, 33, 50, 51 and 52 and the resistance 14
Is not a trimming resistor, the configuration is equivalent to that of the first embodiment.

It is clear that the present invention can be applied even when the contact resistance described above is the contact impedance. In addition, the Hp measurement terminal and the Lc measurement terminal also have a small impedance of the lead wire at the tip of the measurement terminal and a spreading resistance in a bulk when measuring four terminals of a material such as a solid or a liquid with a probe. The influence of the shunt impedance can be eliminated. Although the embodiment of the present invention has been described above, the present invention is not limited to the illustrated style, arrangement, and the like, and a change in the configuration is allowed as needed without losing the gist of the present invention.

[0023]

According to the four-terminal pair measuring method of the present invention, it is possible to measure electric parts with good reproducibility without worrying about contact resistance between a measuring object and a measuring terminal at a high frequency, and it is useful for practical use. It is.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a second embodiment of the present invention.

FIG. 3 is a diagram showing an example of a conventional impedance measuring device.

FIG. 4 is a diagram showing the effect of contact resistance on an Hp measurement terminal.

FIG. 5 is a diagram illustrating the effect of contact resistance on an Lc measurement terminal.

[Explanation of symbols]

 10: Hc measurement terminal 11: Coaxial cable 12: Resistance 13: Signal source 14: Resistance 20: Hp measurement terminal 21: Coaxial cable 22: Voltmeter 23: Amplifier circuit 24: Triaxial cable 25: Resistance 26: Resistance 27: Contact resistance at Hp measurement terminal 28: Capacitance of coaxial cable 30: Lp measurement terminal 31: Coaxial cable 32: Amplifier 33: Resistance 40: Lc measurement terminal 41: Coaxial cable 42: Ammeter 43: Current source 44: Coaxial Cable 45: Coaxial cable 46: Outer shield braid 47: Amplifier circuit 48: Triaxial cable 49: Triaxial cable 50: Resistance 51: Resistance 52: Resistance 53: Contact resistance at Lc measurement terminal 54: Electrostatic force of coaxial cable Capacitance 60: measurement target 61: measurement target terminal 62: measurement target end 63: ground terminal

Claims (8)

[Claims] An apparatus for measuring an impedance of a measurement object by measuring a current and a voltage applied to the measurement object, comprising: a center conductor, an intermediate conductor, and an outer conductor insulated from each other; A first cable for guiding the applied voltage to voltage measuring means, and the voltage measuring means;
A second cable having a center conductor, an intermediate conductor, and an outer conductor insulated from each other, and for guiding a current applied to the object to be measured to a current measuring means; When,
And at least one of the voltage measurement circuit and the current measurement circuit includes means for maintaining an intermediate conductor of the cable included in the circuit at a potential substantially equal to a central conductor. An impedance measuring device, comprising: 2. A voltage measuring circuit comprising: a triaxial cable having one end of a center conductor connected to one end of the object to be measured; a non-inverting input connected to the other end of the center conductor of the triaxial cable; A first amplifier having an inverting input and an output connected to an intermediate conductor; and voltage measuring means connected between the intermediate conductor and the outer conductor of the triaxial cable. The first amplifier comprises: 2. The impedance measuring apparatus according to claim 1, wherein an intermediate conductor and a center conductor of the triaxial cable are controlled to have substantially the same potential. 3. A current measuring circuit comprising: first and second center conductors having one ends connected to the other ends of the object to be measured; and first and second center conductors surrounding the first and second center conductors, respectively. A cable having a second intermediate conductor and an outer conductor surrounding the first and second intermediate conductors; and a current measurement connected between the other end of the first central conductor and the first intermediate conductor. A series circuit of means and a variable current source; an inverting input connected to the other end of the second central conductor; a non-inverting input connected to the second intermediate conductor; and an output connected to an input of the variable current source. A second amplifier, a coaxial cable having one end of a center conductor connected to the other end of the object to be measured, an inverting input connected to the other end of the center conductor of the coaxial cable, and a non-inverting input connected to the outer conductor. Output is at a connection point between the variable current source and the first intermediate conductor. The third, which is continued
The impedance measuring apparatus according to claim 1, further comprising an amplifier, wherein the third amplifier controls the first intermediate conductor and the first center conductor to have substantially the same potential. 4. An H for guiding a measurement signal to one end of an object to be measured.
c cable, Hp having one end of a center conductor connected to one end of the object to be measured.
A triaxial cable, a first amplifier having a non-inverting input connected to the other end of a center conductor of the Hp triaxial cable, and an inverting input and output connected to an intermediate conductor, and an intermediate portion of the Hp triaxial cable. A voltage measuring means connected between the conductor and the outer conductor, for measuring a voltage applied to the object to be measured, first and second center conductors each having one end connected to the other end of the object to be measured, An Lc cable including first and second intermediate conductors surrounding the first and second center conductors and connected to each other, and an outer conductor surrounding the first and second intermediate conductors, respectively; A series circuit of a variable current source connected between the other end of the first intermediate conductor and the first intermediate conductor, and a current measuring unit for measuring a current applied to the measurement object; Opposite to the other end An input is connected, a non-inverting input is connected to the second intermediate conductor, an output is connected to an input of the variable current source, and one end of a center conductor is connected to the other end of the object to be measured. Lp
A coaxial cable, an inverting input is connected to the other end of the center conductor of the Lp coaxial cable, a non-inverting input is connected to the outer conductor, and an output is connected to a connection point between the variable current source and the first intermediate conductor. And a third amplifier, wherein the first amplifier controls the intermediate conductor and the center conductor of the Hp cable to have substantially the same potential, and the third amplifier controls the intermediate conductor and the first intermediate conductor of the Lc cable. An impedance measuring apparatus using a four-terminal-pair measuring method, wherein one central conductor is controlled to have substantially the same potential. 5. A triaxial cable having one end of a center conductor connected to one end of the object to be measured, a non-inverting input connected to the other end of the center conductor of the triaxial cable, A first amplifier having an inverting input and an output connected to an intermediate conductor; a coaxial cable having one end of a center conductor connected to the intermediate conductor of the triaxial cable; and a second end of the center conductor of the coaxial cable and an outer conductor. And voltage measuring means connected between the triaxial cable and the intermediate conductor and the center conductor of the triaxial cable are controlled to have substantially the same potential by the first amplifier. The impedance measuring device according to any one of the preceding claims. 6. A current measuring circuit comprising: a first terminal having one end of a center conductor connected to the other end of the object to be measured;
A triaxial cable; a series circuit of current measuring means and a variable current source connected between the other end of the center conductor of the first triaxial cable and the intermediate conductor; A second triaxial cable having one end connected to the center conductor and one end connected to the middle conductor of the first triaxial cable; and a second end connected to the other end of the center conductor of the second triaxial cable. A second amplifier having an input connected thereto, a non-inverting input connected to the intermediate conductor, and an output connected to the input of the variable current source; and a coaxial cable having one end of a center conductor connected to the other end of the object to be measured. An inverting input is connected to the other end of the center conductor of the coaxial cable, a non-inverting input is connected to the outer conductor, and the output is variable with the intermediate conductor of the first triaxial cable. A third amplifier connected to a connection point with a flow source, wherein the third amplifier controls the intermediate conductor and the center conductor of the first triaxial cable to have substantially the same potential. The impedance measuring device according to claim 1, wherein 7. An H for guiding a measurement signal to one end of an object to be measured.
c cable, Hp having one end of a center conductor connected to one end of the object to be measured.
A triaxial cable, a first amplifier having a non-inverting input connected to the other end of a center conductor of the Hp triaxial cable, and an inverting input and an output connected to an intermediate conductor, and a middle of the Hp triaxial cable. An Hp coaxial cable in which one end of a center conductor is connected to a conductor; and a voltage measurement connected between the other end of the center conductor of the Hp coaxial cable and an external conductor, for measuring a voltage applied to the object to be measured. Means, Lc having one end of a center conductor connected to the other end of the object to be measured.
A first triaxial cable, a variable current source connected between the other end of the center conductor of the Lc first triaxial cable and an intermediate conductor, and a current applied to the object to be measured. An Lc second triaxial having one end of a center conductor connected to the other end of the object to be measured, and one end of an intermediate conductor connected to an intermediate conductor of the Lc first triaxial cable. A second amplifier having an inverting input connected to the other end of the center conductor of the Lc second triaxial cable, a non-inverting input connected to the intermediate conductor, and an output connected to the input of the variable current source. Lp having one end of a center conductor connected to the other end of the object to be measured.
A coaxial cable, an inverting input connected to the other end of the center conductor of the Lp coaxial cable, a non-inverting input connected to the outer conductor, and an output connected to the intermediate conductor of the Lc first triaxial cable and the variable current source And a third amplifier connected to a connection point between the Hp triaxial cable and the intermediate conductor and the center conductor of the Hp triaxial cable are controlled to have substantially the same potential by the first amplifier. An impedance measuring apparatus using a four-terminal pair measuring method, wherein an intermediate conductor and a center conductor of the Lc first triaxial cable are controlled to have substantially the same potential. 8. The impedance measuring apparatus according to claim 5, wherein a resistor is connected at an end of the cable on the measuring means side so as to match the impedance of the cable.