Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
1999-07-27
2003-06-17
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S117000, C324S600000
Reexamination Certificate
active
06581016
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a measurement of impedance and, more particularly, to an apparatus for measuring impedance and a method used therein.
DESCRIPTION OF THE RELATED ART
In the following description, term “impedance” means an absolute value of the impedance of an electric circuit, a value of the real part, a value of the imaginary part and a ratio therebetween. A typical example of the apparatus for measuring impedance is disclosed in Japanese Patent Publication of Unexamined Application No. 61-266965.
FIG. 1
illustrates the prior art apparatus for measuring impedance. Although the references are different,
FIG. 1
is corresponding to
FIG. 1
disclosed in the Japanese Patent Publication of Unexamined Application.
The prior art apparatus measures the impedance of a target circuit
100
. The target circuit
100
is assumed to be a capacitive element, and has an admittance Y expressed as Y=G+jB where G is a conductance and B is susceptance. The prior art apparatus comprises a source of alternating current
102
, a current-to-voltage converter
103
, a phase discriminator
104
, a phase shifter
105
, a phase discriminator
106
, a comparator
107
, a switching unit
108
and an analog-to-digital converter
110
. The source of alternating current
102
applies a voltage e to the target circuit, and a current ig flows out from the target circuit
100
into the current-to-voltage converter
103
. The amount of current is expressed as e(G+jB). The current-to-voltage converter
103
converts the current ig to an output voltage ey, and the output voltage ey is equal to −R×ig. The output voltage ey is applied to the phase discriminators
104
and
106
.
The phase discriminator
104
multiplies the voltage ey by the output voltage e of the source of alternating current
102
, and extracts the direct current component. The phase discriminator
104
outputs a dc voltage ea proportional to the conductance G. On the other hand, the other phase discriminator
106
multiplies the voltage ey by the output voltage of the phase shifter
105
. The phase shifter
105
is supplied with the voltage e of the source of alternating current
102
, and introduces a phase lag of 90 degrees between the voltage e and the output voltage. For this reason, the phase discriminator
106
outputs a dc voltage eb proportional to the susceptance B.
The switching unit
108
has two input nodes
109
a
and
109
b
. The dc voltage ea is applied to the input terminal
109
a
, and the other dc voltage eb is applied to the other input terminal
109
b
. The switching unit
108
selectively supplies the dc voltages ea and eb to the analog-to-digital converter
110
, and the analog-to-digital converter
110
converts the dc voltages ea/eb to a digital signal.
The prior art measuring apparatus further comprises a microcomputer
111
, an ac voltage-to-dc voltage converter
112
, an analog-to-digital converter
113
and display units
114
a
/
114
b
. The output voltage ey is supplied to the ac voltage-to-dc voltage converter
112
, and the ac voltage-to-dc voltage converter
112
produces a dc voltage from the output voltage ey. The dc voltage is proportional to absolute value of the admittance Y. The dc voltage is supplied to the analog-to-digital converter
113
, and is converted to a digital signal.
The analog-to-digital converters
110
and
113
are connected to the microcomputer
111
. The microcomputer
111
calculates the conductance G and the susceptance B on the basis of the digital signal supplied from the analog-to-digital converter
110
, and the displays
114
a
/
114
b
indicate the conductance G, the susceptance B, respectively. The microcomputer calculates the absolute value of the admittance on the basis of the digital signal supplied from the analog-to-digital converter
113
.
The comparator
107
behaves as follows. The dc voltage ea is compared with the dc voltage eb. If the admittance Y is much greater than the conductance G, i.e., Y>>G, the susceptance B is expressed as
B
={square root over ( )}(
Y
2
−G
2
)
≈Y
For this reason, the microcomputer
111
ignores the digital signal converted from the dc voltage eb, and calculates the susceptance B on the basis of the digital signal supplied from the analog-to-digital converter
113
.
On the other hand, when the admittance Y is much greater than the susceptance B, i.e., Y>>B, the conductance G is expressed as
G
={square root over ( )}(
Y
2
−B
2
)
≈Y
For this reason, the microcomputer
111
ignores the digital signal converted from the dc voltage ea, and calculates the conductance G on the basis of the digital signal supplied from the analog-to-digital converter
113
. This is because of the fact that the ac voltage-to-dc voltage converter
112
is much higher in accuracy than the phase discriminators
104
/
106
. In fact, the error introduced by the phase discriminators
104
/
106
is of the order of 0.1 to 0.2 percent. On the other hand, the error introduced by the ac voltage-to-dc voltage converter
112
is of the order of 0.01 percent. Thus, the microcomputer
111
gives the priority to the digital signal supplied from the ac voltage-to-dc voltage converter
112
through the analog-to-digital converter
113
, and enhances the accuracy.
The Japanese Patent Publication of Unexamined Application further discloses an apparatus for measuring an impedance of an inductive element. The resistance R, the reactance X and the impedance Z are measured in a similar manner to that described hereinbefore. The microcomputer also gives the priority to the digital signal converted from the dc voltage representative of the impedance Z, and calculates the resistance R or the reactance X on the basis of the digital signal under the condition of Z>>X or Z>>R.
Although the priority given to the ac voltage-to-dc voltage converter
112
fairly improves the accuracy of the measurement, i.e., either conductance or susceptance under the conditions of Y>>G or Y>>B, the measurement still contains the susceptance or the conductance calculated on the basis the digital signal converted from the dc voltage eb or ea. When the admittance Y is not much greater than the conductance G and the susceptance B, both of the conductance G and the susceptance B are calculated on the basis of the digital signals converted from the dc voltages ea and eb. Thus, the prior art apparatus still has a problem in the accuracy of the measurement. This is the first problem inherent in the prior art apparatus.
The second problem is a noise component due to the dc offset voltage in the phase discriminators
104
/
106
. The prior art apparatus is connected to various kinds of electric circuits
105
, and the phase discriminators
104
and
106
require a dc amplification for the dynamic range. The dc voltages ea and eb contain the dc offset voltage, and the dc offset voltage is transferred to the digital signal through the analog-to-digital conversion. Thus, the digital signal contains the noise component, and the noise component deteriorates the measurement. This is the second problem inherent in the prior art apparatus.
The third problem is the phase shifter
105
. Although the phase shifter
105
targets the phase lag for 90 degrees, the phase shifter can not shift the output voltage e by 90 degrees at all times. This means that the analog multiplication is not accurate.
The same problems are encountered in the prior art apparatus used for an inductive element.
SUMMARY OF THE INVENTION
It is therefore an important object of the present invention to provide an apparatus for accurately measuring an impedance.
It is also an important object of the present invention to provide a method for accurately measuring an impedance.
To accomplish the object, the present invention proposes to digitize a signal processing for determining an impedance.
In accordance with one aspect of the present invention, there is provided an apparatus for measuring an impedance of an object comprising a port connecte
Barbee Manuel L.
Hoff Marc S.
NEC Electronics Corporation
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