Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – With coupling means
Reexamination Certificate
2001-01-17
2002-07-02
Sherry, Michael J. (Department: 2829)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
With coupling means
Reexamination Certificate
active
06414476
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to electrical measurements in general and in particular, to a broadband current detector and an instrument for measuring impedance and apparatus for measuring power that uses the same.
BACKGROUND OF THE INVENTION
Current measurement is a basic measurement and measurement of current flowing through floating lines is widely used. There are also many cases where, besides simply measuring current, current is measured as one of several measurements of physical and chemical quantities other than electricity including measurements of quantities related to power and impedance.
The method whereby the current to be measured is introduced through a balanced-to-unbalanced transformer (referred to below as a balun), such as transformer coupling, etc., to a current detector or voltage detector, which are unbalanced apparatuses, has been used for resultant determination of current with said current detector or voltage detector. However, ideal transformer coupling cannot be used for determinations of current within a wide frequency range of, for instance, 1 MHz to 1 GHz, so that a transmission path-type balun with relatively good frequency properties is used.
A transmission path-type balun is made, for instance, by coiling a coaxial line around a ferrite core, with one terminal pair of said coaxial line serving as the pair of input terminals and the other terminal pair serving as the pair of output terminals. The coupling coefficient of the center conductor and the sheath of the coaxial line are very close to 1 and therefore, excellent frequency properties are achieved. There is a reduction in applied voltage to the component to be measured due to self-inductance of the balun and therefore, as a means for preventing this, the coaxial line is coiled around the ferrite core in order to increase self-impedance and alleviate said reduction. For instance, an example of the use of a transmission path-type balun is given in Japanese Kokai Patent No. 9[1997]-318671.
FIG. 1
is a simplified circuit diagram of an instrument for measuring impedance that is a preferred example of using a current detector that uses this type of transmission path-type balun. Complex impedance Zx of the component to be measured
18
is determined as the vector ratio (v1/i1) of current i
1
flowing through said component
18
and voltage v
2
applied across said component
18
. Incidentally, the current flowing through capacitor
34
and the current flowing through another parasitic impedance will have an effect on i
1
, but these currents are disregarded in the description of the present invention to simplify the description. Direct-current power source
12
and alternating current power source
10
, power source resistance
14
, direct-current detection resistance
16
and component to be measured
18
are connected in-series. Current detection resistance
16
is such that the pair of input terminals of transmission path-type balun
20
represents the end. One pair of the output terminals of balun
20
is direct-current coupled with the terminal on the power source resistance
14
side of current detection resistance
16
via balun
20
and coupled to reference potential point
4
(often has ground potential) via capacitor
24
. The other pair of output terminals of balun
20
is direct-current coupled with the terminal on side of current detection resistance
16
of the component to be measured
18
via balun
20
and coupled to reference potential point
4
via capacitor
30
and resistance
32
.
Apparatus for measuring voltage
36
measures voltage V
1
that is produced between the terminals of resistance
32
by current i
1
, which has been introduced to resistance
32
via balun
20
, and determines the value of current i
1
. Moreover, the voltage v
1
between the terminals of component to be measured
18
is measured by apparatus for measuring voltage
38
via capacitor
34
and measurement V
2
is obtained. Impedance Zx=v
1
/
1
of component to be measured
18
is obtained by multiplying a predefined coefficient A by ratio V
2
/V
1
of measurements V
2
and V
1
. Power consumption in the component to be measured
18
is obtained by multiplying a predefined B by a product of V
2
and V
1
. The ratio between current i
2
to i
1
that produces voltage V
1
and i
1
must be stabilized for stability of coefficients A and B after the calibration for measurements. The reason why this stability is lost is that there are changes in the values of the component to be measured as well as fluctuations in balun properties due to changes in temperature, etc.
The ratio between currents i
1
and i
2
in
FIG. 1
is calculated by the following formula:
i
1
/
i
2
=−{
R
1
+
R
2
+
Zc
3
)/
R
2
}×
N
1
/
N
2
(Formula 1)
Here, N
1
={1+Zc
1
/(R
1
+R
2
+Zc
3
)+(Zc
1
/Z
1
)×(R
3
+Zc
3
)/(R
1
+R
2
+Zc
3
)}, N
2
={1−(Zc
1
/Z
1
)×(Zx/R
2
)} and R
1
, R
2
and R
3
are the resistance values of resistance's
14
,
16
, and
32
, respectively; Zc
1
and Zc
3
are the impedance values of capacitors
24
and
30
, respectively, and Zx and Z
1
are the impedance value of component to be measured
18
and the self-impedance value of balun
20
, respectively.
The self-inductance of the above-mentioned transmission path-type coaxial balun is dependent on the magnetic permeability of the ferrite core and therefore, is unstable with changes in temperature. Therefore, an attempt will be made to investigate the effect of the value Z
1
of self-impedance on current ratio i
1
/i
2
. The denominator in formula 1 becomes a function of impedance Zx of the object to be measured and therefore, the case where the impedance of the component to be measured is 500&OHgr; will be studied as an example. First, a capacitor and resistance are used, whose temperature coefficient of the component values less than 100 ppm/°C. can be easily obtained, and therefore, changes in the impedance of these components can be disregarded. However, the self-impedance of the balun is dependent on the magnetic permeability of the core that is used in this balun and therefore, is about 0.5%/°C. The absolute self-impedance value of the balun changes by 10% with a change in temperature of 20° C.
When typical impedance values (R
1
=R
2
=R
3
=50&OHgr;, Zc
1
=Zc
3
=−j0.5) &OHgr;, Zx=500&OHgr;, Z
1
=j100&OHgr;; (here, j is an imaginary number) are substituted in above-mentioned (formula 2), it is clear that a change of 0.5% is produced in the value of i
1
/i
2
with a change of 10% in self-inductance Z
1
of the balun. This type of change can lead directly to errors in measurements of impedance.
While, it is clear that when Zc
1
=0 (that is, when C
1
is reduced), N
1
=0 and N
2
does not =0 then and changes in the value of i
1
/i
2
are not produced with a change in self-impedance Z
1
of the balun. However, direct current cannot be applied to the component to be measured with a structure wherein Zc
1
=0.
Although the case where 500&OHgr; is the impedance Zx of component to be measured
18
was studied here, the change in i
1
/i
2
when 500&OHgr; is replaced by 50&OHgr; becomes approximately 0.1%. Thus, this amount of change in i
1
/i
2
is greatly dependent on the value of the component to be determined and measurement errors will increase therefore so-called 3-point correction may not be correctly performed. Moreover, temperature correction is also dependent on the absolute self-impedance of the balun and is not realistic.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a current detector with which alternating current can be detected with stability over a broad band, even if direct current has been added.
Another object of the present invention may be to present a high-precision instrument for measuring impedance that uses this current detector.
Yet another object of t
Agilent Technologie,s Inc.
Kurian Roshni
Sherry Michael J.
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