Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Magnetic saturation
Reexamination Certificate
1999-10-06
2001-11-20
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Magnetic saturation
C324S071600, C324S248000, C505S160000, C505S162000, C505S190000, C505S191000
Reexamination Certificate
active
06320369
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a superconducting current measuring circuit and a superconducting current measuring device using a superconductor and suited for measuring a current waveform with high time accuracy.
2. Description of the Related Art
There have been conventionally known some techniques for providing means for measuring the waveform of a current flowing through wiring.
FIG. 1
is a typical view showing a conventional current measuring method. If a value Z of impedance of a wiring
51
is known, a voltage V between a first end
53
and a second end
54
of the wiring
51
is measured by a semiconductor sampler
52
, which can measure a voltage with time accuracy of several pico-seconds. Using the measured voltage V, a current I is obtained according to a formula of I=V/Z. This is an ordinary method, which method will be referred to as “the first prior art” hereinafter.
There is also known a method of measuring a current waveform by means of a magnetic sensor using electromagnetic inductance, as described in the Transactions of The Institute of Electrical Engineers of Japan, Vol.117-A, No.5, May, 1997, pages 523 to 530. This method will be referred to as “the second prior art” hereinafter.
FIG. 2
is a typical view showing a measuring method according to the second prior art. In this method, the output voltage of a magnetic sensor
55
using electromagnetic induction is measured by a spectrum analyzer
56
. Next, the waveform of a current flowing through a wiring
14
on a measurement target
13
is obtained as the deconvolution of a sensor factor which is a ratio of a received magnetic field with each frequency to an output voltage.
Furthermore, there is proposed a measuring method using a superconducting loop (Japanese Patent Unexamined Application Publication No. 7-135099). This method will be referred to as “the third prior art” hereinafter.
FIG. 3
is a typical view showing a measuring method according to the third prior art. In this method, a magnetic field generated due to an ion beam (flow of charged particles=current) is detected by a superconducting loop (DC-SQUID)
59
including Josephson junctions
57
and
58
and an ion beam current is measured. In the third prior art, the voltage between both ends of the superconducting loop
59
biased with a current source
60
so as to provide a constant voltage state is measured by a voltmeter
61
. The displacement of the voltage generated when a magnetic field is applied in this state from the initial voltage value is fed back to the current source
60
and bias current is adjusted to thereby make voltage constant. The magnitude of the applied magnetic field is measured from the variation of the bias current at this time and an ion beam current value is obtained.
A superconducting sampler is described in the IEICE TRANSACTIONS ON ELECTRONICS, VOL. E80-C, No. 10, October, 1997, pages 1226 to 1232.
FIG. 4A
is a circuit diagram showing the superconducting sampler described therein. In addition, the Extended Abstract of the 45th Applied Physics Association Meeting of Japan, meeting No. 28a-Z-11, March, 1998, page I-225 describes that if the superconducting sampler mentioned in the IEICE TRANSACTIONS ON ELECTRONICS, VOL. E80-C, No. 10, October, 1997, pages 1226 to 1232 is used, it is possible to measure current waveforms with time accuracy of pico-seconds.
FIG. 4B
is a timing chart showing the operation of the superconducting sampler shown in FIG.
4
A.
First, a feedback current I
f
is supplied to the first input terminal
171
, thereafter, a measurement target current I
S
is inputted. If a trigger current I
tr
is supplied from the second input terminal
172
at certain timing, a first Josephson junction
173
is switched on. As a result, an SFQ (Single Flux Quantum) enters a first superconducting loop including the first Josephson junction
173
, a second Josephson junction
174
, a third Josephson junction
175
and a first inductance
178
. Following this, a first circulating superconducting current flows through the first superconducting loop.
In addition, an SFQ opposite in direction to the above SFQ enters a second superconducting loop including the first Josephson junction
173
and a second inductance
179
. If the critical current value of the second Josephson junction
174
is set lower than the first circulating superconducting current, the current flowing through the second Josephson junction
174
falls while rising by the switching of the second Josephson junction
174
. As a result, a pulse current I
P
occurs and flows into the third Josephson junction
175
.
The third Josephson junction
175
is referred to as a comparator junction. Since the feedback current I
f
and the measurement target current I
S
already flow through this junction, the feedback current I
f
, the measurement target current I
s
and the pulse current I
P
are added up to one another. If the sum of the three currents is equal to or higher than the critical current of the third Josephson junction
175
, the third Josephson junction
175
is switched on. As a result, a third circulating superconducting current flows through a third superconducting loop including the third Josephson junction
175
, a third inductance
180
and a coupling inductance
181
coupled with a readout SQUID (Superconducting quantum interference device). The third circulating superconducting current causes the generation of voltage between both ends of the readout SQUID including the fourth Josephson junction
176
and the fifth Josephson junction
177
. If the sum of the three currents is less than the critical current value of the third Josephson junction
175
, the junction
175
is not switched on and no voltage is generated between both ends of the readout SQUID.
If negative currents are carried as the second circulating superconducting current and the third circulating superconducting current at the end of each measurement cycle, the first Josephson junction
173
and the third Josephson junction
175
are switched and the superconducting sampler is reset.
The above operation is repeatedly conducted while changing the value of the feedback current I
f
, and the lowest feedback current I
f
with which an output voltage occurs is obtained. The obtained value is compared with the value of the lowest feedback current I
f
with which an output voltage occurs while the measurement target current I
S
is 0, thereby obtaining the value of the measurement target current I
S
at timing at which the pulse current I
P
occurs. Next, the timing for supplying a trigger current I
tr
is changed, timing at which a pulse current I
P
occurs is shifted and the same measurement operation is repeated. In this way, the waveform of the measurement target current I
S
can be measured using the superconducting sampler shown in FIG.
4
A. The above-stated measuring method will be referred to as “the fourth prior art” hereinafter.
The first prior art shown in
FIG. 1
is, however, applicable only to a case where the impedance of wiring is known. Normally, there are many cases where the impedance of wiring is not known. As for, for example, in regard to wiring of a semiconductor large scale integrated circuit (LSI), since contact holes exist and wiring structure is complicated, the inductance of the wiring is normally unknown. In this case, a current waveform cannot be measured using the semiconductor sampler.
The second prior art shown in
FIG. 2
has disadvantages in that detection sensitivity for low frequency components is low and waveform reproducibility is not satisfactory since electromagnetic induction is employed. Besides, the measurable upper limit frequency which is determined with an L/R time constant where R is the resistance of a sensor and L is an inductance, is limited to as low as 1 GHz.
The third prior art shown in
FIG. 3
has disadvantages in that it takes long time to feed back a bias current for keeping the voltage of the SQUID constant and measurement time accuracy is consider
Hidaka Mutsuo
Tahara Shuichi
Brown Glenn W.
Hamdan Wasseen H.
McGinn & Gibb PLLC
NEC Corporation
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