Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2000-04-07
2002-12-24
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C505S846000
Reexamination Certificate
active
06498483
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device and a method for adjusting a working point of a superconducting quantum interference device (hereinafter referred to as SQUID). In particular, the invention relates to a device and a method for achieving automatic adjustment of a working point of a SQUID.
2. Description of the Background Art
FIG. 23
is a circuit diagram showing a conventional control circuit for a SQUID with a flux locked loop (hereinafter referred to as FLL). Referring to
FIG. 23
, the conventional SQUID FLL circuit includes a SQUID
81
having two Josephson junctions formed at predetermined positions and a constant current source
88
supplying constant current to SQUID
81
. Magnetic flux to be measured is input from a pickup coil (not shown) to SQUID
81
. Voltage output through both ends of SQUID
81
is converted by a transformer
83
a
, amplified by a preamplifier
83
b
and then output from an output portion through a multiplier
84
and an integrator
85
. A modulation signal at 40 kHz is added by a modulating unit
87
to the output from multiplier
84
for feedback to a field application coil
82
adjacent to SQUID
81
. Accordingly, external magnetic flux detected at SQUID
81
is cancelled.
The output of SQUID
81
is input partially to the X axis of an oscilloscope
90
and the output of preamplifier
83
b
is input to the Y axis of oscilloscope
90
.
According to the method above for adjusting the working point of the SQUID by the conventional SQUID control circuit, the amplitude should be adjusted to the maximum by using oscilloscope
90
with a predetermined bias current Ib being applied to the SQUID, and consequently a problem arises of time-consuming adjustment before use. Further, since the adjustment must be made manually, automatic adjustment of the SQUID is impossible.
FIG. 24
is a circuit diagram showing a conventional non-modulation type SQUID control circuit with a flux locked loop (FLL). Referring to
FIG. 24
, the non-modulation type SQUID control circuit is basically identical to the modulation type control circuit in
FIG. 23
except that the former does not include modulating unit
87
.
In the initial setting of the non-modulation type SQUID control circuit, a magnetic field bias current is adjusted at an optimum value by watching a waveform on an oscilloscope
90
.
According to the method above for adjusting the working point of the SQUID by the conventional SQUID control circuit of non-modulation type, the field bias current applied to a field application coil
82
should be adjusted to allow the amplitude of the waveform of an output signal from the SQUID to attain the maximum by watching oscilloscope
90
with a predetermined bias current Ib applied to the SQUID, and consequently a problem arises of time-consuming adjustment before use. In addition, since the adjustment must be made manually, automatic adjustment of the SQUID is impossible.
Although the SQUID can detect an extremely weak magnetic flux when used for a magnetometer, for example, the SQUID cannot fully fulfill its function due to the influence of noise such as thermal noise caused by an amplifier employed and the like, due to stability concern of circuits such as amplifier, and the like. In order to solve this problem, a circuit according to the synchronous detection system as shown by the block diagram of
FIG. 25
has been employed.
Referring to
FIG. 25
, an output of a SQUID element
91
detecting magnetic flux is input to a lock-in amplifier
93
via a step-up transformer
92
. Lock-in amplifier
93
performs synchronized detection based on a synchronizing signal supplied from an oscillator
87
and supplies the result of detection to a DC amplifier
95
. The synchronizing signal from oscillator
87
is also superimposed on an output current from DC amplifier
95
and applied to a feedback coil
96
. Accordingly, coil
96
generates a magnetic field on which alternating field corresponding to the synchronizing signal is superimposed, this magnetic field is detected by SQUID element
91
and thus a flux locked loop is formed. DC amplifier
95
outputs a signal proportional to the flux density detected by SQUID
91
.
A heater
97
is provided in the vicinity of SQUID element
91
. Magnetic flux is trapped within a hole of SQUID element
91
when a magnetic field existing in cooling of SQUID element
91
attains a temperature equal to or less than the critical temperature of the superconductor. The trapped flux is released by increasing the temperature by heater
97
to a temperature of at least the critical current. Heater
97
is driven by a power supply circuit
98
and current is supplied to heater
97
as required.
As a conventional heater for releasing magnetic flux, wire dedicated to the heater is processed to form the heater or a resistor having a capacity corresponding to the power supplied to the heater has been employed. Therefore, a resistor having an allowable input greater than 1 W, if 1 W of input power is required for releasing the magnetic flux, or fabrication of a heater having such an allowable input is necessary.
FIG. 26
shows an outside view of a superconducting magnetic sensor using a resistor with an allowable input of 1 W. Referring to
FIG. 26
, superconducting magnetic sensor
74
includes a SQUID element
75
of about 5 mm×5 mm in size and a resistor
76
of 1 W having a length of about 10 mm that is placed adjacently to SQUID element
75
, which are provided on a chip carrier
77
with a diameter of about 30 mm. Resistor
76
occupies a large area on chip carrier
77
as shown in FIG.
26
.
The conventional heater employed must have a large allowable input, so that the SQUID cannot be made compact.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a device and a method which are easy to use for adjusting a working point of a SQUID.
Another object of the present invention is to provide a device and a method for adjusting a working point of a SQUID which improve precision of SQUID adjustment and enable automatic adjustment.
A further object of the present invention is to provide a device and a method for adjusting a working point of a non-modulation type SQUID which improve precision of SQUID adjustment and enable automatic adjustment.
A further object of the present invention is to provide an easy-to-use SQUID magnetometer capable of easily performing the entire operation including working point adjustment in a short period of time.
A further object of the present invention is to provide a compact heater for a superconducting magnetic sensor.
The objects above are accomplished by a SQUID working point adjustment device including following elements. Specifically, a SQUID working point adjustment device according to one aspect of the invention includes a SQUID, a unit for supplying a bias current to the SQUID, a unit for applying an alternating source at a predetermined frequency to the SQUID to which the bias current is being supplied to generate magnetic field, a unit for picking out from the generated field a signal corresponding to a half period, and a unit for determining an optimum value of the bias current based on the signal corresponding to change of the field.
According to another aspect of the invention, a method of adjusting a working point of a SQUID includes the steps of supplying a bias current to the SQUID, applying an alternating source at a predetermined frequency to the SQUID to which the bias current is being supplied to generate magnetic field, picking out from the generated field magnetic field corresponding to a half period, and determining an optimum value of the bias current based on an output corresponding to change of the picked out field.
The magnetic field is generated at the SQUID to which the bias current is being applied, and the optimum value of the bias current is determined based on an output corresponding to change of the field. Accordingly, the optimum value of the bias current can automatically be determined. Further,
Hirano Tetsuya
Itozaki Hideo
Nagaishi Tatsuoki
Foley & Lardner
Lefkowitz Edward
Sumitomo Electric Industries Ltd.
Zaveri Subhash
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