JP2552245B2 - SQUID magnetometer - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to SQUID (Super
rconducting QuantumInterf
The present invention relates to an SQUID magnetometer that measures a minute change in magnetic field (flux change) due to a biomagnetic signal or the like by using an erence device (superconducting quantum interference device).

[0002]

2. Description of the Related Art An SQUID magnetometer using a dc-SQUID (voltage output type SQUID) including two Josephson junctions in a superconducting ring can detect a minute magnetic field of about several tens pT. It is used for measuring biomagnetic signals such as magnetic and magnetoencephalograms, as well as for high-precision ammeters and voltmeters. The SQUID used for such magnetic measurement is characterized in that when an external magnetic flux acts on the SQUID ring, a variable output that repeats a sine waveform every quantum magnetic flux Φ0 cycle is obtained, and the linearity of this variable output is good. When the operating point is set to, the voltage output approximately proportional to the amount of change in magnetic flux can be obtained.

Then, FLL (Fax Locked Loo) utilizing such characteristics of dc-SQUID is used.
The SQUID magnetometer based on the p) method is widely known.
According to this FLL method, by negatively feeding back the SQUID output accompanying the change in the magnetic flux acting on the SQUID ring to the SQUID ring, the magnetic flux acting on the SQUID ring is fixed to a constant value, and the feedback required for fixing this magnetic flux is fixed. The amount of change in magnetic flux is measured by the amount. However, dc-
Since dΦ / dV in the Φ-V characteristic of the SQUID is a very small amount, the noise (especially 1 / F noise) of the preamplifier that preamplifies the SQUID output has a considerable effect, which improves the measurement accuracy. I can't do it. So FL
In the L-type SQUID magnetometer, a method of applying AM modulation to negative feedback is adopted to reduce 1 / F noise by a preamplifier.

As described above, the FLL requiring the AM modulation
Since the SQUID magnetometer of the type has a complicated structure, as a method of reducing the influence of preamplifier noise with a relatively simple structure, an APF (Additional Po
A method using a passive feedback circuit is known. This APF method, as shown in FIG.
The SQUID has a structure in which L series circuits are connected in parallel to the SQUID.
Dc-SQ corresponding to the magnetic flux Φex acting on the ID ring 10
The output of the UID is the positive feedback coil 11 of the RL series circuit.
The feedback is performed via (self-inductance Lf), and the Φ-V characteristic is changed by feedback based on the mutual inductance Ma between the SQUID ring 10 and the positive feedback coil 11. Note that S
When the magnetic flux fed back to the QUID ring is positive feedback, the directions of the magnetic flux Φex and the magnetic field may be matched, and when the negative feedback is used, the directions of the magnetic flux Φex and the magnetic field may be reversed. Positive feedback and negative feedback can be set by the relative relationship between the direction of current in the coil and the direction of the current in the coil and the SQUID ring.

The SQUID output with respect to changes in magnetic flux is shown in FIG.
Although it has the property that the same waveform is repeated in the unit magnetic flux Φ 0 cycle as shown in (shown by the dashed line in the figure), the positive output as described above causes the maximum output in the unit magnetic flux cycle due to the feedback magnetic flux acting. The point is displaced (indicated by the solid line in the figure). That is, dV /
An area where dΦ is large and an area where dΦ is small are generated.
Therefore, if the magnetic flux is locked by the FLL so as to set the operating point in the most linear region of the large dV / dΦ, the SQUID output change amount with respect to a minute magnetic flux change can be increased. The noise (white noise and 1 / F noise) has a negligible effect on measurement accuracy, AM modulation is not required, and the SQUID magnetometer of the FLL method has a simple structure and a high slew rate.

[0006]

[Problems to be Solved by the Invention] However, the APF
The FLL-type SQUID magnetometer using a circuit requires two feedback coils, which reduces the degree of freedom in coil arrangement and requires a high manufacturing technology. Therefore, the present invention has a high degree of freedom in coil arrangement and can be easily manufactured by a manufacturing technique conventionally used.
It is an object of the present invention to provide a FLL type SQUID magnetometer using a PF circuit.

[0007]

In order to solve the above-mentioned problems, the SQUID magnetometer according to the first aspect of the present invention is an SQUID reed meter.
Voltage output corresponding to the change in magnetic flux acting on the ring (1) is obtained.
The output of dc-SQUID is
Input minute changes in the external magnetic flux acting on the coil (2)
Input to SQUID ring via coil (9), external
SQUID magnetometer for measuring minute changes in magnetic flux
The output of the dc-SQUID to the negative feedback coil (7)
To the SQUID ring via SQ
FLL that fixes the magnetic flux acting on the UID ring to a fixed value
Negative feedback used in the system is performed, and dc-SQUID
Output is picked up via positive feedback coil (8)
To dc-SQUID magnetic flux-voltage
Do the positive feedback used in the APF method to increase the conversion rate
did.

Further, the SQUID magnetometer according to claim 2
Responds to the change in magnetic flux acting on the SQUID ring (1).
Using the output of dc-SQUID that can obtain
The external magnetic flux acting on the pickup coil (2).
A small change is sent to the SQUID via the input coil (9).
SQU to measure the minute change in external magnetic flux
In an ID magnetometer, multiple auxiliary input coils (example
For example, by connecting the first and second input coils 9a, 9b) in series
The input coil (9) is constructed, and the dc-SQUID
Output to SQUID ring via negative feedback coil (7)
Magnetic flux that feeds back and acts on the SQUID ring
When the negative feedback used in the FLL method of fixing
Both output the dc-SQUID to the input coil
SQUID link via the auxiliary input coil
Feedback to the dc-SQUID flux-electric
Positive feedback used for APF method to increase pressure conversion rate
I chose

[0009]

Therefore, according to the SQUID magnetometer of claim 1,
For example, by providing positive feedback to the pickup coil,
When performing positive feedback to the input coil with high magnetic flux transmission efficiency
Magnetic flux noise due to the APF circuit
Hard to get

According to the SQUID magnetometer of claim 2,
The auxiliary input coil that makes up the input coil.
By using a positive feedback coil, the magnetic flux noise by the APF circuit
Is difficult to be fed back.

[0011]

The SQUID magnetometer according to the present invention will be described below with reference to the accompanying drawings.

In FIG. 1, a bias current is supplied to the SQUID ring 1 from a DC power source Vb via a resistor Rb, and the external magnetic flux received by the pickup coil 2 is input to the SQUID ring 1 from the input coil 3. The voltage change generated across the SQUID ring 1 according to the input magnetic flux is preamplified by the preamplifier 4 and then the operational amplifier 5a
And an integrating amplifier 5 composed of a parallel-connected capacitor 5b
It is inverted and amplified through and is used to measure the amount of change in magnetic flux.
The output of the integrating amplifier 5 is fed back from the feedback coil 6 to the SQUID ring 1 via the negative feedback resistor Rf, and the magnetic flux lock by the FLL method is performed. Therefore,
The output of the SQUID ring 1 is supposed to be fed back from the feedback coil 6 to the SQUID ring 1 via the positive feedback resistor Ra, and by applying this positive feedback,
The magnetic flux-voltage conversion rate in the SQUID output is improved. In this embodiment, the polarities of the output of the preamplifier 4 and the output of the integrating amplifier 5 are inverted, so that the resistor Rf and the feedback coil 6 perform negative feedback, and the resistor Ra and the feedback coil 6 perform positive feedback. You can

According to the above-described SQUID magnetometer, since positive feedback can be performed by the APF circuit without separately providing a positive feedback coil, the degree of freedom in coil arrangement is high and the SQUID magnetometer is manufactured by a conventionally used manufacturing technique. The SQUID magnetometer of the FLL method using the APF circuit is possible. The reason why the APF type positive feedback is used as the negative feedback coil for the FLL without using the input coil 3 is as follows.

For example, an input coil (self-inductance Li) for inputting the external magnetic flux received by the pickup coil (self-inductance LP) to the SQUID ring.
Is connected in parallel with the SQUID ring via the resistors Ra1 and Ra2 to feed back the SQUID output from the input coil to the SQUID ring to form an APF circuit, while the SQUID output changes via the preamplifier. It is assumed that the circuit configuration is used for measurement as an output and also used for negative feedback for FLL via a resistor Rf and a negative feedback coil. Note that the resistance Ra1
And the resistance Ra2 is 1 / of the resistance Ra used in the APF circuit.
Set to 2 respectively.

Even in the virtual circuit as described above, the APF
Although a positive feedback coil for the circuit is not required separately, the noise peculiar to the APF circuit increases in this virtual circuit. That is, as the mutual inductance Ma between the positive feedback coil and the SQUID ring in the APF circuit increases, the magnetic flux noise Φa in the APF circuit.
, The minimum magnetic field resolution is reduced, and the sensitivity of the magnetometer is inevitably reduced. To explain this principle, the positive feedback ratio βa of the APF circuit is expressed by the following equation. As is clear from this equation, the positive feedback coil and the SQ
As the mutual inductance Ma with the UID ring increases, the positive feedback rate βa naturally increases. In addition, dV / dΦ
Is the magnetic flux-voltage conversion rate of SQUID when the APF circuit is not provided.

[0016]

[Equation 1]

The noise Φa of the APF circuit is expressed by the following equation, and as is clear from this equation, βa / (1
It can be seen that the magnetic flux noise Φa increases as −βa) increases (βa increases). In addition,
In the equation, Va is the resistance Ra (Ra1 + Ra) of the APF circuit.
It is the voltage noise caused by 2).

[0018]

[Equation 2]

As mentioned above, the positive feedback coil and the SQUI
If mutual inductance with D ring is small, APF
While the magnetic flux noise Φa of the circuit can be suppressed small, the efficiency of magnetic flux transmission from the input coil to the SQUID ring cannot be increased unless the mutual inductance between the input coil and the SQUID ring is increased. That is, in order to realize the input coil and the positive feedback coil for the APF circuit with a single coil,
There is no choice but to accept an increase in magnetic flux noise or a decrease in magnetic flux transmission efficiency.

On the other hand, the SQUI according to the above embodiment
In the D magnetometer, the positive feedback coil for the APF circuit is F
Since it is also used as the negative feedback coil for the LL circuit,
It is not necessary to keep the mutual inductance between the input coil 3 and the SQUID ring 1 low, and the magnetic flux transmission efficiency of the SQUID can be sufficiently increased. Further, the feedback magnetic flux from the negative feedback coil to the SQUID ring 1 does not need to take a large value (several 10 pT to 100 p).
(About T), the positive feedback ratio from the positive feedback coil to the SQUID ring 1 can be suppressed to a small value, and an SQUID magnetometer with less magnetic flux noise (higher minimum magnetic field resolution) can be obtained.

In the SQUID magnetometer of the above embodiment, a change in the external magnetic flux is detected via the pickup coil 2 and input from the input coil 3 to the SQUID ring 1. However, the present invention is not limited to this. Instead of Dire that can improve the efficiency of magnetic flux transmission
You may comprise using ctCoupling type SQUID.

FIG. 2 shows the SQUID according to the present invention.
In another embodiment of the magnetometer, the output of the integrating amplifier 5 is fed back from the negative feedback coil 7 to the SQUID ring 1 via the negative feedback resistor Rf, and the output of the SQUID ring 1 is fed from the positive feedback coil 8 to the pickup coil 2. To give feedback.

Even in such a case, since the positive feedback can be performed by the APF circuit without separately providing the positive feedback coil, the degree of freedom of the coil arrangement is high and the APF circuit can be manufactured by the conventionally used manufacturing technique. This is a FLL type SQUID magnetometer using a circuit. Further, as in the above-described embodiment, the coupling coefficient between the positive feedback coil 8 and the pickup coil 2 can be suppressed to a low level without reducing the mutual inductance between the input coil 3 and the SQUID ring 1, so that the magnetic flux transmission efficiency can be reduced. There is also an effect that a SQUID magnetometer having high magnetic flux noise and high magnetic flux noise can be obtained. The output of the preamplifier 4 may be fed back from the positive feedback coil 8 to the pickup coil 2.

FIG. 3 shows the SQUID according to the present invention.
In another embodiment of the magnetic flux meter, the output of the preamplifier 4 is fed back from the feedback coil 6 to the SQUID ring 1 via the positive feedback resistor Ra, and the SQUID output from the preamplifier 4 and the integrating amplifier 5 is fed back to the negative feedback resistor. R
Feedback is made from the positive feedback coil 6 to the SQUID ring 1 via f. If the positive feedback resistor Ra is composed of a variable resistor as in this embodiment, dV / dΦ can be changed, and it becomes easy to adjust the magnetic flux-voltage conversion rate suitable for the magnetic flux change, which is a practical effect. High S
It becomes a QUID magnetometer.

FIG. 4 shows the SQ according to the present invention.
Is another embodiment of a UID magnetometer input coil 9 made of the second input coil 9b serving sub input coil of the auxiliary input coil serving first input coil 9a and self-inductance Li2 the self-inductance Li1 connected in series
Is used, for example, the first input coil 9a of the input coil 9 is used as a positive feedback coil for the APF circuit. To do so, the first input coil 9a is connected in parallel to the SQUID ring 1 via the resistor Ra1 and the resistor Ra2. Even in such a case, since the positive feedback can be performed by the APF circuit without separately providing the positive feedback coil, the flexibility of the coil arrangement is high, and the APF circuit that can be manufactured by the manufacturing technique conventionally used is used. It becomes the SQUID magnetometer of the FLL system. Also,
Since the coupling coefficient between the first input coil 9a as the positive feedback coil and the pickup coil 2 can be suppressed to a low level without reducing the mutual inductance between the input coil 9 and the SQUID ring 1, the magnetic flux transmission efficiency is high. There is also an effect that an SQUID magnetometer with less noise can be obtained. The output of the preamplifier 4 may be fed back from the first input coil 9a to the pickup coil 2. Further, the number of coils forming the input coil 9 is not limited to two, and a larger number of input coils may be used. By thus, as the mutual inductance between the positive feedback coil and the SQUID ring has a desired value, the number of connections of the auxiliary input coil to be used for positive feedback it is also possible to variably set.

[0026]

As described above, the S according to claim 1
According to the QUID magnetometer, the positive feedback used for APF is
Magnetic flux transmission efficiency by applying to the up coil
Compared to the case where positive feedback is applied to a high input coil,
The magnetic flux noise due to the circuit is less likely to be fed back.
Therefore, the minimum magnetic field resolution is not reduced. Obey
Therefore, the APF circuit can be used without separately providing a positive feedback coil.
Since positive feedback can be performed well,
It has a high degree of freedom and is made possible by the manufacturing technology conventionally used.
FLQ type SQ using APF circuit that can be manufactured by
It becomes possible to provide a UID magnetometer.

Further, in the SQUID magnetometer according to claim 2,
According to this, the auxiliary input coil that constitutes the input coil
The magnetic flux generated by the APF circuit is
Since it is difficult for noise to be fed back, the minimum magnetic
It does not reduce the field resolution. Therefore, positive feedback carp
Good positive feedback by APF circuit without separately providing
Since there is a high degree of freedom in coil placement,
Both can be manufactured by the manufacturing technology conventionally used.
FLQ type SQUID magnetometer using APF circuit
Can be provided.

[0028]

[Brief description of drawings]

FIG. 1 SQ in which a single feedback coil performs positive feedback and negative feedback
It is a circuit diagram which shows schematic structure of a UID magnetometer.

FIG. 2 is an SQ that performs positive feedback to a pickup coil.
It is a circuit diagram which shows schematic structure of a UID magnetometer.

FIG. 3 is a circuit diagram showing a schematic configuration of an SQUID magnetometer in which positive feedback and negative feedback are performed by a single feedback coil, and an SQUID output amplified by a preamplifier is used for positive feedback.

FIG. 4 is a circuit diagram showing a schematic configuration of an SQUID magnetometer configured to perform positive feedback using a first input coil of an input coil configured by connecting a first input coil and a second input coil in series.

FIG. 5 is a circuit diagram showing an outline of an APF circuit.

FIG. 6 is a characteristic curve diagram showing a Φ-V characteristic of SQUID output.

[Explanation of symbols]

 1 SQUID ring 2 Pickup coil 3 Input coil 6 Feedback coil 7 Negative feedback coil 8 Positive feedback coil

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kunio Kazami 2-1200 Takezai Gakuendai, Inzai-cho, Inba-gun, Chiba Prefecture Inside the Superconducting Sensor Laboratory Co., Ltd. (72) Masayuki Ueda Takesei-gakuen, Inzai-cho, Inba-gun, Chiba Prefecture Taiwan 2-1200 Superconductivity Sensor Laboratory (72) Inventor Kenichi Okajima Takenishi Gakuen, Inzai-cho, Inba-gun, Chiba Taiwan 2-1200 Superconductivity Sensor Laboratory (72) Inventor Takanori Komuro Inzai-gun, Chiba Prefecture Machitake Nishi Gakuendai 2-1200 Examiner, Superconducting Sensor Laboratory Co., Ltd. Hiroshi Ogawa (56) Reference JP-A-4-232482 (JP, A) International Publication 91/18298 (WO, A) D. DRUNG, ET AL. : AP PL. PHYS. LETT. 57 (4), 23 JULY 1990, PP. 406-408

Claims (3)

(57) [Claims] 1. A magnetic flux change acting on a SQUID ring.
Use the output of dc-SQUID that can obtain the corresponding voltage output.
The external magnetic flux acting on the pickup coil.
Input to SQUID ring via input coil
SQUID magnetometer that measures the minute change in external magnetic flux
, The output of the dc-SQUID is sent to the S
SQUID ring is fed back to the QUID ring.
Used for the FLL method that fixes the magnetic flux acting on the magnet to a fixed value
The negative feedback is performed and the output of dc-SQUID
Feedback to pickup coil via return coil
To increase the flux-voltage conversion rate of dc-SQUID A
The feature is that the positive feedback used in the PF method is performed.
SQUID magnetometer. 2. The change of magnetic flux acting on the SQUID ring
Use the output of dc-SQUID that can obtain the corresponding voltage output.
The external magnetic flux acting on the pickup coil.
Input to SQUID ring via input coil
SQUID magnetometer that measures the minute change in external magnetic flux
At Connect multiple sub-input coils in series and
The dc-SQUID output as a negative feedback coil.
To the SQUID ring via SQ
FLL that fixes the magnetic flux acting on the UID ring to a fixed value
Negative feedback used in the system is performed, and dc-SQUID
The output is the auxiliary input coil that constitutes the above input coil.
Feedback to the SQUID ring via
APF method to increase the flux-voltage conversion rate of dc-SQUID
S characterized by performing positive feedback used in the equation
QUID magnetometer. 3. A dc-SQUID output to a preamplifier
Therefore, the previous stage amplification is performed and the output of the preamplifier is used for positive feedback.
According to claim 1 or claim 2, characterized in that
The SQUID magnetometer shown.