JP2869775B2 - SQUID magnetometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a SQUID (Superconducting Quantum Interfer) utilizing the Josephson effect.
ence Device: Superconducting quantum interference device)
It relates to an ID magnetometer. Here, the SQUID is a DC current as a bias current which is maintained in a low-temperature state in a heat insulating container (cryostat or the like) by liquid helium, liquid nitrogen, or the like, and is applied to a SQUID loop which is a superconducting loop including a Josephson junction in the loop. Drive by applying
When a magnetic flux from the outside is coupled and applied in the ID loop,
A circulating current is induced in the SQUID loop, and the quantum interference effect at a Josephson junction in the loop causes the SQ
This element measures a minute magnetic flux change such as biomagnetism by utilizing the fact that the UID loop operates as a transducer that converts a small change in the applied external magnetic flux into a large change in the output voltage.

[0002]

2. Description of the Related Art Conventionally known dc-SQUID magnetometers include a dewar (or cryostat) for storing liquid helium and a SQUID operating in liquid helium.
It consists of a probe, an amplifier and a controller operating at room temperature, and the SQUID probe in liquid helium and the amplifier at room temperature are connected by a coaxial cable. Such SSQ
The magnetic flux resolution of the ID magnetometer is 10 ^-5 φ. / Hz ^1/2 (in the left formula, φ indicates magnetic flux quantum), which is extremely high sensitivity, and is disadvantageous in that it is susceptible to external noise and induction noise. In addition, usually, a feedback circuit for a linear operation called FLL (Flux Locked Loop) is used to control the zero-order method to be established.

[0003]

However, in the above-mentioned conventional SQUID magnetometer, large-amplitude or high-speed extraneous magnetic noise that the FLL circuit does not follow cannot be obtained.
When added to the QUID or when electrical pulse noise is added to the circuit, the operation of the FLL loop is lost (referred to as "lock loss"), and the subsequent linear operation is hindered. When the magnetic flux is measured by restarting the power supply for the same reason after turning off the power supply, the SQUID φ
Because you do not know which phase on the -V characteristic to lock,
The continuity of the measured value before power off is not guaranteed.

[0004] The present invention has been made to solve the above problems, and has a dynamic characteristic of SQUID,
In particular, it is an object of the present invention to provide a SQUID magnetometer capable of improving frequency characteristics and noise resistance performance and further ensuring continuity of measurement.

[0005]

According to a first aspect of the present invention, there is provided a SQUID magnetometer for transmitting a plurality of SQUIDs to a plurality of SQUIDs and transmitting a magnetic flux to the plurality of SQUIDs. Is a SQUID magnetometer provided with a plurality of input coils different from each other and a plurality of input coils connected in series with each other, wherein the ratio of the magnetic flux transmission rates of the plurality of input coils is a ratio of relatively prime natural numbers. It is configured as represented by

[0006]

According to the present invention having the above structure, a plurality of pickups having different sensitivities or SQUIS having a magnetic flux transmission rate are provided.
Since the magnetic flux to be measured can be evaluated on different scales by ID and the phase difference can be directly detected within a range of 2π, FL
Compared to the magnetic flux detection system using L,
Even if the phase is disturbed due to the mixing of noise, it is restored at the next moment. Further, if a sufficiently large dynamic range is secured, the reproducibility of the output after the power is restored can be maintained even if the power is cut off for a long time. Therefore, once calibration can be performed in an absolute zero magnetic field environment, the conventional FLL method can be used as an absolute magnetometer that outputs an absolute value, whereas the conventional FLL method outputs only a relative value from the time of magnetic flux lock. In addition, since the phases between SQUIDs in a zero magnetic field match, even if calibration in an absolute zero magnetic field environment is not possible, this can also be used as an absolute magnetometer if the effect of the trapped magnetic flux is sufficiently small.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of the phase detection unit in the SQUID magnetometer according to the first embodiment of the present invention. As shown in the figure, this phase detection unit includes two SQUIDS1 and S2
, Two input coils L1 and L2, external input terminals Ta and Tb, and voltage terminals T1 to T3. The input coils L1 and L2 are S
They are coupled to QUIDS1 and S2 with mutual inductances M1 and M2. J11 to J22 are Josephson junctions.

FIG. 2 shows a second embodiment of the present invention.
FIG. 3 shows a configuration of a phase detection unit in the ID magnetometer, in which two SQUIDS1 and S2, two input coils L1 and L2, two positive feedback coils La1 and La2, a positive feedback resistor Ra1 and Ra2 and the external input terminal Ta
, Tb and voltage terminals T1 to T3. The input coils L1 and L2 are
DS1 and S2 are coupled by mutual inductances M1 and M2. The positive feedback coils La1 and La2 are connected to SQUIDS1 and S2 respectively by mutual inductances Ma1 and Ma2.
Are joined by J11 to J22 are Josephson junctions.

[0009] This phase detecting section is provided for each of the SQUIDS1, SQUIDs.
2 is configured to independently apply positive feedback, and the following conditional expression (dV / dφ) × (Ma1 / Ra1) ≧ 1 (1) (dV / dφ) × (Ma2 / Ra2) ≧ 1 (2) By configuring the positive feedback circuit so as to satisfy the following condition, the influence of noise of a preamplifier described later is reduced. In the above equations (1) and (2), dV / dφ
Is the flux voltage conversion rate of a single SQUID.

FIG. 3 shows a circuit configuration of a SQUID magnetometer provided with the phase detector shown in FIG. 1 and for obtaining a phase signal from these SQUIDs S1 and S2. As shown in the figure, the SQUID magnetometer 101 includes differential amplifiers 1 and 2, comparators 3 and 4, track-hold circuits 5 and 6, a subtractor 7, an attenuator 8,
A / D converter 9 as a discretizing means and D / A converter 10
, An adder 11, a bias current source 12, a signal source 13 which is a magnetic flux modulation means, and resistors 14A and 14B.

The differential amplifiers 1 and 2 amplify the output voltages of SQUIDS1 and S2. The comparators 3 and 4 detect the phase by comparing with the comparison voltage Vb. The bias current source 12 is a current source for supplying a bias current ib to the SQUIDS1 and SQUIDS1 and S2. The signal source 13
A ramp-shaped voltage (modulation signal) such as a triangular wave or a sawtooth wave is generated to modulate the two SQUIDS1 and S2. Resistance 14
A and 14B are resistors for limiting the current. The track hold circuits 5 and 6 sample the voltage of the signal 13 at the inversion timing of the comparators 3 and 4. The subtracter 7 performs a phase comparison. The attenuator 8 adjusts the value of the track hold circuit 5 to an appropriate value. The A / D converter 9 quantizes (discretizes) the phase difference output from the subtractor 7. The D / A converter 10 returns the output of the A / D converter 9 to an analog value. The adder 11 performs final phase synthesis.

Although not shown in FIG. 3, the voltage sampled by the track and hold circuit 5 is quantized by an A / D converter 9 and taken into a processor (not shown).
Even if the calculation is performed as a digital value, the same configuration and effect as described above can be obtained.

Next, the operation of the SQUID magnetometer will be described. First, the phase relationship of the SQUID magnetometer for absolute phase detection using the above configuration will be described. Now, the relationship between the output voltage and the input magnetic flux of the SQUID is sinusoidal, and the first periodic function sin
(2πφ / φ). Here, φ. Is a magnetic flux quantum. At this time, two SQUIDs that are not magnetically coupled to each other are connected to coils (for example, M1 and M2, respectively) having different coupling states (magnetic flux transmission rates).
And )), The magnetic flux is input to the SQUID.
A second periodic function having a period of (M1 -M2) is obtained. In this case, if the former (first periodic function) is considered as a vernier scale and the latter (second periodic function) is considered as a main scale, a two-step magnetic field measurement becomes possible.

When the above relationship is expressed by an equation, SQUID
The coupling inductances of the input coils L1 and L2 of S1 and SQUIDS2 are M1 and M2, the current flowing through these input coils is I, and the output voltages are V1 and V2.
Assuming that 2, if the offset is ignored, V1 and V2 become
The following equation V1 = sin (2πM1 I / φ) (3) V2 = sin (2πM2 I / φ) (4)

Therefore, the phase difference between the output voltages V1 and V2 is (2πI / φ.) × (M1−M2), and the value of I × (M1−M2) is Nφ. (N: natural number), the phase matches.

FIG. 4 shows that M 1 −M 2 = φ from the viewpoint of SQUIDS 2. / 6, and M1-M2 = φ when viewed from SQUIDS1. / 7. FIG. 5 shows the relationship between the magnetic flux φ input to the two input coils L1 and L2, the phase of each SQUID, and the phase difference from FIG.

As shown in FIG. 5, the input magnetic flux φ is
6φ for IDS2. , SQUIDS1 for 7φ. , The mutual phase difference changes by 2π, that is, by one period. Since the output of the SQUID is a periodic function, it is 1φ for only one phase. Although the magnitude or the amount of change of the input magnetic flux cannot be determined, the phase difference is set as a coarse scale and what is φ. In this example, 6φ is used as an identification standard for determining whether or not the phase is the same. From 7φ. Can be determined. Here, the natural numbers 6 and 7 are relatively prime, and the difference is 1.

According to the same principle, (M1-M2) / M1
If the numerical value is set so that is smaller, the range of the magnetic flux that can be determined can be expanded. For example, M1: M2 = 10
01: 1000, 1000φ. The dynamic range has been expanded up to that point). Where the natural number 1
000 and 1001 are disjoint and the difference is 1.

Since such a phase relationship is determined by the input current and the coupling inductance, compared with a magnetic flux detection system using the FLL, even if there is a momentary interruption of the power supply or a disturbance of the phase due to the incorporation of noise, the next instantaneous change occurs. Can be found to be repaired. Further, if a sufficiently large dynamic range is secured, the reproducibility of the output after the power is restored can be maintained even if the power is cut off for a long time. Therefore, once calibration can be performed in an absolute zero magnetic field environment, it can be used as an absolute magnetometer. Even if calibration in an absolute zero magnetic field environment is not possible, if the influence of the trapped magnetic flux is sufficiently small, the phases between the SQUIDs in the zero magnetic field coincide with each other.

Next, a phase detection circuit using the above configuration will be described. The modulation signal source 13 shown in FIG.
1φ of QUIDS1 and S2. When a current is injected with a ramp wave such as a triangular wave or a sawtooth wave having the above amplitude, if the output impedance of the signal source connected to the external input terminals Ta and Tb is sufficiently large, the phases of SQUIDS1 and S2 are linearly modulated. .

FIG. 6 shows the change over time (FIGS. 6 (B) and 6 (C)) of the output voltages of SQUIDS1 and S2 together with the change over time of the modulated magnetic flux signal (FIG. 6 (A)). S
After the outputs of QUIDS1 and S2 are amplified to appropriate levels by the differential amplifiers 1 and 2, the levels are compared by the comparators 3 and 4 with reference to the comparison voltage Vb.

Vb is equivalent to the SQUID offset voltage, and DCb omitted in the above equations (1) and (2).
By using the value corresponding to the component as the reference value, the comparator is inverted at the point where the phase change rate (dV / dφ value) is the largest. As shown in FIG. 6 (D), the track and hold circuits 5 and 6 are configured to perform sampling at the voltages A and B of the modulation signal in accordance with the inversion (here, the rising point) of the comparators 3 and 4. In this example,
When the phase of the SQUID output voltage is S1 by π / 2 and S2
Indicates a case where the phase difference is shifted by π, but the phase difference changes from 0 to 2π according to the input current I.

The voltage at the sampled points A and B is determined by each SQUI
Since the difference is proportional to the phase of D, a voltage corresponding to the phase difference can be obtained by taking the difference with the subtractor 7. This is shown in FIG. A / D converter 9 performs A / D conversion, and D / A converter 1
When D / A conversion is performed at 0, the phase difference takes a quantized value as shown in FIG. The minimum resolution of this phase difference is φ. LSB (Least Signicant Bit: Least Significant Bit) is set so that Specifically, L of the D / A converter
In order to combine the SB and the phase signal (see FIG. 7A),
The maximum voltage is 1φ by the attenuator 8. Equivalent (or 2π)
, And the values are added by the adder 11, the value of 1 LSB or less of the phase difference is interpolated as shown in FIG. From several hundred to several thousand φ. Over a wide dynamic range.

The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and has substantially the same configuration as the technical idea described in the claims of the present invention, and any device having the same function and effect can be realized by the present invention. It is included in the technical scope of the invention.

For example, in each of the above embodiments, the SQUI
Although the example in which the phase detection unit of the ID magnetometer is composed of two SQUIDs and two input coils has been described, the present invention is not limited to this, and three or more SQUIDs and three
It may be composed of more than two input coils.

In each of the above embodiments, the SQUID
Although the example in which the D magnetometer is provided with a plurality of input coils has been described, the present invention is not limited to this.
D may also serve as a pickup that directly takes in magnetic flux, and the ratio of the sensitivity of these pickups or the ratio of the effective area of these pickups may be used as the ratio of the magnetic flux transmission rate.

[0027]

As described above, according to the present invention,
With a plurality of pickups with different sensitivities or SQUIDs with magnetic flux transmissibility, the magnetic flux to be measured can be evaluated on different scales, and the phase difference can be directly detected within the range of 2π. In comparison, even if there is an instantaneous interruption of the power supply or phase disturbance due to the incorporation of noise, it is restored at the next instant. Further, if a sufficiently large dynamic range is secured, the reproducibility of the output after the power is restored can be maintained even if the power is cut off for a long time. Therefore, once calibration can be performed in an absolute zero magnetic field environment, the conventional FLL method can be used as an absolute magnetometer, while only the relative value from the time of magnetic flux lock is output. In addition, since the phases between SQUIDs in a zero magnetic field coincide with each other, even if such calibration cannot be performed, if the influence of the trapped magnetic flux is sufficiently small, this can also be used as an absolute magnetometer. Therefore, the dynamic characteristics of SQUID,
In particular, it is possible to provide a SQUID magnetometer capable of improving frequency characteristics and noise resistance performance and further ensuring measurement continuity.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a phase detector in a SQUID magnetometer according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration of a phase detection unit in a SQUID magnetometer according to a second embodiment of the present invention.

FIG. 3 is a diagram showing an overall configuration of a SQUID magnetometer according to a first embodiment of the present invention.

FIG. 4 is a diagram (1) illustrating an operation of the SQUID magnetometer shown in FIG. 3;

FIG. 5 is a diagram (2) for explaining the operation of the SQUID magnetometer shown in FIG. 3;

FIG. 6 is a diagram (3) for explaining the operation of the SQUID magnetometer shown in FIG. 3;

FIG. 7 is a diagram (4) for explaining the operation of the SQUID magnetometer shown in FIG. 3;

[Explanation of symbols]

 1, 2 Differential amplifier 3, 4 Comparator 5, 6 Track hold circuit 7 Subtractor 8 Attenuator 9 A / D converter 10 D / A converter 11 Adder 12 Bias current source 13 Signal source 14A, 14B Resistance 101 SQUID magnetic flux J11-J22 Josephson junction L1, L2 Input coil S1, S2 SQUID T1-T3 Voltage terminals

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01R 33/035 ZAA H01L 39/22 ZAA

Claims (4)

(57) [Claims] A plurality of SQUIDs; and a plurality of SQUIDs.
A SQUID magnetometer comprising a plurality of input coils transmitting a magnetic flux to an ID and having different magnetic flux transmissivities, the plurality of input coils being connected in series with each other, wherein each of the magnetic flux transmissivities of the plurality of input coils is The SQUID magnetometer is characterized in that the ratio is expressed by a ratio of disjoint natural numbers. 2. The SQUID magnetometer according to claim 1, wherein the difference between the ratios of the respective magnetic flux transmission rates is one. 3. The SQUID magnetometer according to claim 1, further comprising a resistor and a coil for applying a positive feedback to each of the plurality of SQUIDs. 4. The method according to claim 1, further comprising: applying a ramp-shaped modulation voltage to the plurality of
Magnetic flux modulating means for inputting to the ID, and the plurality of SQUIDs
A comparator that detects the phase of the voltage output of the comparator, a track-hold circuit that samples the ramp-shaped modulation voltage with the output of the comparator, and a subtractor that calculates the difference between the ramp-shaped modulation voltages held by the track-hold circuit. Discretizing means for discretizing the output subtracted by the subtracter, an amplitude adjuster for adjusting the amplitude of the sampled ramp-shaped modulation voltage, and the discretized output and the amplitude-adjusted voltage. 4. An apparatus according to claim 1, further comprising an adder for adding.
The SQUID magnetometer according to the item.