JPH0763835A - Squid fluxmeter - Google Patents

Abstract

PURPOSE:To provide an SQUID fluxmeter, which can remove the temperature fluctuating component of an element parameter with the simple circuit constitution, can perform the measurement even if the characteristics of two Josephson junctions of the SQUID ring are different, and has the excellent S/N in a white region. CONSTITUTION:A means (2, VR1, R1) supplies a rectangular-wave current bias into a DC-SQUID ring 1. A means (2, VR2, 4, 5, VR) supplies the magnetic flux signal, wherein the rectangular-wave shaped magnetic signal in synchronization with the rectangular-wave current bias signal and a DC magnetic flux signal are overlapped, into the SQUID ring 1 through a feedback coil 3. An amplifier circuit 7 amplifies the output of the SQUID ring 1. An integrator 8 integrates the amplified outputs and obtains the measured output of the magnetic flux. A circuit means (9, S) feeds back the output through the feedback coil 3. These parts are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetometer for measuring extremely weak magnetic flux, and more particularly to a magnetometer using DC-SQUID.

[0002]

2. Description of the Related Art As a magnetometer for measuring extremely weak magnetic flux, a magnetometer using a DC-SQUID is generally used. As the method of the drive / measurement circuit of the DC-SQUID magnetometer, the conventional modulation method, AC bias method, S
The HAD method and the Philips method are known.

In the modulation method, a DC bias current is applied to the DC-SQUID ring, and an AC magnetic flux is applied as a modulation signal from the AF oscillator to the SQUID ring via the modulation coil. In that state, the output of the SQUID ring is amplified, the phase detection is performed in comparison with the output of the AF oscillator, the magnetic flux measurement output is obtained through the integrating amplifier, and the output is sent through the modulation coil to the SQUID.
Feedback is provided to the ring (eg IEEE Transaction on Magnetics Vol.25, No.2March
1989, p1008 Y. Miki et al., "Low Frequency in Flux
DC SQUIDS ").

In the AC bias system, a bias current that changes positive and negative in a rectangular wave shape is supplied to the SQUID ring, and a rectangular magnetic flux signal having a frequency that is an integral multiple of the frequency of the bias current is used as a modulation signal through a modulation coil. Apply. Then, after subtracting the DC component riding on the output from the output of the SQUID ring in that state and amplifying it, the signal is synchronized with the rectangular wave-shaped magnetic flux signal applied to the SQUID ring, and the AC A special rectangular wave-shaped reference voltage signal whose phase is shifted by 180 ° is detected for each half cycle of the bias current, and this is integrated through an integrator to obtain a magnetic flux measurement output, and the output is also sent to the SQUID via a modulation coil. Adopt feedback to the ring (eg 1988 American Institute
of Phys-ics, p239 S.kuriku et al., "Effect of alte
rnating bias current on the low-frequency noise in
dc SQUID's ").

In the SHAD method, the bias current supplied to the SQUID ring is changed so as to take three values of positive, zero, and negative in a constant cycle, and the modulation current supplied to the SQUID ring is supplied via a modulation coil. The magnetic flux signal is also changed so as to take three values in synchronization with this. Then, after amplifying the output of the SQUID ring in this state, phase detection of this is performed by a rectangular wave signal having a half cycle of the bias current and the magnetic field signal for modulation, and the signal after the detection is integrated. Thus, the magnetic flux measurement output is obtained, and this output is fed back to the SQUID ring via the modulation coil (for example, Appl.
Phys. Lett. 49 (20), 17 November 1986, p1393 V. Fogli
etti et al., "Low-frequency noise in low 1 / f noise
dc SQU-ID's ").

Further, in the Phillips method, SQ
A rectangular wave bias current that changes positively and negatively at a constant cycle is supplied to the UID ring, and the magnetic flux signal for modulation also has a rectangular wave shape of the same cycle and the phase is shifted by 4 / π. From the output of the SQUID ring in this state, the DC component on the output is subtracted and amplified, and then the signal is detected by a reference signal obtained by multiplying the bias current and the magnetic flux signal for modulation, and further, The magnetic flux measurement output is obtained by integrating the signal after the detection, and this output is similarly fed back to the SQUID ring via the modulation coil (for example, IEEE T
ransaction on Magnetics Vol.27, No.2, March 1991
p2797 O.Doessel et al., "A Modular Lownoise 7-Chann
el SQUID-Magnetometer).

[0007]

By the way, DC-SQ
The following two factors can be mentioned as factors of the low frequency noise of the UID. One is the fluctuation of the magnetic flux due to the magnetic flux trap and the other is the fluctuation of the temperature of the device parameter.

Of these, the former magnetic flux fluctuation is called in-phase component fluctuation, and as shown by the broken line in FIG. 5A, the SQUID in the state where the fluctuation component shown by the solid line is not included.
With respect to the output, the φ-V (magnetic flux-output voltage) characteristics change in the same direction in the supply state of the positive and negative bias currents. Therefore, this fluctuation does not differ from the input signal magnetic flux, and its component cannot be removed by the magnetometer itself.

On the other hand, the temperature fluctuation of the latter element parameter is called an anti-phase component fluctuation, and as shown by the broken line in FIG. 7B, it is positive and negative with respect to the SQUID output containing no fluctuation component. The φ-V characteristics change in opposite directions when the bias current is supplied. The temperature fluctuation of the element parameter can be removed by considering the circuit system of the magnetometer, and the low frequency noise can be reduced.

That is, the above-mentioned conventional DC-SQUI
Among the D circuit methods, the modulation method cannot remove the temperature fluctuation of the element parameter, but the other three circuit methods can remove the temperature fluctuation of the element parameter to reduce the low frequency noise. Become.

However, among the conventional circuit systems capable of removing the temperature fluctuation of the element parameter, the AC bias system requires the frequencies of the magnetic flux signal for modulation and the bias current to be different, and a reference of a special shape is required. In addition to the drawback that the circuit configuration becomes complicated, such as the need to form a working voltage, the characteristics of the two Josephson junctions of the DC-SQUID ring differ from each other to some extent or more, and positive and negative bias currents are supplied. When the φ-V characteristic is asymmetric with respect to the φ axis in the state, there is a problem that measurement becomes impossible.

On the other hand, in the SHAD method, even if the characteristics of the two Josephson junctions are different from each other and the φ-V characteristics are asymmetrical, the SQUID output is φ-V across zero.
The median value of the magnetic flux signal for modulation is set appropriately because the values of the positive current bias side curve and the curve of the negative current bias side of the curve, which are point-symmetrical with respect to the median value of the magnetic flux signal for modulation, are repeated. However, since the bias current is zero in the 1/2 cycle, the signal reading duty becomes 1/2.
There is a drawback that the S / N in the white area is worse,
Further, since it is necessary to form such a special bias current and a magnetic flux signal for modulation, there is a drawback that the circuit configuration becomes complicated as in the former case.

In the Philips system, the circuit configuration is relatively complicated because it detects the second harmonic, and the S / N in the white region does not deteriorate unlike the SHAD system. However, even in this system, two Josephson junctions are used. When the characteristics are different and the φ-V characteristics are asymmetric, it cannot be measured.

The present invention has been made in view of the above circumstances, and can eliminate the temperature fluctuation of the element parameter based on a simple circuit configuration, and can realize the two Josephson junctions of the SQUID ring. The object of the present invention is to provide an SQUID magnetometer which can be measured even if the characteristics are different and has a good S / N ratio in the white region.

[0015]

A structure for achieving the above object will be described with reference to FIG. 1 which is an embodiment drawing, and a SQUID magnetometer of the present invention is a DC-SQUID.
A current bias source (rectangular wave oscillator 2, variable resistor VR1, resistor R1) for supplying a rectangular-wave-shaped driving current bias Ib that changes positively and negatively at a predetermined frequency with respect to the ring 1, and D
Feedback coil 3 for C-SQUID ring 1
A magnetic flux signal supply means (rectangular wave oscillator 2, variable resistor VR2, adder 4) for supplying a magnetic flux signal φm obtained by superimposing a rectangular magnetic flux signal synchronized with the current bias Ib and a predetermined DC magnetic flux signal φm ₀ via , DC power supply 5, Variable resistance VR
3), an amplifier circuit 7 that amplifies the output of the DC-SQUID ring 1, an integrator 8 that integrates the amplified output and outputs a magnetic flux detection signal, and an output of the integrator 8 that is fed back to the feedback coil 3. It is characterized in that it is provided with circuit means (switch S, voltage-current converter 9 and the like) for feeding back through.

[0016]

The .phi.-V characteristics when positive and negative current biases Ib and -Ib are supplied to the SQUID ring 1 are as shown by the solid lines in FIG. 2 and FIG. 3 in the absence of fluctuation components. Current bias is Ib and -I
It is changed at a constant period between b and the magnetic flux signal for modulation is synchronized with (φm ₀ + Δφm) and (φm ₀ −Δφm)
The SQUID output V is changed to (φm ₀ −Δφm
) And voltage values corresponding to (φm ₀ + Δφm) are alternately repeated. That is, the SQUID output V is
The values of the positive-current bias side curve and the negative-current bias side curve of the φ-V curve at point-symmetrical positions centered on the median value φm ₀ of the magnetic flux signal for modulation are repeated. As shown in (3), it responds to the external magnetic flux, and the antiphase component fluctuation due to the temperature fluctuation of the element parameter can be removed. In addition, since the SQUID output V repeats the point-symmetrical voltage value on the positive and negative current bias curves of the φ-V curve as described above, the SQUID is similar to the SHAD method.
The measurement can be performed even if the characteristics of the two Josephson junctions in the ID ring 1 do not match.

[0017]

1 is a block diagram showing a circuit configuration of an embodiment of the present invention. In the DC-SQUID ring 1 (hereinafter, simply referred to as SQUID ring 1) having two Josephson junctions 11 and 12 in the superconducting loop, an output from the rectangular wave oscillator 2 is connected to a variable resistor VR1 and a resistor R1. A rectangular wave-shaped current bias Ib converted into a current is supplied via the. The frequency of this current bias Ib is 10k.
It is desirable to set it to about Hz.

A feedback coil 3 magnetically coupled to the SQUID ring 1 is disposed near the SQUID ring 1, and a magnetic flux signal for modulation is supplied to the SQUID ring 1 via the feedback coil 3. With
The magnetic flux measurement output V ₀ described later is fed back.

The magnetic flux signal φ m for modulation is obtained by adding the DC signal generated by the DC power source 5 and the variable resistor VR3 and the rectangular wave signal generated by the rectangular wave oscillator 2 and the variable resistor VR2 by the adder 4. , And is formed by the current flowing through the feedback coil 3 through the resistor R2. Therefore, the magnetic field signal φm for this modulation is
DC magnetic flux bias φm ₀ determined by adjusting VR3,
The signal is a signal in which a rectangular wave-shaped magnetic flux signal is superimposed in synchronization with the current bias Ib and having an amplitude ± Δφm determined by adjusting VR2.

The output V of the SQUID ring 1 is the amplifier circuit 7
After being amplified by, it is integrated by the integrator 8 and becomes the magnetic flux measurement output V ₀ . This output V ₀ is converted into a current signal by the voltage-current converter 9 via the switch S, and then fed back to the SQUID ring 1 via the feedback coil 3.

Next, the adjusting method of the above embodiment of the present invention will be described. Prior to use, the current bias Ib is first determined by adjusting the variable resistor VR1, but the amplitude ± I is adjusted so that the amplitude of the φ-V curve of the SQUID ring 1 is maximized.
Establish b.

Next, the modulation magnetic flux signal is determined. In this case, in the reset state in which the current bias Ib is supplied and the switch S is opened, the DC magnetic flux bias φm ₀ is first adjusted by adjusting the variable resistor VR3. After that, the amplitude ± Δφm is determined by adjusting the variable resistor VR2. φm ₀
Is a position where the positive and negative φ-V curves are symmetrical, and the amplitude ± Δ
φm is a position where the inclination of the φ-V curve is steepest with respect to φm ₀ (see FIGS. 2, 3 and 4 (A) shown below).

After the above adjustment is completed, the switch S is closed and used in the locked state. 2 and 3 show S
Two Josephson junctions 11 and 12 on the QUID ring 1
In the case where the characteristics are matched, the change in the φ-V characteristic (A) and the time chart (B) showing the signal waveform of each part are shown together during operation with respect to the signal magnetic field and during operation with respect to the anti-phase component noise. The operation of the embodiment of the present invention will be described below with reference to these figures.

As shown in FIG. 2A, the φ-V characteristic of the SQUID ring 1 is as shown by the solid line in the state where there is no fluctuation component and no signal magnetic field, and when an external magnetic field is applied, the φ-V characteristic is obtained. The characteristics change in phase as shown by the broken line.

The output V of the SQUID ring 1 supplied with the rectangular wave current bias Ib as shown in FIG. 2B and modulated by the rectangular wave magnetic flux signal .phi.m synchronized with this is the positive current bias side output V. φ on the φ-V curve = (φ
m ₀ −Δφ m) and φ on the negative current bias side
The value with the voltage corresponding to φ = (φm ₀ + Δφm) on the −V curve is repeated. When there is no fluctuation component and no signal magnetic field, the output V of the SQUID ring 1 is V1 as shown by the solid line.
And V2 in the form of a rectangular wave, in this case, the output of the integrator 8, that is, the magnetic flux measurement output V ₀ becomes zero as shown by the solid line, but when the signal magnetic field enters, the SQU
The ID output V changes in a rectangular wave shape between V1 'and V2' as shown by the broken line, and the magnetic flux measurement output V ₀ has a value according to the input magnetic field as shown by the broken line.

On the other hand, when there is temperature fluctuation of the element parameter, the φ-V characteristic of the SQUID ring is shown in FIG.
As shown in (A), the state shown by the solid line without the fluctuation component and the signal magnetic field changes to the opposite phase as shown by the broken line.

In this case, the output of SQUID ring 1 is
As shown in FIG. 3B, the state shown by the solid line changes to the state shown by the broken line, that is, a rectangular wave signal that changes between V1 "and V2". In this state, the magnetic flux measurement output V
₀ remains zero and does not change. Therefore, the temperature fluctuation of the element parameter does not affect the measurement output, and the low frequency noise is reduced.

In the above embodiment, the point to be particularly noted is that the current bias signal and the modulating magnetic flux signal may be signals synchronized at the same frequency, and moreover, the phase detection using the reference signal is not particularly performed. By simply amplifying and integrating the output of the SQUID ring 1, a magnetic flux measurement signal can be obtained with an extremely simple circuit configuration, and low-frequency noise caused by temperature fluctuation of element parameters can be reliably removed. This is a point that can be done. Further, according to the configuration of the embodiment of the present invention, as shown below, the SQUID
The magnetic flux can be measured even when the two Josephson junctions in the ring 1 have different characteristics.

If the characteristics of the two Josephson junctions 11 and 12 in the SQUID ring 1 do not match, φ
As for the −V characteristic, as illustrated in FIG. 4A, the curve when positive and negative current bias is supplied is asymmetric with respect to the φ axis. In the embodiment of the present invention, the output of the SQUID ring 1 is the voltage corresponding to φ = (φm ₀ −Δφm) on the φ-V curve on the positive current bias side and φ− on the negative current bias side, as described above. The value corresponding to φ = (φm ₀ + Δφm) on the V curve is repeated, and each voltage value has a fluctuation component and a signal with φ = φm ₀ on the φ axis as the center point. It is the voltage value at the point-symmetrical position of each φ-V curve when positive and negative current bias is supplied in the absence of a magnetic field. Here, as shown in FIG. 4 (A), the φ-V characteristic of the SQUID ring has a positive / negative curve that is generally point-symmetric with respect to any point on the φ-axis. By setting the magnitude of the DC magnetic flux bias so that the modulation magnetic flux signal has a median value φ _m0 , the signal magnetic field can be measured as shown by the broken line in FIG. 4B. If there is a temperature fluctuation of the element parameter, the influence thereof can be canceled in the same manner as in the case of FIG. 3 described above although not shown.

[0030]

As described above, according to the present invention,
A rectangular-wave driving current bias that changes positively and negatively at a predetermined frequency is supplied to the DC-SQUID ring, and a rectangular-wave magnetic flux signal synchronized with the current bias is supplied to the DC-SQUID via a feedback coil. The magnetic flux signal that is superimposed on the DC magnetic flux signal of is supplied to take out the output of the SQUID ring, and the DC-SQUI
The output of the ID ring is amplified and then integrated to form a magnetic flux measurement signal, and the output of the integrating circuit is fed back through the feedback coil. Therefore, a pulse oscillator of one frequency is used for current bias and modulation. Magnetic flux signal can be formed, and in particular, a reference signal is not generated to detect the output of the SQUID ring,
It is possible to obtain the magnetic flux measurement signal based on an extremely simple circuit configuration and remove the temperature fluctuation component of the element parameter to reduce the low frequency noise. Moreover, SQUID
The output of the ring is a repetitive waveform of the voltage value at a point symmetric position around a point corresponding to the DC magnetic flux signal on the φ axis on the positive and negative side φ-V curves when positive and negative current bias is supplied. Therefore, it is possible to perform measurement even with an SQUID ring having a positive and negative asymmetrical φ-V characteristic as in the conventional SHAD method, and at the same time, as in the SHAD method, there is a dead zone in which the output of the SQUID ring is not read. Since there is no such time zone, S in the white area
/ N does not deteriorate.

[Brief description of drawings]

FIG. 1 is a block diagram showing a circuit configuration of an embodiment of the present invention.

FIG. 2 shows φ-V when a signal magnetic field is applied to the embodiment of the present invention.
Graph (A) showing characteristic changes and time chart (B) of signal waveforms at various parts

FIG. 3 is a graph (A) showing a change in the φ-V characteristic when there is a reverse phase fluctuation in the embodiment of the present invention, and a time chart (B) of the signal waveform of each part.

FIG. 4 shows φ-when a signal magnetic field enters when the characteristics of the SQUID ring 1 in the embodiment of the present invention are positive and negative asymmetrical.
A graph showing changes in V characteristics (A) and a time chart of signal waveforms at various parts (B)

FIG. 5 is an explanatory diagram of low frequency noise in DC-SQUID.

[Explanation of symbols]

 1 SQUID ring 11, 12 Josephson junction 2 Square wave oscillator 3 Feedback coil 4 Adder 5 DC power supply 7 Amplifying circuit 8 Integrator 9 Current-voltage converter R1, R2 Resistance VR1, VR2, VR3 Variable resistance S switch

Claims (1)

[Claims] 1. A magnetic flux measurement apparatus by a null method using a DC-SQUID, wherein a current bias for supplying a rectangular-wave-shaped driving current bias that changes positively and negatively at a predetermined frequency to a DC-SQUID ring. Source and the DC-S
A magnetic flux supply means for supplying a magnetic flux signal for modulation in which a rectangular-wave-shaped magnetic flux signal synchronized with the current bias and a predetermined DC magnetic flux signal are superposed on the QUID via a feedback coil, and the output of the DC-SQUID ring is amplified. SQUID magnetic flux, comprising: an amplifying circuit, an integrating circuit that integrates the output of the amplifying circuit and outputs a magnetic flux measurement signal, and circuit means that feeds back the output of the integrating circuit through the feedback coil. Total.