JPH07253455A - Squid fluxmeter - Google Patents

Abstract

PURPOSE:To tilt the flux-current characteristic of a SQUID fluxmeter and increase the flux-voltage conversion factor of the fluxmeter so as to reduce flux noise by making the injecting position of a bias current asymmetric with respect to inductances. CONSTITUTION:Although the graph showing the flux-voltage characteristic of a SQUID fluxmeter usually becomes a symmetric sine wave, the graph becomes to rise steeply and the characteristic becomes to tilt as a whole, since the injecting position 2 of a bias current IB is asymmetric with respect to inductances 3 and 4. In the graph showing the flux-flux vs voltage conversion factor characteristic obtained from the graph showing the flux-voltage characteristic, the flux-voltage conversion factor is a measure representing dV/dPHI. The ratio of external magnetic fluxes PHI to the magnetic flux quantum PHI0 is shown as the abscissa and the voltage (microvolts) is shown as the ordinate. Since the graph showing the flux-voltage characteristic is inclined, the value of the flux-voltage conversion factor (dV/dPHI) in the graph showing the flux-flux vs voltage conversion factor characteristic also increases. Therefore, the amplitude of the output voltage of a SQUID(superconducting quantum interference device) becomes larger and the SQUID does not produce much noise.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a medical diagnostic device (specifically, a magnetocardiogram,
A device for measuring biomagnetism such as brain magnetic waves and eye muscle magnetic fields), a device for measuring geomagnetism, or a device for measuring physical properties (specifically, a device for measuring magnetic susceptibility of a substance), and a communication device (specifically magnetic device). SQUID (Superconducting Quantum Interfer) suitable for dynamic signal transmission interface)
ence Device: superconducting quantum interference device) Here, SQUID means that when a magnetic flux is applied to the superconducting loop 21 from the outside, a circulating current is induced in the loop, and the applied external magnetic flux is caused by a quantum interference effect in the Josephson junction in the loop. This is an element for measuring a minute change in magnetic flux by utilizing the fact that a minute change appears as a large change in the circulating current.

[0002]

2. Description of the Related Art In a conventional SQUID magnetometer, there is known a method of measuring a magnetic field by adding a modulating magnetic flux of a certain frequency to a periodic function of a magnetic flux voltage characteristic and detecting its output voltage at the same frequency. There is. That is, the external magnetic flux is coupled to the SQUID loop through the pickup coil and the input coil, and the modulation coil is arranged close to the SQUID loop. The output voltage of SQUID is passed through an impedance matching circuit and a preamplifier to PSD (Phase Sensitiv
e Detector: Phase discriminator) for lock-in detection,
The first derivative of the Φ-V curve is obtained. When this output is added to the above modulation coil and negative feedback is performed, Φ−
The magnetic field that is stable at the point (peak or valley) where the first derivative of the V curve becomes zero can be obtained by monitoring the above feedback amount at the output. The above method is FLL (Flux Locked Loop)
It is called the law and is a kind of the so-called "zero-level method". However, as described above, in the method of applying a modulation magnetic flux of a certain frequency to the periodic function of the magnetic flux voltage characteristic (Φ-V characteristic) and detecting the output voltage at the same frequency, an oscillator for applying modulation, A phase discriminator or the like for detection is required, and the electronic circuit becomes complicated. In addition, in this method, as shown in FIG.
The injection position 12 of B was symmetrical with respect to the inductances 13 and 14 of the SQUID loop 11. On the other hand, D. Drung et al. Proposed a method of directly comparing the output voltage of the SQUID with a reference voltage without applying modulation, and prevented the circuit from becoming complicated. Further, in order to increase the magnetic flux voltage conversion coefficient, an APF (Additional Positive Feedback) coil is added.

[0003]

[Problems to be Solved by the Invention] However, the above-mentioned D. Dru
In the SQUID magnetometer of the method of ng et al., as shown in FIG. 6, a circuit in which a resistor R and a superconducting coil 18 are connected in series is used.
The configuration in which the output terminal of the QUID 11 is connected in parallel is adopted, and as shown in FIG. 7, the magnetic flux-voltage characteristic, which normally has a symmetrical sine waveform, is inclined as a whole, but the current noise generated by the resistor R is SQUID SQ as magnetic flux noise
There was a drawback that it was tied to the UID output. In the above, VA is the output voltage of SQUID and Φex is S
External magnetic flux applied from the outside of the QUID 11 and M are mutual inductances. The present invention has been made to solve the above problems, and an object of the present invention is to provide an SQUID magnetometer capable of reducing magnetic flux noise without complicating an electronic circuit.

[0004]

In order to solve the above problems, in the SQUID magnetometer according to the present invention, the bias current injection position of the SQUID loop is provided at an asymmetrical position so that the voltage magnetic flux characteristic is inclined and the output voltage is reduced. A magnetic flux lock loop is constructed by comparing with a reference voltage.

[0005]

According to the present invention having the above-mentioned structure, by making the injection position of the bias current asymmetrical, the magnetic flux voltage characteristic is inclined, the magnetic flux voltage conversion coefficient (dV / dΦ) is increased, and the magnetic flux noise is reduced. can do.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an equivalent circuit diagram showing the configuration of the SQUID loop 1 in the SQUID magnetometer which is an embodiment of the present invention.

The SQUID loop 1 is an SQUID loop having inductances 3 and 4 and Josephson junctions 5 and 6, and a part of the SQUID loop is grounded at a position 7. The SQUID loop 1 is biased at a position 2 which is asymmetric with respect to the inductance. The current IB is arranged to be injected.

FIG. 2 is a magnetic flux-voltage characteristic graph showing the relationship (Φ-V characteristic) between the output voltage V and the external magnetic flux Φex which is the magnetic flux applied from the outside of the SQUID 1. The horizontal axis of the graph is the ratio of external magnetic flux Φ and magnetic flux quantum Φo Φ / Φ
o, the vertical axis of the graph is voltage (unit: microvolt).

The magnetic flux-voltage characteristic graph of FIG. 2 usually has a symmetrical sine waveform, but in this embodiment, the bias current I
Since the injection position 2 of B is asymmetric with respect to the inductance, the rising edge of the waveform becomes steep and the magnetic flux-voltage characteristic is inclined as a whole.

FIG. 3 is a graph of the magnetic flux-magnetic flux voltage conversion coefficient characteristic obtained from the above magnetic flux-voltage characteristic graph.
Here, the magnetic flux voltage conversion coefficient is an amount indicating dV / dΦ. The horizontal axis of the graph is the ratio Φ / Φo of the external magnetic flux Φ and the magnetic flux quantum Φo, and the vertical axis of the graph is the voltage (unit: microvolt).

As described above, since the magnetic flux-voltage characteristic graph of this embodiment shown in FIG. 2 is inclined, the magnetic flux voltage conversion coefficient (dV / d) in the magnetic flux-magnetic flux voltage conversion coefficient characteristic graph is shown.
The value of Φ) is also increasing.

Next, the conventional SQUID and the SQ of this embodiment
Compare UIDs. The SQUID output voltage when the magnetic flux voltage characteristic is inclined by the conventional APF method is shown in b of FIG.
The SQUID output voltage in the case where the magnetic flux voltage characteristic is inclined by making the injection position 2 of the bias current IB asymmetric with respect to the inductance as in the present embodiment is shown in FIG.
, Respectively. As you can see from Figure 4, SQUID
The output voltage amplitude of 1 is larger in this embodiment than in the case of the conventional APF method. This is because the configuration of this embodiment has less noise than the APF method.

The present invention is not limited to the above embodiment. The above-mentioned embodiment is an exemplification, has substantially the same configuration as the technical idea described in the scope of the claims of the present invention, and has any similar effect to the present invention. It is included in the technical scope of the invention.

[0014]

As described above, according to the present invention having the above-described structure, by making the injection position of the bias current asymmetrical, the magnetic flux voltage characteristic is inclined, and the magnetic flux voltage conversion coefficient (dV / dΦ). , And the magnetic flux noise can be reduced.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram of an SQUID loop in an SQUID magnetometer that is an embodiment of the present invention.

FIG. 2 is a diagram showing a magnetic flux-voltage characteristic of the SQUID magnetometer shown in FIG.

FIG. 3 is a diagram showing a magnetic flux-magnetic flux voltage conversion coefficient characteristic of the SQUID magnetometer shown in FIG. 1.

FIG. 4 is a diagram showing the voltage amplitude of the SQUID magnetometer shown in FIG. 1 in comparison with the voltage amplitude of a conventional SQUID magnetometer.

FIG. 5 is an equivalent circuit diagram of a SQUID loop in a conventional SQUID magnetometer.

FIG. 6 is a circuit diagram showing the configuration of the SQUID magnetometer proposed by Drung et al.

FIG. 7 is a diagram showing characteristics of the conventional SQUID magnetometer shown in FIG. 7.

[Explanation of symbols]

 1 SQUID loop 2 Bias current injection position 3,4 Inductance 5,6 Josephson junction 7 Grounding position 11 SQUID loop 12 Bias current injection position 14 Input coil 15,16 Josephson junction 17 Grounding position 18 Superconducting coil

Claims (1)

[Claims] 1. A SQUID magnetometer characterized in that a bias current injection position of the SQUID loop is provided at an asymmetrical position to incline the voltage magnetic flux characteristic, and a magnetic flux lock loop is formed by comparing the output voltage with a reference voltage. .