JPH09222469A - Multi-channel squid fluxmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multi-channel SQUID fluxmeter which achieves signifi cant minimization of the circuitry while reducing the number of connection wires for connection of a sensor SQUIDs and an electronic circuits. SOLUTION: A bias current to be supplied to a plurality of sensor SQUIDs/is switched by a multiplexer 4 to take out an output current in a time sharing manner while the output current is applied to an input coil 6 of a plurality of SQUIDs 5 and moreover, the bias current supplied to the SQUID 5 is switched by a multiplexer 9 to take out the output current in a time sharing manner. At the same time, the output current is applied to an input coil 19 of a SQUID array 20 to obtain an output signal from the SQUID array 20.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an SQUID magnetometer utilizing the Josephson effect, and more particularly to a circuit structure for receiving an electric signal obtained from a plurality of SQUIDs with a simple electronic circuit. It is a thing.

[0002]

2. Description of the Related Art Conventionally known SQUID magnetometers are composed of a dewar (or cryostat) that stores liquid helium, an SQUID probe that operates in liquid helium, and an amplifier and controller that operate at room temperature. The SQUID probe in helium and the room temperature amplifier are connected with a coaxial cable.

Normally, a zero-order method is controlled to be valid by forming feedback for linear operation called FLL (flux lock loop) through the room temperature amplifier. When performing magnetic measurement at a large number of points such as biomagnetic measurement at the same time, several tens to several hundreds of SQUIDs are arranged in an array in the dewar to obtain the spatial distribution of the magnetic field.

At this time, in order to configure the FLL, each SQU
The same number of FLL circuits are connected to the ID. This is because the flux lock is necessary to guarantee the linearity of the output voltage with respect to the input flux as well as its linearity with respect to time. That is, the temporal change of the spatial distribution of the magnetic field cannot be measured without the same number of FLL circuits as the sensors.

As described above, when the sensor and the electronic circuit correspond to each other in a one-to-one relationship, a total of six bias current supply lines, signal voltage output lines, and feedback signal lines for the one-channel magnetometer are SQUID and FLL circuits. Needed during.

When the number of wires is large, heat invasion into the dewar increases, so that the SQUID having a large number of channels is used.
In the magnetometer, there has been a problem that the running cost is increased due to an increase in the evaporation amount of the refrigerant such as helium. (25
A 6-channel SQUID magnetometer requires 6 x 256 wires)

Further, since the number of connection lines increases as the number of connection lines increases, it is not preferable in terms of wiring cost and reliability.
Furthermore, as the number of FLL circuits increases, the load also increases in terms of weight and power consumption.
These were big problems.

For example, in a 256-channel SQUID magnetometer, the current consumption of the FLL circuit is several amperes,
The FLL circuit cannot be installed in the magnetically shielded room for performing biomagnetic measurement originating from a minute biocurrent.

[0009]

Therefore, it is necessary to derive a small SQUID output voltage of several tens of μV to the FLL circuit outside the magnetic shield room with a good S / N ratio.
There is a new problem that the electromagnetic shield of the wiring system is burdened and RF noise is easily mixed.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art, and a plurality of dc-
In a multi-channel SQUID magnetometer using SQUID (hereinafter SQUID) as a sensor for magnetic field detection, first switch means for turning on / off the bias current of each of these sensors SQUID and the voltage output of each sensor SQUID are taken out as a current. A first coupling resistor, a first coil electrically connected in series with the first coupling resistor, and a SQUID magnetically coupled to the first coil.
Alternatively, a plurality of first adders composed of SQUID arrays, second switch means for turning on and off the first adders, and a second voltage output of the first adder as a current A coupling resistor, a second coil electrically connected in series with the second coupling resistor, and an SQUID or SQUID magnetically coupled to the second coil.
The present invention provides a multi-channel SQUID magnetometer including a second adder including a D array.

The present invention is an SQU having a magnetic flux voltage conversion rate dV / dΦ.
Resistor Rn and coupling coil in ID or SQUID array
A negative feedback circuit that is composed of Mn and has a feedback ratio β = (dV / dΦ) × Mn / Rn ≧ 1 and has a linear operating point Φ−
The present invention provides a multi-channel SQUID magnetometer provided with a magnetic flux bias circuit for locating the magnetic flux bias circuit at approximately the midpoint of the slope of the V curve.

According to the present invention, the magnetic flux bias and the current bias are synchronized with the first and second switch means, and each SQ is synchronized.
A multi-channel SQUID magnetometer configured to set a bias point according to a UID.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to the drawings. FIG. 1 is a circuit diagram showing one embodiment of the present invention. In the figure, 1 is a sensor S for detecting a magnetic field.
It is a QUID, and the pickup coil and the shunt resistor are omitted. Although the negative feedback circuit described with reference to FIG. 2 is also omitted, the negative feedback circuit may not necessarily be attached. It should be noted that a plurality of sensors SQUID1 are installed at different locations for signals to be detected.

Reference numeral 2 denotes a limiting resistor for supplying a bias current to each sensor SQUID 1, which is connected to each distribution terminal of the analog multiplexer 4. Each switch of the multiplexer 4 is exclusively on / off controlled so as to sequentially supply a bias current to each SQUID. The multiplexer 4 may be replaced with another analog switch or mechanical switch.

The sensor SQUID1, which is switched on by the multiplexer 4, is transferred to the voltage state because the bias current is supplied. Therefore, the output voltage corresponding to the input magnetic flux is generated. On the other hand, the sensor SQ in the off state
Since UID1 is in the superconducting state, no voltage is generated.
In this way, the voltage generated by the sensor SQUID1 is transmitted to the SQUID5 at the next stage by the value limited by the coupling resistor 3 through the input coil 6.

At this time, each coupling resistor 3 is usually sufficiently larger than the wiring resistance for supplying a current to the input coil 6, and the current addition is established. Therefore, the sensor SQUID1
All the fluctuations in the voltage generated in 1 flow into the input coil 6 as a current. Therefore, depending on the strength of the magnetic field, SQUI
A voltage is generated across D5. That is, only the sensor sensor SQUID1 in the ON state of the bias current can selectively transmit the signal to the SQUID5 of the next stage.

Here, the magnetic field sensitivity of the sensor SQUID1 including a pickup coil (not shown) (the magnetic field-magnetic flux conversion coefficient when the shielding current proportional to the magnetic field is transferred as the magnetic flux in the SQUID ring by the pickup coil) is given. Letting η, the magnetic flux voltage conversion rates of the sensors SQUID1 and SQUID5 be dV / dΦ1 and dV / dΦ2, the resistance value of the coupling resistor 3 be R1, and the coupling of the input coil 6 and SQUID5 be M1, the input magnetic field fluctuation ΔB The output voltage fluctuation ΔVo1 of SQUID5 is expressed by the following equation 1
Given in.

[0018]

[Equation 1]

Reference numeral 7 is a current limiting resistor for supplying a bias current to the SQUID 5, which is connected to one of the terminals of another multiplexer 9. This SQUID 5 is also selected by shifting to the voltage state only when the multiplexer 9 is turned on, and is magnetically coupled to the SQUID of the third stage by the coupling resistor 8 or the SQUID array 20 by the input coil 19.

At this time, the resistance value of the coupling resistor 8 is R2, the input coil 19 and the SQUID, or the SQUID.
When the coupling of the array 20 is M2 and the magnetic flux voltage conversion rate of the SQUID array 20 is dV / dΦ3, the output voltage fluctuation ΔVo2 is given by Equation 2 below.

[0021]

[Equation 2]

Further, reference numerals 10, 11 and 13, 12 denote a magnetic flux input coil and a current limiting resistor for giving a bias magnetic flux for setting the operating points of the sensors SQUID1, SQUID5, respectively, together with the voltage setting power supplies 14, 15, respectively. It is adjusted so that the operating point is near the midpoint of the slope of the SQUID Φ-V curve.

Similarly, in order to set the operating point of the SQUID array 20, a magnetic flux input coil 21, which is a bias magnetic flux setting circuit, a voltage setting power source 22, and a current limiting resistor 23.
Is provided.

When there are a plurality of blocks indicated by 17 in the above configuration, the sensor sensor SQUI of each block
As for D1 and SQUID5 for addition, one sensor SQUID1 can be selected by the two multiplexers 4 and 9. For example, if the switch 4a of the multiplexer 4 is on and the switch 9a of the multiplexer 9 is on, the sensor SQUID1 shown in FIG. 1 is selected.

In this way, as many sensor SQUIDs 1 as the number of contacts m × n of the multiplexer are selected. 2
Reference numeral 4 denotes a resistor for setting the bias current of the SQUID array 20, which is set near the critical current value together with the bias setting power supply 16. Similarly, although each SQUID array 20 is also connected so that the bias current is set by the same bias setting power supply 16, these may be set by different power supplies.

The output of the SQUID array 20 is amplified by the voltage amplifiers 25 and 26 operating at room temperature, the optimum bias voltage is applied by the bias setting power supply 29, and then the sample hold circuit 27 outputs a signal synchronously with each switch. Hold.

Reference numeral 28 is a sequence control circuit for synchronously operating the multiplexers 4 and 9 and the sample hold circuit 27. The control sequence of the sequence control circuit 28 is omitted since it is only for selecting the sensors SQUID1 and SQUID5 and holding the output of the selected SQUID. The held output voltage is output to a reading circuit such as an AD converter (not shown).

On the other hand, since the operating point of the SQUID and the bias current value may differ for each SQUID, in this case, the bias setting power sources 14, 15, 16, 22, 29, etc.
It may be replaced with a DA converter or the like, and the output voltage value may be set to an optimum value for each SQUID according to the switch sequence.

Further, in this embodiment, the influence of the current noise of the coupling resistor can be suppressed to a small level. For example,
In FIG. 1, assuming that the current noise of the coupling resistor 3 is In,
When m sensors SQUID1 are combined, In × √m noise current is transferred to SQUID5 of the next stage.

Therefore, if the number of channels is large, the S / N ratio is low if the configuration has only two stages. However, the three-stage configuration as in this embodiment allows the values of m and n to be kept small even if the number of channels is large, which is also advantageous in terms of the S / N ratio.

2, 3 and 4 explain the above construction in more detail. As for the output of the SQUID, as shown by the thin line in FIG. 4, the output voltage changes sinusoidally with respect to the input magnetic flux. Therefore, a magnetic flux lock circuit, which is usually called a FLL circuit, is used to perform feedback correction such that the zero position method is established so that the output voltage becomes linear with respect to the change in the input magnetic flux.

However, such correction has a problem that the lock is skipped as described in the prior art. Therefore,
By providing a negative feedback circuit composed of the negative feedback resistor 30 and the coil 31 as shown in FIG. 2, the output voltage is linearized. At this time, the resistance value of the resistor 30 is Rn, and the coil 31 and S
Mutual coupling inductance of QUID32 is Mn, SQUI
Assuming that the magnetic flux voltage conversion rate of D32 is dV / dΦ, the feedback rate β is given by Equation 3 below.

[0033]

(Equation 3)

Here, when β ≧ 0, the Φ-V curve is deformed so that the slope of the slope becomes gentle as shown by the thick line in FIG. 4 (as β increases). The magnetic flux voltage conversion rate after the feedback at this time is given by Equation 4 below.

[0035]

(Equation 4)

Then, in the range of β from 1 to 3,
The sinusoidal Φ-V curve has a sawtooth shape, and the slope of the thick line shown in FIG. 4 has increased linearity. In the absence of feedback, the linear range is Φ1 is Φo / 2 or less, whereas the linear part is Φo depending on the value of β (in the vicinity of 2) such as Φ2.
It is enlarged to the vicinity. At this time, by applying a magnetic flux bias to the center of the linear portion (shown in FIG. 4), the magnetic field applied from the outside fluctuates around this point.

Normally, the magnitude of the magnetic field to be measured is about several 10 pT in the case of biomagnetism measurement, but the magnetic field sensitivity η is several nT including the transfer efficiency from the pickup coil to the SQUID.
/ Φo, the signal dynamic range is Φo /
10 or less. On the other hand, the direct-current magnetic field fluctuation is about 50 nT / Φo due to the diurnal variation of the earth's magnetism, and is about 1/100 in the environment in the magnetic shield room, so the fluctuation of the bias point is smaller than Φo.

Therefore, it can be seen that even if the linearity is obtained, no problem occurs in the measurement even in the open loop. The linearization by such negative feedback is performed by the sensors SQUID1 and SQUI of FIG.
By applying to ID5, magnetic field-voltage conversion and voltage transfer can be performed while maintaining linearity.

In FIG. 3, the output voltage is linearized by providing the same negative feedback also in the SQUID array. That is, a negative feedback circuit including the negative feedback resistor 33 and the coil 34 is provided. At this time, the resistance 3
When the resistance value of 3 is Rn, the mutual coupling inductance of the coil 34 and the SQUID array 35 is Mn, and the magnetic flux voltage conversion rate of the SQUID array 35 is dV / dΦ, the feedback rate β is given by the above-described Equation 3.

Although the embodiments of the present invention have been described above, the present invention is not limited to the embodiments shown here, and is not limited to the modifications described in the claims. It can be implemented appropriately. For example, in the embodiment of the present invention, the current bias is set in synchronization with the first and second switch means to set the bias point according to each SQUID, but the magnetic flux bias is similarly set to the above first. Each SQUI in synchronization with the first and second switch means
The bias point may be set according to D.

[0041]

As described above, in the multi-channel SQUID magnetometer of the present invention, the sensor SQUID is selected by the multiplexer and is temporarily received by another SQUID, and this addition SQUID is selected by another multiplexer. In addition, since the SQUID array in the subsequent stage performs additional amplification and the room-temperature amplifier amplifies the voltage output in a time-division manner, multiple sensors are amplified with a single amplifier at a high S / N ratio. Therefore, the number of wirings can be made extremely smaller than the case where FLL circuits are provided by the number of sensors.

Further, if the multiplexer is installed in the cryostat like the SQUID, the number of wires is further reduced. Therefore, the heat invasion of the cryostat is reduced, so that the running cost of helium is reduced, and further, the circuit scale can be reduced.

On the other hand, in this embodiment, the influence of the current noise of the coupling resistor can be suppressed to a small level, and can be kept small even if the number of channels is large, which is also advantageous in terms of the S / N ratio.

Furthermore, since the measurement is performed in an open loop, even if the measurement is temporarily hindered by an excessive input or power source noise, it is possible to continue the measurement, which is a great effect.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a main part of a multi-channel SQUID magnetometer according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a configuration of a negative feedback circuit of SQUID.

FIG. 3 is a circuit diagram showing a configuration of a negative feedback circuit of the SQUID array.

FIG. 4 is a characteristic diagram showing input / output characteristics of SQUIDs.

[Explanation of symbols]

1 Sensor SQUID 2, 7, 12, 13, 23, 24 Current limiting resistor 3, 8 Coupling resistor 4, 9 Multiplexer 5 Next stage SQUID 6, 19, 31, 33 Input coil 10, 11, 21 Flux input coil 14, 15, 22 Voltage setting power supply 16 Bias current supply voltage source 17, 18 Block 20, 35 SQUID array 25, 26 Voltage amplifier 27 Sample hold circuit 28 Timing control circuit 29 Bias power supply 30, 33 Negative feedback resistance

Claims (3)

[Claims] 1. A plurality of dc-SQUIDs (hereinafter referred to as SQUs).
ID) as a sensor for magnetic field detection
In the QUID magnetometer, first switch means for turning on / off the bias current of each of the sensors SQUID, and each of the sensors SQUI.
A first coupling resistor for extracting the voltage output of D as a current, a first coil electrically connected in series with the first coupling resistor, and an SQ magnetically coupled to the first coil.
A plurality of first adders each consisting of a UID or SQUID array, a second switch means for turning the first adder on and off, and a second voltage output of the first adder as a current Coupling resistor, a second coil electrically connected in series with the second coupling resistor, and a SQUID magnetically coupled to the second coil or S
A multi-channel SQUID magnetometer comprising a second adder composed of a QUID array. 2. Negative feedback such that the feedback ratio β = (dV / dΦ) × Mn / Rn ≧ 1 is constituted by a SQUID having a magnetic flux voltage conversion ratio dV / dΦ or a SQUID array which is composed of a resistor Rn and a coupling coil Mn. The multi-channel SQUID magnetometer according to claim 1, further comprising a circuit, and a magnetic flux bias circuit for locating the operating point at a substantially middle point of a slope of a linearized Φ-V curve. 3. A bias point according to each SQUID is set by synchronizing at least one of a magnetic flux bias and a current bias with the first and second switch means. The multi-channel SQUID magnetometer according to any one of claims 1 and 2.