JP2929173B2 - Low noise SQUID magnetometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a SQUID magnetometer utilizing the Josephson effect, and more particularly to a circuit configuration of a magnetometer which reduces noise of an amplifier directly connected to the SQUID.

[0002]

2. Description of the Related Art Conventionally, a known SQUID magnetometer comprises a dewar (or cryostat) for storing liquid helium, an SQUID probe operating in liquid helium, an amplifier and a controller operating at room temperature, and a liquid helium. SQUID probe and the room temperature amplifier are connected by a coaxial cable.

Normally, a feedback for a linear operation called FLL (flux lock loop) is formed through the room temperature amplifier to control the zero-order method. When simultaneously performing magnetic measurement at many points such as biomagnetic measurement, tens to hundreds of SQUIDs are arranged in an array in a dewar to obtain a spatial distribution of a magnetic field.

At this time, since the same number of FLL circuits are connected to each SQUID, there is a problem that the circuit scale becomes large in a large-scale system having a large number of channels.

[0005] Usually, a lock amplifier technique of modulating magnetic flux and applying FLL is used, but this requires a complicated circuit configuration. Therefore, a direct connection, non-modulation FLL circuit has been proposed to reduce the size of the circuit, and some FLL circuits have been put to practical use.

On the other hand, since the output voltage of the SQUID is as small as several tens of μV, low-frequency components become a problem even in current noise and voltage noise of an electronic circuit. Several Hz or more for biomagnetic measurement
Since the band of several tens of Hz contains important information, the 1 / f noise of the low frequency component of the circuit noise and the drift of the amplifier are S
/ N ratio was deteriorated.

This is a problem peculiar to a DC amplifier that was not found in an AC amplifier used in a modulation type. In particular, a SQUID called an APF adopted in a direct connection type is subjected to positive feedback to reduce a magnetic flux voltage conversion rate. In the method of taking the noise matching with the amplifier by improving the noise, the dynamic resistance of the SQUID increases, so that the current noise of the amplifier has been a problem.

[0008]

Further, the series SKU
Even when the output voltage is increased by a method such as an ID array, the dynamic resistance increases in proportion to the number of connected SQUIDs, so the same problem occurs. It has a problem that it is not suitable for applications handling very low frequency signals.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of such problems of the prior art, and has been made in view of the above problem.
D (hereinafter referred to as SQUID) in a magnetometer using a sensor for detecting a magnetic field, wherein a first SQ which is a sensor SQUID is used.
A UID, a resistor for extracting an output voltage of the first SQUID as a current, a coil directly connected to the resistor, and a second SQUID or SQUID magnetically coupled to the coil;
An ID array and the second SQUID or SQUID
An amplifier operating at room temperature that directly receives the voltage output of the D array;
Switch means for turning on / off the bias current of the first SQUID; and a driving device for the switch means, and subtraction means for subtracting an output voltage in an off state from an output voltage in an on state of the switch means. The present invention provides a low-noise SQUID magnetometer provided with.

The present invention relates to the above-mentioned S of the magnetic flux voltage conversion rate dV / dΦ.
The QUID array is composed of a resistor Rn and a coupling coil Mn,
A negative feedback circuit is provided so that the feedback ratio β = (dV / dΦ) × Mn / Rn ≧ 1, and the operating point is a Φ−V curve (magnetic flux voltage conversion curve)
And a low-noise SQUID magnetometer provided with a magnetic flux bias circuit for positioning the magnet at a substantially middle point of the slope.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The operation principle of the present invention will be described below. First, the magnetic flux voltage conversion rate of the first SQUID that is the sensor SQUID is (dV / dΦ) 1, the second SQUID or the magnetic flux voltage conversion rate of the SQUID array is (dV / dΦ) 2, and the second SQUID Alternatively, the dynamic resistance of the SQUID array is Rd.

Further, the resistance value of the coupling resistor is Rl, the mutual inductance of the coupling coil is Ml, and the low frequency components (1 / f noise including temperature drift) of the current noise and the voltage noise of the room temperature amplifier, respectively. In, Vn.

That is, when the magnetic flux input to the first SQUID is ΔΦ, the output ΔVon of the second SQUID is given by the following equation (1).

[0014]

(Equation 1)

Here, the first term of Equation 1 is a signal component. At this time, the fluctuation of the amplifier (1 / f including temperature drift)
Noise) has a time-series correlation within a very short period of time. Therefore, on the data when the bias current of the first SQUID is turned on / off at high speed, they are adjacent to each other or close in time. In the data, the second and third terms can be regarded as equal.

Accordingly, when the voltage output at the time of OFF is ΔVoff, when the bias current is OFF, (dV / dΦ) 1 = 0, so that it is given by the following equation (2).

[0017]

(Equation 2)

Then, the data of Equations 1 and 2 are temporarily stored, and the subtraction result is given by ΔV, which is given by Equation 3 below.

[0019]

(Equation 3)

At this time, since other than ΔΦ is a constant, it is understood that components other than the signal can be removed.

FIG. 1 is a circuit diagram of a low noise SQUID magnetometer according to an embodiment of the present invention. In the figure, reference numeral 1 denotes one or more sensors SQUID for detecting a magnetic field, and a pickup coil and a shunt resistor are omitted from the drawing.
Although the negative feedback circuit described in FIG. 2 is also omitted, the negative feedback circuit does not necessarily have to be included.

Reference numerals 2 and 3 denote limiting resistors for supplying a bias current to each SQUID. The current flowing through 2 is turned on / off by the output of an open / close control signal source 5 using a switch 4 (analog switch or mechanical switch).
Controlled.

Reference numeral 6 denotes a power supply for supplying a bias current. 7 is a SQUID or multiple SQUIDs
FIG. 1 shows a configuration example of a SQUID array in which D is connected in series. FIG. 1 shows a configuration example of a SQUID array. The linearity near the operating point of the V characteristic is improved.

Further, these SQUIDs asymmetrically inject a bias current without using a negative feedback, so that Φ−
A linear region may be used by modifying the V characteristic.

Then, the sensor S whose switch 4 is turned on
Since the bias current is supplied to QUID1, it transitions to the voltage state. Therefore, an output voltage is generated according to the input magnetic flux.

On the other hand, the sensor SQUID in the OFF state
No voltage is generated because 1 is in the superconducting state. The voltage generated by the sensor SQUID1 in this way is transmitted to the next-stage SQUID array 7 through the input coil 10 by the value limited by the resistor 9.

Here, the magnetic field sensitivity of the sensor SQUID1 is η, the magnetic flux voltage conversion rates of the sensor SQUID1 and the SQUID array 7 are dV / dΦ1, dV / dΦ2, the resistance value of the resistor 9 is R, the input coil 10 and the SQUID array. 7
Is Mn, the output voltage Vo of 7 with respect to the input magnetic field B is given by the following equation (4).

[0028]

(Equation 4)

Numeral 11 denotes a magnetic flux input coil for applying a bias magnetic flux for setting the operating point of the sensor SQUID1, and a current limiting resistor 12 and a voltage setting power supply 13 together with Φ−
The adjustment is made so that the operating point is near the middle point of the slope of the V curve. Similarly, in order to set the operating point of the SQUID array 7, a magnetic flux input coil 14, a current limiting resistor 15, and a bias magnetic flux setting circuit indicated by a voltage setting power supply 16 are provided.

The output of the SQUID array 7 is amplified by voltage amplifiers 17 and 18. At this time, since the output of the SQUID array 7 has a bias voltage, the output voltage fluctuation range is set by the bias voltage adjusting power supply 19.

When the output of the sensor SQUID1 is turned ON / OFF, the voltage amplifier 18 alternately outputs the amplified value of the voltage represented by the above-described formulas 1 and 2, so that the voltage is output by one of the sample and hold circuits 20a and 20b. Each value of the voltage is temporarily held and subtracted by the differential amplifier 21 to obtain an output corresponding to the value of Expression 3.

The sample and hold circuits 20a, 20b
Although not shown in the figure, the same effect can be obtained even if the differential amplifier 21 is constituted by a subtractor using a flying capacitor.

The details of the above-described embodiment will be described below with reference to FIGS. 2, 3 and 4. First, the output of the SQUID changes in a sinusoidal manner with respect to the input magnetic flux as shown by the thin line in FIG.

Therefore, feedback correction is performed in a magnetic flux lock circuit usually called a FLL circuit so that the zero-order method is established so that the output voltage is linear with respect to a change in input magnetic flux. However, such correction has the problem of lock jump as described in the disadvantages of the prior art.

Therefore, the output voltage is linearized by providing a negative feedback circuit composed of a resistor and a coil as shown in FIG. The resistance value of the resistor 22 is Rn, and the coil 23 and SQ
The mutual coupling inductance of UID is Mn, SQUID2
Assuming that the magnetic flux voltage conversion rate of No. 4 is dV / dΦ, the feedback rate β is given by the following equation (5).

[0036]

(Equation 5)

Here, when β ≧ 0, the Φ-V curve is deformed as shown by the thick line in FIG. The magnetic flux-to-voltage conversion rate after the feedback at this time is given by Expression 6 below.

[0038]

(Equation 6)

At this time, when β is in the range of 1 to 3,
The sinusoidal Φ-V curve has a sawtooth shape, and the slope of the bold line shown in FIG. 4 has increased linearity. In addition, when there is no feedback, the linear range is such that Φ1 is Φo / 2 or less.
o 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 fluctuation of the externally applied magnetic field fluctuates around this point.

Usually, the magnitude of the magnetic field to be measured is about several tens pT in the case of biomagnetic measurement, but the magnetic field sensitivity η is about several nT / Φo including the transmission efficiency from the pickup coil to the SQUID. The dynamic range of the signal is less than Φo / 10.

On the other hand, the DC magnetic field fluctuation is about 50 nT / Φo due to the diurnal variation of the geomagnetism, and is about 1/100 under the environment in the magnetically shielded room. Therefore, as long as linearity can be obtained, there is no problem in measurement even in an open loop.

FIG. 3 is intended to linearize the output voltage by providing a similar negative feedback also in the SQUID array. When a negative feedback circuit including a negative feedback resistor 26 and a coil 25 is provided in the SQUID array 27, noise matching with an amplifier operating at room temperature is reduced in exchange for linearization.

That is, if the sensor SQUID1 of FIG. 1 that has been subjected to negative feedback is directly connected to the voltage amplifier 17, the signal-to-noise ratio is reduced. This means that if the noise of the amplifier is Vn,
This is because the input converted magnetic field noise is given by Vn / (dV / dΦ) ′.

The sensor SQUID1 in the SQUID array 7
This is why the S / N ratio is improved by amplifying the output voltage.

As described above, the present invention has been described based on the embodiments. However, the present invention is not limited to the above-described embodiments, and may be implemented in any manner unless the configuration described in the claims is changed. it can. For example, instead of the sample-and-hold circuit, the voltage output may be AD-converted and stored as digital data, and the ON / OFF data may be subtracted in time series.

[0047]

As described above, in the low-noise SQUID magnetometer according to the present invention, since low-frequency noise can be reduced, particularly low signal components can be measured even in biomagnetic measurement. Further, by using the SQUID array, it is possible to further amplify ultra-low noise, and since the electronic circuit has an extremely simple configuration, a low cost and a high S / N ratio can be secured.

On the other hand, since the measurement is performed in an open loop, even if the measurement is temporarily hindered by an excessive input or power supply noise, an excellent effect is obtained such that the measurement can be continued continuously.

[Brief description of the drawings]

FIG. 1 shows a low noise SQUID according to an embodiment of the present invention.
It is a principal part circuit diagram of a magnetometer.

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

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 a SQUID.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sensor SQUID 2, 3, 12, 15 Current limiting resistor 4 Switch 5 Switching control signal source 6 Voltage source for bias current supply 7, 27 SQUID array 8, 22, 26 Negative feedback resistor 9 Coupling resistor 10, 22, 25 Input Coil 11,14 Magnetic flux input coil 13,16 Voltage setting power supply 17,18 Voltage amplifier 19 Bias power supply 20a, 20b Sample hold circuit 21 Differential amplifier

Claims (2)

(57) [Claims] 1. A magnetometer using a dc-SQUID (hereinafter referred to as SQUID) as a sensor for detecting a magnetic field, comprising: a first SQUID that is a sensor SQUID; and a resistor that extracts an output voltage of the first SQUID as a current. A coil directly connected to the resistor, a first SQUID array magnetically coupled to the coil, an amplifier operating at room temperature which directly receives the voltage output of the first SQUID array, and a bias current of the first SQUID Switch means for turning on / off the switch, and a driving device for the switch means.
A low-noise SQUID magnetometer provided with subtraction means for subtracting the output voltage in the F state. 2. The SQUID having a magnetic flux voltage conversion rate dV / dΦ.
The array consists of a resistor Rn and a coupling coil Mn, and the feedback rate β
= (dV / dΦ) × Mn / Rn ≧ 1 Provide a negative feedback circuit so that
2. The low-noise SQUID magnetometer according to claim 1, further comprising a magnetic flux bias circuit for setting an operating point at a substantially middle point of a slope of a Φ-V curve (magnetic flux-voltage conversion curve).