JPH11118894A - Squid fluxmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simplify the constitution of a SQUID(superconducting quantum interference device) fluxmeter designed to drive a SQUID by AC bias method without magnetic flux modulation and to decrease the number of parts requiring adjustments. SOLUTION: A SQUID fluxmeter has a differential amplifier 8 comparing a SQUID output signal output as an AC signal as the result of an AC bias current with signal amplitude at a flux fixing point output from an amplitude regulator 10, to calculate the difference between them, and has a lock-in amplifier 9 obtaining a detection signal through the lock-in detection of the calculated difference at the same frequency as the AC bias current, the obtained signal being integrated by an integrator 11, so that a feedback signal to be fed back to a SQUID 6 is output by a voltage-to-current converter 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a SQUID magnetometer, and more particularly, to a SQUID magnetometer for driving a SQUID using an AC bias method.

[0002]

2. Description of the Related Art Conventionally, in SQUID magnetometers,
A method of using an alternating current as a bias current of a SQUID to remove a unique 1 / f noise of the SQUID (Bias Current Reversa)
l) is known. In particular, the SQUID made of a high-temperature superconducting material has a larger critical current fluctuation at the junction than a SQUID made of Nb or the like,
The driving of the SQUID by an AC bias is required.

[0003] In addition, the driving of the SQUID by a method combining the AC bias method and the magnetic flux modulation method may significantly complicate the circuit configuration and reduce the reliability. When driving the SQUID, it is preferable not to perform magnetic flux modulation. FIG. 5 shows a configuration of a conventional SQUID magnetometer in which the SQUID is driven by the AC bias method without performing the magnetic flux modulation, and the time chart of the voltage of each part is as shown in FIG.

The SQUID magnetometer supplies an output signal from an oscillator 31 to an inversion buffer 32 so as to shift the phase of the AC bias current by 180 degrees from the phase of the current in other circuit units. The signal is supplied to the amplitude adjuster 33 to adjust the signal amplitude, and the signal whose amplitude is adjusted is supplied to the voltage-current converter 34 to obtain an AC bias current, and the SQUID ring 3 of the SQUID 36
6a. The output signal from the offset adjuster 35 for making the upper and lower waveforms symmetrical to each other is also supplied to the voltage-current converter 34.

[0005] The SQ from the SQUID ring 36a
The UID output signal is amplified by a preamplifier 37, further amplified by an amplifier 38, and supplied to a voltage adder 39. Further, the output signal from the oscillator 31 is output to the amplitude adjuster 4.
0 to adjust the signal amplitude, and the signal whose amplitude has been adjusted is also supplied to the voltage adder 39. Then, the output from the voltage adder 39 is supplied to the integrator 41 to obtain the output of a phase locked loop (FLL) circuit.

In addition, the output from the integrator 41 is supplied to a voltage adder 42, and the output signal from the oscillator 31 is supplied to an amplitude adjuster 43 to adjust the signal amplitude. The voltage is supplied to the voltage adder 42. Further, the output from the voltage adder 42 is converted to a voltage-current converter 44.
To obtain a feedback current and supply it to the feedback coil 36b of the SQUID 36.

The operation of the SQUID magnetometer having the above configuration is as follows. The output from the oscillator 31 is shown in FIG.
6, the output signal from the inverting buffer 32 is supplied to the oscillator 31 as shown in FIG.
Are inverted in phase with respect to the output from. FIG.
The middle (F) and (G) are the positive and negative Φ of SQUID 36, respectively.
/ V characteristic is shown. Here, the amplitude Φ0 / 2 is output from the feedback coil 36b in accordance with the cycle of the bias modulation.
, The phases of the positive and negative Φ / V characteristic periods are matched with each other. Then, an input magnetic flux Φ1 shown in (H) in FIG. 6 is given.

Therefore, the SQUID output signal is determined based on the point on the positive / negative Φ / V characteristic corresponding to the input magnetic flux Φ1, and is amplified by the preamplifier 37 to obtain the signal waveform shown in FIG. Can be In addition, by supplying the output signal from the oscillator 31 to the amplitude adjuster 40 to adjust the signal amplitude, a signal waveform shown in FIG. 6D is obtained. Then, by supplying the signal waveform shown in FIG. 6C and the signal waveform shown in FIG. 6D to the voltage adder 39, the AC waveform caused by the AC bias is removed, and the signal is input to the SQUID 36. A signal proportional to the magnetic flux Φ1 is detected (see (E) in FIG. 6).

[0009]

The SQ having the configuration shown in FIG.
When the UID magnetometer is used, there is a disadvantage that the number of adjustment points is increased as compared with the SQUID magnetometer that drives the SQUID by the DC bias method for performing the magnetic flux modulation. Here, the adjustment is generally performed by an operator visually, and the more subtle and difficult the manual adjustment, the more difficult it is to automate the adjustment. Therefore, it is difficult to substitute control by digital equipment or the like.

[0010] In consideration of such points and the increase in the number of channels, there is a strong demand for reducing the number of adjustment points as much as possible.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and simplifies the configuration of a SQUID magnetometer which drives a SQUID by an AC bias method without performing magnetic flux modulation. The purpose is to reduce the number of adjustments.

[0012]

According to a first aspect of the present invention, there is provided a SQUID magnetometer for supplying an AC current to a SQUID as a bias current and driving the SQUID without performing flux modulation. A signal processing means for removing an AC component and an offset voltage caused by an AC bias from a SQUID output signal output as an AC signal due to the above, to obtain a signal proportional to the input magnetic flux of the SQUID; and integrating the obtained signal. And a feedback signal calculating means for calculating a feedback signal to be fed back to the SQUID.

According to a second aspect of the present invention, in the SQUID magnetometer, the signal processing means compares a SQUID output signal output as an AC signal due to an AC bias current with a signal amplitude at a fixed magnetic flux point to calculate a difference. And a detecting means for obtaining a detection signal by lock-in detecting the calculated difference at the same frequency as the AC bias current.

According to a third aspect of the present invention, as the signal processing means, the SQUID output signal, which is output as an AC signal due to the AC bias current, is lock-in detected at the same frequency as the AC bias current, and the detection signal is output. The present invention employs a device that includes a detection unit that obtains the signal and a comparison unit that calculates a difference by comparing the detected signal with a signal amplitude at a fixed magnetic flux point.

[0015]

According to the SQUID magnetometer of the first aspect, the SQUID magnetometer
In supplying the AC current as a bias current to the ID and driving the SQUID without performing the magnetic flux modulation, the signal processing means converts the SQUID output signal output as an AC signal due to the AC bias current from the SQUID output signal. A signal proportional to the input magnetic flux of the SQUID is obtained by removing an AC component and an offset voltage caused by the AC bias, and the obtained signal is integrated by a feedback signal calculating means to calculate a feedback signal to be fed back to the SQUID. can do.

Therefore, the configuration for supplying the AC current to the feedback coil and the configuration for inverting the sign of the AC bias can be omitted, thereby simplifying the overall configuration and reducing the number of adjustment points. it can. In the case of the SQUID magnetometer of claim 2, the comparing means compares the SQUID output signal output as the AC signal due to the AC bias current with the signal amplitude at the fixed magnetic flux point to calculate the difference, and the detecting means The calculated difference is lock-in detected at the same frequency as the AC bias current to obtain a detection signal, and the feedback signal calculation means can calculate the feedback signal to be fed back to the SQUID by integrating the detection signal. .

Therefore, the configuration for supplying the AC current to the feedback coil and the configuration for inverting the sign of the AC bias can be omitted to simplify the overall configuration and reduce the number of adjustment points. it can. In the case of the SQUID magnetometer according to claim 3, the detection means performs lock-in detection on the SQUID output signal output as an AC signal due to the AC bias current at the same frequency as the AC bias current to obtain a detection signal. Means for comparing the detected signal with the signal amplitude at the magnetic flux fixed point to calculate a difference, and integrating the calculated difference by the feedback signal calculating means to calculate a feedback signal to be fed back to the SQUID. .

Therefore, the configuration for supplying the AC current to the feedback coil and the configuration for inverting the sign of the AC bias can be omitted, so that the overall configuration can be simplified and the number of adjustment points can be reduced. it can.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the SQUID magnetometer according to the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a block diagram showing one embodiment of the SQUID magnetometer of the present invention. The SQUID magnetometer supplies an oscillation output voltage of an oscillator 1 to an amplitude adjuster 2 to adjust a signal amplitude, and outputs a signal whose amplitude has been adjusted to a voltage-current converter 4.
AC bias current is obtained by supplying
It supplies to the SQUID ring 6a of D6. An offset adjuster 5 for making the upper and lower waveforms symmetrical to each other.
Is also supplied to the voltage-current converter 4.

The SQUID from the SQUID ring 6a
The ID output signal is amplified by the preamplifier 7 and supplied to the non-inverting input terminal of the differential amplifier 8. Furthermore, oscillator 1
Is supplied to the amplitude adjuster 10 to adjust the signal amplitude, and the signal whose amplitude has been adjusted is supplied to the inverting input terminal of the differential amplifier 8. Then, the differential amplifier 8 outputs a difference signal (a signal from which the offset voltage has been removed) between the output signal from the preamplifier 7 and the signal whose amplitude has been adjusted by the amplitude adjuster 10. This difference signal is detected by the oscillation output voltage of the oscillator 1 being supplied to the other input terminal of the lock-in amplifier 9 being supplied to one input terminal, thereby detecting the difference signal and removing the AC component caused by the AC bias. SQ
A signal proportional to the input magnetic flux Φ1 to the UID ring 6a is obtained. Then, the output from the lock-in amplifier 9 is supplied to the integrator 11 to obtain the output of a phase lock loop (FLL) circuit.

The output from the integrator 11 is supplied to the voltage-current converter 12 to obtain a feedback current,
It is supplied to the feedback coil 6b of ID6. Next, the operation of the SQUID magnetometer having the above-described configuration will be described with reference to the waveforms of the respective sections shown in FIG. The oscillation output voltage of the oscillator 1 (see FIG. 2A) is supplied to the amplitude adjuster 2 to adjust the signal amplitude, and the signal whose amplitude has been adjusted and the output signal from the offset adjuster 5 are subjected to voltage-current conversion. Vessel 4
AC bias current is obtained by supplying
Since the power is supplied to the SQUID ring 6a of D6, FIG.
As shown in the middle (F) and (G), positive and negative Φ /
V characteristics are obtained.

In this state, if the input magnetic flux Φ1 is supplied to the SQUID ring 6a as shown in FIG. 2H, the SQUID is determined based on the positive and negative Φ / V characteristic points corresponding to the input magnetic flux Φ1. The output signal is determined and the preamplifier 7
, A signal waveform shown in FIG. 2B is obtained. In addition, by supplying the output signal from the oscillator 1 to the amplitude adjuster 10 to adjust the signal amplitude, FIG.
The signal waveform shown in the middle (C) is obtained. Then, by supplying the signal waveform shown in FIG. 2B and the signal waveform shown in FIG. 2C to the differential amplifier 8, the offset voltage at the lock point is removed. )reference}.
Further, by supplying the signal from which the offset voltage has been removed to the lock-in amplifier 9, the AC waveform caused by the AC bias is removed, and the input magnetic flux Φ1 to the SQUID 6 is removed.
(See (E) in FIG. 2). Then, this signal is supplied to the integrator 11 to obtain an FLL circuit output. Further, the output of the FLL circuit is a voltage −
The current is converted into a feedback current by the current converter 12,
It is supplied to the feedback coil 6b of the SQUID 6. Therefore, the input magnetic flux Φ1 supplied to the SQUID ring 6a is canceled by the magnetic flux generated from the feedback coil 6b due to the feedback current.

By repeating the above operation, the input magnetic flux Φ1 can be measured. SKU with the above configuration
The ID magnetometer uses a conventional SQUID
The number of magnetometers can be reduced from three to two. Compared with a conventional SQUID magnetometer, although a lock-in amplifier needs to be provided, it is necessary to supply an AC current to an inversion buffer and a feedback coil. Since the components can be omitted, the configuration of the entire SQUID magnetometer can be simplified.

FIG. 3 is a block diagram showing another embodiment of the SQUID magnetometer of the present invention. The SQUID magnetometer supplies the oscillation output voltage of the oscillator 1 to the amplitude adjuster 2 to adjust the signal amplitude, and supplies the amplitude-adjusted signal to the voltage-current converter 4 to obtain an AC bias current. , SQUID 6 to the SQUID ring 6a. The output signal from the offset adjuster 5 for making the upper and lower waveforms symmetrical to each other is also supplied to the voltage-current converter 4.

The SQUID from the SQUID ring 6a
The ID output signal is amplified by the preamplifier 7 and the oscillator 1
Is supplied to the other input terminal of the lock-in amplifier 9 which is supplied to one input terminal, thereby detecting and outputting a detection signal. Then, the detection signal is supplied to one input terminal of the voltage adder 13. The output signal from the offset adjuster 14 (a signal obtained by inverting the sign of the DC voltage output at the lock point) is supplied to the other input terminal of the voltage adder 13. Further, an output signal from the voltage adder 13 is supplied to the integrator 11 to obtain a phase locked loop (FLL) circuit output.

The output from the integrator 11 is supplied to the voltage-current converter 12 to obtain a feedback current,
It is supplied to the feedback coil 6b of ID6. Next, the operation of the SQUID magnetometer having the above-described configuration will be described with reference to the waveforms of the respective sections shown in FIG. The oscillation output voltage of the oscillator 1 (see FIG. 4A) is supplied to the amplitude adjuster 2 to adjust the signal amplitude, and the signal whose amplitude has been adjusted and the output signal from the offset adjuster 5 are subjected to voltage-current conversion. Vessel 4
AC bias current is obtained by supplying
Since the power is supplied to the SQUID ring 6a of D6, FIG.
As shown in the middle (F) and (G), positive and negative Φ /
V characteristics are obtained.

In this state, if the input magnetic flux Φ1 is supplied to the SQUID ring 6a as shown in FIG. 4H, the SQUID is determined based on the positive and negative Φ / V characteristic points corresponding to the input magnetic flux Φ1. The output signal is determined and the preamplifier 7
, A signal waveform shown in FIG. 4B is obtained. The output signal from the preamplifier 7 is supplied to the lock-in amplifier 9 to be detected, and an AC component caused by an AC bias is removed (see (C) in FIG. 4).

Thereafter, the offset adjuster 14 outputs a signal obtained by inverting the sign of the DC voltage previously output at the operating point {see FIG. 2D}, and the voltage adder 13 9 and the output signal from the offset adjuster 14 to remove the offset voltage at the lock point,
A signal proportional to the input magnetic flux Φ1 to ID6 is detected {see FIG. 4 (E)}. Then, this signal is supplied to the integrator 11 to obtain an FLL circuit output. Also, F
The output of the LL circuit is converted into a feedback current by the voltage-current converter 12 and supplied to the feedback coil 6b of the SQUID 6. Therefore, the input magnetic flux Φ1 supplied to the SQUID ring 6a is canceled by the magnetic flux generated from the feedback coil 6b due to the feedback current.

By repeating the above operation, the input magnetic flux Φ1 can be measured. SKU with the above configuration
The ID magnetometer uses a conventional SQUID
The number of magnetometers can be reduced from three to two. Compared with a conventional SQUID magnetometer, although a lock-in amplifier needs to be provided, it is necessary to supply an AC current to an inversion buffer and a feedback coil. Since the components can be omitted, the configuration of the entire SQUID magnetometer can be simplified.

[0030]

According to the first aspect of the present invention, the configuration for supplying an AC current to the feedback coil and the configuration for inverting the sign of the AC bias can be omitted, and the overall configuration can be simplified. This has a specific effect that the number of adjustment points can be reduced.

The second aspect of the invention has the same effect as the first aspect. The third aspect of the invention has the same effect as the first aspect.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of a SQUID magnetometer of the present invention.

FIG. 2 is a diagram showing waveforms at various parts of the SQUID magnetometer of FIG. 1;

FIG. 3 is a block diagram showing another embodiment of the SQUID magnetometer of the present invention.

FIG. 4 is a diagram showing waveforms at various parts of the SQUID magnetometer of FIG. 3;

FIG. 5 is a block diagram showing a configuration of a conventional SQUID magnetometer.

FIG. 6 is a diagram showing waveforms at various parts of the SQUID magnetometer of FIG. 5;

[Explanation of symbols]

 6 SQUID 8 Differential amplifier 9 Lock-in amplifier 10 Amplitude adjuster 13 Voltage adder 14 Offset adjuster

Claims (3)

[Claims] An SQUID magnetometer that supplies an AC current as a bias current to a SQUID (6) and drives the SQUID (6) without performing magnetic flux modulation, as an AC signal due to the AC bias current. Removing the AC component and the offset voltage caused by the AC bias from the output SQUID output signal,
(6) signal processing means (8) (9) (10) (13) (14) for obtaining a signal proportional to the input magnetic flux, and a feedback signal to be fed back to the SQUID by integrating the obtained signal. SCU including feedback signal calculating means (11) and (12) for calculating
ID magnetometer. 2. The signal processing means (8), (9) and (10).
Means for comparing a SQUID output signal output as an AC signal due to an AC bias current with a signal amplitude at a fixed magnetic flux point to calculate a difference; 2. The SQUID magnetometer according to claim 1, further comprising a detection means (9) for obtaining a detection signal by performing lock-in detection at the same frequency as the current. 3. The signal processing means (9), (13) and (1)
4) a detecting means (9) for lock-in detecting a SQUID output signal output as an AC signal due to the AC bias current at the same frequency as the AC bias current to obtain a detection signal; 2. The SQUID magnetometer according to claim 1, further comprising comparison means (13) and (14) for calculating a difference by comparing with the signal amplitude in (1).