JPH0660934B2 - DC SQUID magnetometer - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to a DC-SQUID magnetometer, and in particular to DC S
The present invention relates to high sensitivity and stable operation of a QUID magnetometer.

[Prior art]

SQUID (Super Conducting Quantum Interference Device ... Superconducting
Quantum Interference Device) is a highly sensitive magnetometer utilizing the superconducting Josephson effect. Among them, DC
-SQUID is widely used because of its low noise, for example, Journal of Row Template (J
ournal of Low Temperature Physics), Vol.25, Nos.1 /
2, pp99,1976. This DC-SQUI
D is a superconducting ring including two Josephson elements as shown in FIG. 10 (a), and is used for detecting the input magnetic flux linked to the ring. Here, since the relationship between the current I flowing through the rings in parallel and the terminal voltage V changes depending on the interlinking magnetic flux Φ of the ring as shown in FIG.
When biased with I _b , the terminal voltage becomes a periodic function with respect to the magnetic flux as shown in FIG. 10 (b). This cycle is a magnetic flux quantum Φ _o (2 × 10 ^-15 Wb), and the relationship between the interlinkage magnetic flux and the terminal voltage is called the V-Φ characteristic. The slope of this V-Φ characteristic becomes the sensitivity to the input magnetic flux. Further, the V-Φ characteristic changes with the bias current I _b as shown in FIG. 10 (c). Therefore, the bias current _Ib is set so that the maximum V-Φ characteristic is obtained.

[Problems to be Solved by the Invention]

In the above-mentioned conventional technology, it is necessary to set the optimum bias current I _b in order to obtain the maximum sensitivity, but no consideration is given to this setting method, and a pseudo signal is added to the input so that the output becomes maximum. The bias current _Ib was set.
Therefore, there is a problem that the setting work is complicated and takes time. Furthermore, when an abnormal operation of the SQUID occurs, the sensitivity decreases, but it cannot be determined whether the input signal has decreased or the operation of the SQUID is abnormal, and it is necessary to ensure the reliability when using it as a magnetometer. The prior art does not take into consideration at all, and there is a problem in reliability in use. The present invention has been made in view of the above circumstances, and one object thereof is to provide an SQUID magnetometer capable of easily setting an appropriate bias current I _b . Another object of the present invention is to provide an SQUID magnetometer capable of easily detecting an abnormal operation. Still another object of the present invention is to provide a configuration in which it is easy to make the sensitivities of a plurality of SQUID magnetometers uniform.

[Means for Solving the Problems]

The above object of the present invention is achieved by detecting the even harmonic component of the voltage generated at the output terminal by the modulating magnetic field applied to the SQUID.

[Action]

In the DC-SQUID magnetometer according to the present invention, the even harmonic components of the voltage generated at the terminals of the SQUID due to the modulation magnetic field depend on the V-Φ characteristic regardless of the presence or absence of the input signal or the magnitude thereof. The optimum bias current can be set and the SQUID operation abnormality can be detected by detecting the component.

【Example】

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 2 is a diagram for explaining the principle of the present invention. In FIG. 2, the V-Φ characteristic is approximated by a broken line to facilitate the analysis. A sine wave having an amplitude of 2φm is given to this V-Φ characteristic as a modulation signal. Further, if there is an input magnetic field of φ _in , the terminal voltage V _o of the SQUID becomes a half-wave waveform having different amplitudes alternately as shown in the same figure. At this time, each half-wave waveform is approximated as being a half cycle of a sine wave having a different amplitude, and the output voltage V _o (t) is represented by the equation of its Fourier expansion, as shown in equation (1). Further, the double frequency component V _o2 (t) is expressed by the following equation. Here, K is a coefficient and indicates the slope of the V-Φ characteristic, that is, the sensitivity to the input magnetic flux. Further, ω is the angular velocity of the modulated wave. First, from equation (1), it can be seen that the same fundamental frequency component as the modulated wave is proportional to the input magnetic flux φ _in . Therefore, the input magnetic flux is usually detected by amplifying the fundamental wave component and performing phase detection in synchronization with the modulation magnetic flux. On the other hand, paying attention to the even harmonic component, especially the double frequency component, this component is constant regardless of the input magnetic field, and is proportional to the coefficient K and the magnitude φ _m of the modulation magnetic flux. Here, φ _m can be given constant, and K is V−Φ
The slope of the characteristic, that is, the coefficient proportional to the sensitivity,
Eventually, the condition of the V-Φ characteristic can be known by detecting this double frequency component. FIG. 1 shows a first embodiment of the present invention based on the above principle. In the figure, an input coil 3-1 is coupled to an SQUID ring 1 composed of two Josephson junctions connected in parallel by a superconducting wire, and the magnetic flux measured by the detection coil 3-1 is the SQUID input coil 3-1. It occurs again and interlinks with ring 1. Further, an alternating current having a frequency fc generated by an oscillator 6 is passed through the modulation and feedback coil 4, so that a modulation magnetic field is applied from the coil 4 to the ring 1. Further, the bias current I _o is SQUID by the current source 5.
Given to. The terminal voltage of the SQUID is amplified by the amplifier 2, the fundamental wave component of the modulated wave is detected by the synchronous detection circuit 71 and the high frequency cutoff filter 81, and the output for the input signal can be obtained. Further, this output is fed back to the SQUID by the feedback coil so that the magnetic field in the SQUID is kept constant, and as a result, the feedback signal is proportional to the input magnetic flux, so that the output 91 can be obtained. . Although the above is the method that has been conventionally performed, the object of the present invention can be achieved by adding the following means to this method. The output of the amplifier 2 has a frequency 2f that is twice the frequency fc of the modulated wave.
It is input to the synchronous detection circuit 72 operating at c, and its output is further passed through a high frequency cutoff filter 82 to obtain a double frequency detection output 92 indicating the magnitude of the double frequency component. Reference numeral 61 is a circuit for generating a doubled frequency from the output of the oscillator 6. The output of the filter 82 will be proportional to (2) the coefficient terms (4Kφ _m / 3π). Therefore, the time when this output becomes maximum is the time when the sensitivity of the SQUID is maximum, and therefore, the output current I _o of the current source 5 is increased by the current setting means 10 so that this output becomes maximum or a predetermined value. By setting the height, the bias current can be automatically set to the optimum value. Next, a second embodiment is shown in FIG. In the figure, a level discriminating circuit 11 for discriminating the level is provided in the double frequency detection output 92 of FIG. This level discriminating circuit may specifically be a comparator, and outputs the double-frequency detection output 9 below the threshold V _TH.
It is detected whether or not 2, and a warning signal is generated as an operation abnormality of the SQUID when it becomes less than or equal to the threshold value. Next, the current setting means 10 shown in FIG. 1 and FIG.
FIG. 4 shows a first concrete configuration example of the above. In the figure, the double-frequency detection output 92 representing the magnitude of the double-frequency component is converted into a digital value by the A / D converter 110, and the data is stored in the register R (A) 121. The data of the register R (A) 121 is transferred to the register R (B) 122 by the next clock.
Then, the following data is stored in the register R (A) 121 at the same time. The data A and the data B one clock before are compared by the comparator 130. Here, it is assumed that the comparator 130 outputs "1" when A <B. Further, the output of the comparator drives the flip-flop 140, and the output of the flip-flop 140 controls whether the up / down counter 150 counts up or down. The data of this counter 150 is D
It becomes an input signal of the A / A converter, is converted into an analog value and becomes an input signal of the current source 5 shown in FIG. 1 or FIG. 3, and the current source 5 generates a current compared with this signal. . Reference numeral 170 is a clock generation circuit for supplying a clock to each circuit. Here, the current setting means will be described in more detail. The operation of each circuit in FIG. 4 is shown in FIG. First, it is assumed that the registers 121 and 122, the flip-flop 140, and the up / down counter 150 are cleared and are in an initial state. Here, the up / down counter 150 counts up when the control signal is “0” and counts down when the control signal is “1”. Since the output of the flip-flop 140 is "0" at first, the up / down counter 150 counts up, and the output of the D / A converter increases accordingly. The SQUID bias current also increases in proportion to this. At this time, since the register value is A> B, the output of the comparator 130 remains "0" and the output of the flip-flop 140 also remains "0". However, when the bias current increases and the sensitivity of the SQUID becomes maximum, that is, the magnitude of the frequency doubler component becomes maximum, then it starts to decrease. Then,
The output of the register is A <B and the comparator output is "1". As a result, the output of the flip-flop 140 is "1".
Therefore, the up / down counter 150 starts counting down. This output changes the output of the D / A converter and reduces the SQUID bias current. Then, the double frequency component increases again, the output of the register becomes A> B, and the comparator output becomes "0". At the next clock, since the bias current decreases, the double frequency component decreases, A <B, the comparator output becomes “1” again, the output of the flip-flop 140 becomes “0”, and the up / down counter 150 counts. Turn up. After this, the bias current will increase again, and the above-described operation will be repeated. Therefore, the output of the D / A converter converges near the maximum frequency component, that is, near the maximum sensitivity. Therefore, if the input data of the D / A converter is fixed, such as stopping the clock at the stage of convergence, the bias current of the SQUID can be automatically set near the maximum sensitivity. The register 121 in FIG. 4 may be considered as an output latch included in the A / D converter. Next, FIG. 6 shows a second specific configuration example of the current setting means 10. In the figure, the above operation is performed in an analog manner without using an A / D converter. First, the double frequency detection output 92 is the sample / hold circuit (S / H) 11
It is sampled at 1 and its value is held. Next, the data of the S / H 111 is sampled by the sample / hold circuit (S / H) 112 and its value is held by the clock, and at the same time, the next data is sampled by the sample / hold circuit (S / H) 111. The data A and the data B one clock before are compared by an analog comparison 131. The output of the comparator 131 drives the flip-flop 140, and the output of the flip-flop 140 controls whether the up / down counter 150 counts up or down, as in the case of FIG. The operation may be considered in the same manner as described with reference to FIG. Since the current setting means does not require an A / D converter or a register, the circuit is simplified and power consumption can be reduced. The above has described the method of automatically setting the bias current so that the double frequency component is maximum, that is, the sensitivity of the SQUID is maximum. However, when a large number of SQUID magnetometers are used for measurement, all SQUID magnetometers are adjusted to maximum sensitivity rather than being adjusted for each SQUID magnetometer.
It may be convenient for the D magnetometers to have the same sensitivity. Therefore, an example in which the sensitivity is automatically adjusted to a constant value will be described below. FIG. 7 shows a specific configuration example of the current setting means 10 for automatically adjusting to the above-mentioned constant sensitivity. The operation will be described with reference to FIG. Double frequency detection output 92
It is converted into a digital value by the D converter 110, and the data is temporarily held by the register R (A) 121. The comparator 130 compares the data A with the threshold TH generated by the threshold generation circuit 123. Here, this comparator 1
It is assumed that 30 outputs “1” when A> TH. Further, the output of this comparator controls whether the up / down counter 150 counts up or down. Also,
The data of the counter 150 becomes the input signal of the D / A converter and is converted into an analog value to become the input signal of the current source 5 shown in FIGS. 2 and 3. Others are the same as in FIG. Here, the current setting means will be described in more detail. The operation of each circuit in FIG. 7 is shown in FIG. First, the register 121 and the up / down counter 150
Has been cleared and is in the initial state. here,
At first, the bias current does not flow or is a small value, so that A <TH. Therefore, the comparator 130
Output is "0", so up / down counter 15
0 counts up, and the output of the D / A converter increases accordingly. The SQUID bias current also increases in proportion to this. However, the bias current increases and S
When the sensitivity of the QUID increases and the magnitude of the double frequency component exceeds the threshold value, A> TH and the comparator output becomes "1". This causes the up / down counter 150 to start counting down. This output changes the D / A converter output and reduces the SQUID bias current. Then, the double frequency component decreases, A <TH, the comparator output becomes “0”, and the up / down counter 15
0 starts counting up. After this, the bias current will increase again, and the above-described operation will be repeated. Therefore, the output of the D / A converter is such that the double frequency component converges near a predetermined threshold value. Therefore, if the input data of the D / A converter is fixed, such as by stopping the clock at the stage of convergence in this way, SQ
The bias current of the UID can be automatically set so that the sensitivity becomes almost constant. In this case, too, the register 121 may be considered as an output latch included in the A / D converter, as in the case of FIG. Next, another specific configuration example of the current setting means 10 for automatically adjusting to a constant sensitivity is shown in FIG. Similar to FIG. 6, this figure shows the above operation in an analog manner without using an A / D converter. First, the double-frequency detection output 92 is sampled by the sample / hold circuit (S / H) 111 and its value is held. The analog comparator 131 compares the data A with the analog threshold value generated by the threshold value generation circuit 113. The output of the comparator 131 is used as the up / down counter 150.
The control of whether to count up or count down is the same as in FIG. 7, and the operation of each circuit is also the same as in FIG.
It may be considered in the same way as explained in the figure. This current setting means does not require an A / D converter or a register, so that the circuit is simple and the power consumption can be reduced as in the case of FIG. The concrete configuration examples of the current setting means have been described above in relation to the first and second embodiments, but it is clear that this operation can be performed by using a computer such as a microcomputer.

【The invention's effect】

As described above in detail, according to the present invention, the SQUI
The sensitivity of the D magnetometer can be automatically set to the maximum or constant without using a pseudo signal. Also, SQUI
Even when using the D magnetometer, more than SQUID operation can be detected. Therefore, there is an effect that the adjustment is facilitated and the reliability is improved.

[Brief description of drawings]

FIG. 1 is a diagram showing the first embodiment, FIG. 2 is a diagram showing the principle of the present invention, FIG. 3 is a diagram showing the second embodiment, and FIG. 4 is a first concrete example of the current setting circuit. Fig. 5 is the fourth figure
FIG. 6 is a diagram for explaining the operation of the figure, FIG. 6 is a diagram showing a second specific configuration of the current setting circuit, FIG. 7 is a diagram showing a third specific configuration of the current setting circuit, and FIG. FIG. 9 is a diagram for explaining the operation of the figure, FIG. 9 is a diagram showing a fourth specific configuration of the current setting circuit, and FIG. 10 is a diagram for explaining a conventional SQUID magnetometer. DESCRIPTION OF SYMBOLS 1 ... SQUID ring, 2 ... Amplifier, 3 ... Input coil system, 4 ... Modulation and feedback coil, 5 ... Current source, 6 ... Oscillator, 61 ... Double frequency generation circuit, 71, 72 ... Synchronous detection circuit, 81, 82 ... Wide area cutoff filter, 10 ... Current setting means.

 ─────────────────────────────────────────────────── ─── Continuation of the front page Examiner Yoshiyuki Shimonaka (56) References JP-A-3-131781 (JP, A) JP-A-4-99980 (JP, A) JP-A-2-8762 (JP, A) JP-A-63-153484 (JP, A) JP-A-62-102174 (JP, A)

Claims (8)

[Claims] 1. A direct current SQUID including at least two Josephson junctions, a current source for giving a bias current to the SQUID, a feedback coil for giving a modulated magnetic field signal to the SQUID, and an amplifier for amplifying a terminal voltage of the SQUID. A circuit, a means for synchronously detecting the amplified signal and detecting the fundamental wave component of the modulated magnetic field signal by a high-frequency cutoff filter, and a direct current for feeding back the fundamental wave component to the feedback coil and obtaining an output proportional to the input magnetic flux. In the SQUID magnetometer, means for detecting an even harmonic component of the modulated magnetic field signal from the signal of the amplifier circuit between the amplifier circuit and the current source, and a signal for the even harmonic component is maximized. DC SQUID, characterized in that current setting means for controlling
Magnetometer. 2. A means for detecting the even harmonic components, a double frequency generating circuit for obtaining a frequency twice as high as the modulating magnetic field signal, and a synchronous detection circuit operating with a signal output from the double frequency generating circuit, The DC SQUI according to claim 1, comprising a high-frequency cutoff filter for filtering the output signal of the synchronous detection circuit.
D magnetometer. 3. The direct current according to claim 1, further comprising a level determination circuit for detecting an abnormality in SQUID and generating a warning signal, at the output of the means for detecting even harmonic components. SQUID magnetometer. 4. The current setting means, means for storing and comparing the output of the high-frequency cutoff filter based on a clock signal, a counter circuit for counting the clock signal according to a comparison result, and an output of the counter circuit. A DC SQUID magnetometer according to any one of claims 1 to 3, wherein the DC SQUID magnetometer comprises a D / A converter for converting the signal into an analog signal. 5. As the means for storing and comparing, the contents of the memories are compared with two memories for converting analog signals into digital signals based on the clock signal and storing data to be input in time series. The DC SQUID magnetometer according to claim 4, characterized in that 6. The means for storing and comparing as described above, wherein the means for converting an analog signal into digital and storing it and the contents of the memory and a predetermined value are compared. DC SQUID magnetometer. 7. The means for storing and comparing comprises: two sample and hold circuits operating with the clock signal; and an analog comparator for comparing the outputs of the sample and hold circuits. The DC SQUID magnetometer described in the fourth item. 8. The means for storing and comparing is a sample hold circuit operating with the clock signal, and an analog comparator for comparing an output of the sample hold circuit with a predetermined value. The DC SQUID magnetometer according to claim 4.