JPH04136775A - Dc squid fluxmeter - Google Patents

Abstract

PURPOSE:To simply set a proper bias current by detecting the even number higher harmonic component of the voltage generated at an output terminal by the modulated magnetic field applied to an SQUID. CONSTITUTION:The output of an amplifier 2 is inputted to a synchronous detection circuit 72 operated by frequency twice the frequency of a modulated wave and, further, the double frequency detection output 92 showing the magnitude of a double frequency component is obtained from said output from a high-pass interruption filter 82. A numeral 61 shows a circuit generating double frequency from the output of an oscillator 6. When the magnitude of the output current I0 of a current source 5 is set by a current setting means 10 so that the output of the filter 82 becomes the max. or predetermined value, a bias current can be automatically set to the optimum value. By this method, adjustment becomes easy and reliability is enhanced.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a DC-SQUID magnetometer, particularly a DCSQ
6 regarding high sensitivity and stable operation of UID magnetometer

[Conventional technology]

SQUID (Superconducting Clarontum Interference Device...Super)
conducting Quantum I nte
rferenceDevice) is a highly sensitive magnetometer that utilizes the superconducting Josephson effect. among them,
The DC-8QUID is widely used due to its low noise, for example in the Journal of Low Temperature.
urePhysics), Vol. 25. Nos, 1/
2. , pp99°1976. This DC-
The SQUID is a superconducting ring including two Josephson elements as shown in FIG. Here, the relationship between the current flowing in parallel through the ring and the terminal voltage ■ is shown in the same figure (b).
As shown in , it changes depending on the ring flux linkage Φ, so
When biased with a constant current Ib, the terminal voltage becomes a periodic function with respect to the magnetic flux, as shown in FIG. 10(b). This period is a magnetic flux quantum Φo (2X 10-1 SWb), and the relationship between the interlinkage flux and the terminal voltage is called the ■-Φ characteristic. The slope of this ■-Φ characteristic becomes the sensitivity to the input magnetic flux. Moreover, the -Φ characteristic changes depending on the bias current 1b, as shown in FIG. 1o(c). For this reason, the bias current Ib is set so as to obtain the maximum V-Φ characteristic.

[Problem to be solved by the invention] In the above conventional technology, it is necessary to set the optimum bias current 1b 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. Bias current Ib was set so that the output would be maximized. Therefore, there was a problem that this setting work was complicated and took an hour. Furthermore, when an abnormality occurs in the SQUID's operation, sensitivity decreases, but it is not possible to determine whether the input signal has decreased or the SQUID is abnormally operating.
The prior art did not give any consideration to ensuring reliability when used as a magnetometer, and there was a problem with reliability during use. The present invention has been made in view of the above circumstances, and one purpose thereof is to provide a SQUID magnetometer that can easily set an appropriate bias current Ib. Another object of the present invention is to provide a SQUID magnetometer that can easily detect abnormalities in operation. Furthermore, 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 objects of the present invention are achieved by detecting even harmonic components of the voltage generated at the output terminal by the modulated magnetic field applied to the SQUID.

[Effect]

In a 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 modulated magnetic field are V regardless of the presence or magnitude of the input signal.
- Since it depends on the Φ characteristic, by detecting this even harmonic component, the optimum bias current can be set and the SQUI
It is possible to detect abnormalities in the operation of D.

【Example】

Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 2 is a diagram explaining the present invention in detail. In FIG. 2, the V-Φ characteristic is approximated by a broken line to facilitate analysis. A sine wave having an amplitude of 2φ1 is applied to this ■-Φ characteristic as a modulation signal. Furthermore, if there is an input magnetic field φI, the terminal voltage V of the SQUID. As shown in the figure, the waveforms are half-waveforms with alternating amplitudes. At this time,
Approximating each half-wave waveform as a 1/2 period of a sine wave with different amplitude, and using the Fourier expansion formula of the output voltage Vo(t
) is expressed as equation (1). Further, the double frequency component voz(t) is expressed by the following equation. Here, K is a coefficient, which indicates the slope of the -Φ characteristic, that is, the sensitivity to the input magnetic flux. Further, ω is the angular velocity of the modulated wave. First, it can be seen from equation (1) that the same fundamental frequency component as the modulated wave is proportional to the input magnetic flux φ1n. Therefore, the input magnetic flux is usually detected by amplifying this fundamental wave component and performing phase detection in synchronization with the modulated magnetic flux. On the other hand, if we pay attention to the even harmonic components, especially the double frequency components, this component is constant regardless of the input magnetic field, and is proportional to the coefficient and the magnitude φ of the modulated magnetic flux. Here, φ1 can be given constant, and K is ■
Since it is a coefficient proportional to the slope of the -Φ characteristic, that is, the sensitivity, the state 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 ring 1 of the SQUID, which is made up of two Josephson junctions connected in parallel by a superconducting wire, and the magnetic flux measured by the detection coil 3-1 is transmitted to the input coil 3-1 of the SQUID. It is generated again and linked to 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. Also, bias current is supplied by current source 5. is SQUID
given to. The terminal voltage of the SQUID is amplified by the amplifier 2, and the fundamental wave component of the modulated wave is detected by the synchronous detection circuit 71 and the high-frequency cutoff filter 81, so that an output corresponding to the input signal can be obtained. Furthermore, this output is fed back to the SQUID using a feedback coil to keep the magnetic field inside the SQUID constant. Since the feedback signal is proportional to the input magnetic flux, the output 91 can be obtained. The above is a conventional method, but by adding the following means to it, the object of the present invention can be achieved. The output of amplifier 2 has a frequency 2f which is twice the frequency fc of the modulated wave.
The signal is input to a synchronous detection circuit 72 operating at c. Further, the output is passed through a high frequency cutoff filter 82 to obtain a double frequency detection output 92 indicating the magnitude of the double frequency component. In addition,
61 is a circuit that generates a frequency double from the output of the oscillator 6. The output of this filter 82 is the coefficient term (4 in φ
, /3π). Therefore, when this output is at its maximum, the sensitivity of the SQUID is at its maximum, and therefore, the output current of the current source 5 is adjusted by the current setting means 10 so that this output becomes the maximum or a predetermined value. By setting the magnitude of , the bias current can be automatically set to the optimum value. Next, a second embodiment is shown in FIG. In this figure, the double frequency detection output 92 shown in FIG. 1 is provided with a level discrimination circuit 11 for discriminating its level. Specifically, this level discrimination circuit may be a comparator, and detects whether or not the double frequency detection output 92 becomes below the threshold value VTH, and when it becomes below the threshold value, it generates a warning signal as an abnormality in the operation of the SQUID. This is what I did. Next, the current setting means 10 shown in FIGS. 1 and 3
A first specific example of the configuration is shown in FIG. In the figure, a double frequency detection output 92 representing the magnitude of the double frequency component is converted into a digital value by an A/D converter 110,
The data is put into register R(A) 121. With the next clock, the data in register R(A) 121 is sent to register R(B) 122, and at the same time, the data in register R(A) 121 is sent to register R(B) 122.
21 stores the following data. A comparator 130 compares this data A with data B one clock ago. Here, it is assumed that this comparator 130 outputs It 1 )l when A<B. Furthermore, this comparator output drives a flip-flop 140, and this flip-flop 14
The output of 0 controls whether the up/down counter 150 counts up or down. Moreover, the data of this counter 150 becomes an input signal of the D/A converter,
It is assumed that the signal is converted into an analog value and becomes an input signal to the current source 5 shown in FIG. 1 or 3, and the current source 5 generates a current proportional to this signal. Note that 170 is a clock generation circuit for supplying tarock to each circuit. Here, the present current setting means will be explained in more detail. FIG. 5 shows the operation of each circuit in FIG. 4. First of all, registers 121, 1221. It is assumed that the flip-flop 140 and the up/down counter 150 have been cleared and are in an initial state. Here, up/
The down counter 150 counts up when the control signal is 1 (OI+), and counts down when the control signal is rr I I+. Initially, the output of the flip-flop 140 is 11 Q I+, so the up/down counter 150 counts up. , the output of the D/A converter increases accordingly. The bias current of the SQUID also increases in proportion to this. At this time, the value of the register is A>B, so the output of the comparator 130 is '○'' As it is, the output of the flip-flop 140 also remains at 110 I+.However, as the bias current increases and the sensitivity of the SQUID reaches its maximum, that is, the magnitude of the double frequency component reaches its maximum, it begins to decrease. The register output becomes A<B and the comparator output is 1
As a result, the output of the flip-flop 140 becomes rr 1 u, and the up/down counter 150 starts counting down. This output changes the D/A converter output and reduces the SQUID bias current. The frequency component increases again, and the output of the register becomes A>B, and the comparator output becomes "0".At the next clock, the bias current decreases, so the double frequency component decreases, and A<B, and the comparator output becomes "0". becomes "1" again, the output of the flip-flop 140 becomes LIOH, and the up/down counter 150 starts counting up.After this, the bias current increases again, and the above-mentioned operation is repeated. .Therefore, D/A
The output of the converter will converge near the maximum double frequency component, that is, near the maximum sensitivity. Therefore, if the input data of the D/A converter is fixed, such as by stopping the clock at the stage of convergence, the bias current of the SQUID can be automatically set to around the maximum sensitivity. Note that in FIG. 4, the register 12' may be considered as an output latch included in the A/D converter. Next, a second specific example of the configuration of the current setting means 10 is shown in FIG. In the figure, the above operation is performed in an analog manner without using an A/D converter. First, the frequency doubler detection output 92 is output from the sample/hold circuit (S/H) 11.
It is sampled at 1 and its value is held. With the next clock, the data in the S/H 111 is sampled by a sample/hold circuit (S/H) 112 and its value is held, and at the same time, the next data is sampled by the sample/hold circuit (S/H) 111. This data A is compared with data B one clock ago by an analog comparator 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 shown in FIG. The operation can also be considered in the same way as explained in FIG. Since this current setting means does not require an A/D converter or a register, the circuit becomes simple and power consumption can be reduced. The above describes a method of automatically setting the bias current so that the double frequency component is maximized, that is, the sensitivity of the SQUID is maximized. But because. When making measurements using multiple SQUID magnetometers, it may be more convenient for all SQUID magnetometers to have the same sensitivity than for each SQUID magnetometer to be adjusted to its maximum sensitivity. be. Therefore, an embodiment in which the sensitivity is automatically adjusted to a constant level will be described below. FIG. 7 shows a specific example of the configuration of the current setting means 10 for automatically adjusting the above-mentioned constant sensitivity. The operation will be explained according to the figure. Double frequency detection output 92 is A/
The D converter 110 converts the data into a digital value, and the data is temporarily held in the register R(A) 121. This data A and the threshold value TH generated by the threshold value generation circuit 123
The comparator 130 compares the two. Here, it is assumed that this comparator 130 outputs LL I TT when A>TH. Furthermore, the up/down counter 15 is output from this comparator.
Controls whether to count up or down to 0. Further, the data of this counter 150 becomes an input signal of the D/A converter, is converted into an analog value, and becomes an input signal of the current source 5 shown in FIGS. 2 and 3. Others are 4th
It is similar to the figure. Here, the present current setting means will be explained in more detail. FIG. 8 shows the operation of each circuit in FIG. 7. First, it is assumed that the register 121 and the up/down counter 150 have been cleared and are in an initial state. Here, since the bias current is initially not flowing or has a small value, A<TH. Therefore, since the output of the comparator 130 is 110++, the up/down counter 150 counts up, and accordingly the D/down counter 150 counts up.
The output of the A converter increases. In proportion to this, SQU
The ID bias current also increases. However, when the bias current increases, the sensitivity of the SQUID increases, and the magnitude of the double frequency component exceeds the threshold, A>TH and the comparator output becomes “
1". This causes the up/down counter 150
turns into a countdown. 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 IIO++, and the up/down counter 150 starts counting up. Again after this)
The Iasumi flow will increase, and the operations described above will be repeated. Therefore, the double frequency component of the output of the D/A converter converges around a predetermined threshold value. Therefore, by fixing the input data of the D/A converter, such as by stopping the clock at the stage of convergence, it becomes possible to automatically set the SQUID's 1<Iasumi flow so that the sensitivity is almost constant. Note that in this case as well, the register 121 may be considered as an output latch included in the A/D converter, as in the case shown in FIG. Next, FIG. 9 shows another specific example of the configuration of the current setting means 10 for automatically adjusting the sensitivity to a constant level. In this figure, like in FIG. 6, the above operation is performed in an analog manner without using an A/D converter. First, the frequency doubler detection output 92 is a sample/hold circuit (S/
H) It is sampled at 111 and its value is held. This data A and an analog threshold value generated by the threshold value generation circuit 113 are compared by an analog comparator 131. The up/down counter 15 uses the output of this comparator 131.
Controlling whether 0 is a count up or a count down is the same as in Figure 7, and the operation of each circuit is also controlled in the same way as in Figure 8.
You can think of it in the same way as explained in the figure. Since this current setting means does not require an A/D converter or a register, the circuit is simplified and power consumption can be reduced, as in FIG. 6. Although specific configuration examples of the current setting means have been shown above in connection with the first and second embodiments, it is clear that similar operations can be performed using a calculator such as a microcomputer. [Effects of the Invention] As described above in detail, according to the present invention. The sensitivity of the SQUID magnetometer can be automatically set to maximum or constant without using any pseudo signals. Also,
Even when using a SQUID magnetometer, it is possible to detect more than just the SQUID operation. This has the effect of facilitating adjustment and improving reliability.

[Brief explanation 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 diagram showing the first embodiment of the current setting circuit. Figure 5 shows the configuration of the
6 is a diagram showing the second specific configuration of the current setting circuit, FIG. 7 is a diagram showing the third specific configuration of the current setting circuit, and FIG. 8 is a diagram showing the second specific configuration of the current setting circuit. 9 is a diagram illustrating the fourth specific configuration of the current setting circuit,
FIG. 10 is a diagram illustrating a conventional SQUID magnetometer. 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 cutoff filter, 10... Current setting means.

Claims (1)

[Claims] 1. DC SQ including at least two Josephson junctions
a UID, a current source for applying a bias current to the SQUID, means for applying a modulated magnetic field signal to the SQUID;
A DC SQUID comprising means for detecting even harmonic components of the terminal voltage of the QUID, and current setting means for controlling the current value of the current source so that the even harmonic components are maximized. Magnetic flux meter. 2. DC SQ including at least two Josephson junctions
A direct current comprising a UID, a current source for applying a predetermined bias current to the SQUID, and means for applying a modulated magnetic field signal to the SQUID, and further comprising means for detecting an even harmonic component of the terminal voltage of the SQUID. SQUID magnetometer. 3. In the DC SQUID magnetometer according to claim 2,
A DC SQUID magnetometer, further comprising current setting means for controlling the current value of the current source so that the even harmonic components have a predetermined value. 4. The DC SQUID magnetometer according to claim 1, 2 or 3, further comprising a level determination circuit for determining whether the even harmonic component exceeds a predetermined value. . 5. The current setting means samples the amplitude or effective value of the even harmonic components of the terminal voltage of the SQUID, determines whether the even harmonic components tend to increase or decrease from the previous and subsequent sampling data, and based on the determination result. The DC SQUI according to claim 1, 2, 3, or 4, wherein the DC SQUI generates a control signal for controlling the current source.
D magnetometer. 6. The current setting means samples the amplitude or effective value of the even harmonic components of the terminal voltage of the SQUID, and increases the current of the current source until the sampling data reaches a predetermined value, so that the predetermined value is reached. 2. A control signal is generated for controlling the current source so as to reduce the current of the current source when the current exceeds the current value.
, 2, 3 or 4.