JP3387445B2 - SQUID operating point adjustment device and method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device and method for adjusting the operating point of a superconducting quantum interference device (hereinafter abbreviated as SQUID), and more particularly to an SQU capable of automatic adjustment.
The present invention relates to an ID operating point adjustment device and method.

[0002]

2. Description of the Related Art FIG. 7 shows a conventional magnetic flux lock loop (hereinafter, referred to as
It is a circuit diagram which shows the control circuit of SQUID which has FLL. Referring to FIG. 7, a conventional FL for SQUID
The L circuit includes an SQUID 81 in which two Josephson junctions are formed at a predetermined position, and a constant current source 88 that supplies a constant current to the SQUID 81. A magnetic flux to be measured is input to the SQUID 81 from a pickup coil (not shown). The output voltage from both ends of the SQUID 81 is transformed by a transformer, amplified by a preamplifier 83, and output as an output voltage via a multiplier 84 and an integrator 85. A 40 kHz modulation signal is added to the output from the multiplier 84 by the modulator 87, and the output voltage is fed back to the magnetic field applying coil 82 adjacent to the SQUID 81. As a result, the external magnetic flux detected by the SQUID 81 is canceled.

A part of the output from the SQUID 81 is input to the X axis of the oscilloscope 90, and the output from the preamplifier 83 is input to the Y axis of the oscilloscope 90.

[0004]

The adjustment of the operating point of the conventional SQUID control circuit has been performed as described above. Oscilloscope 9 with a predetermined bias current Ib flowing
Since it is necessary to adjust 0 so that the amplitude becomes maximum, there is a problem that it takes a very long time to perform the adjustment before use. Also, because it had to be adjusted manually,
There is a problem that the SQUID cannot be automatically adjusted. The present invention has been made in order to solve the above problems, and provides an SQUID operating point adjustment device and a method thereof, which improves the accuracy of SQUID adjustment and enables automatic adjustment. With the goal.

[0005]

[Solving Means] A SQUI according to the present invention
The operating point adjusting device of D applied the SQUID, a means for supplying a bias current to the SQUID, and a means for applying an AC magnetic field of a predetermined frequency to the SQUID while the bias current was supplied . Output of SQUID modulated by magnetic field
Means for extracting a half cycle of the magnetic flux dependence of
Based on the output corresponding to the extracted magnetic field change, the via
Of the bias current of the bias current.
The means for obtaining the optimum value is the output of the extracted SQUID.
Predetermined frequency from half cycle of magnetic flux dependence
The output voltage of the frequency component that is twice the number and the predetermined frequency
Including the filter means to output the maximum bias current
The appropriate value is fed back to the means for passing the bias current.
Means are further included.

The SQUID operating point adjusting method includes a step of applying a bias current to the SQUID and a step of applying a bias current to the SQUID at a predetermined frequency of a predetermined frequency.
Applying an alternating magnetic field and changing the applied magnetic field
Based on the step of extracting a half cycle of the period in the magnetic flux dependence of the adjusted SQUID output, and the output of the extracted SQUID based on the output corresponding to the change of the extracted magnetic field .
Predetermined frequency from half cycle of magnetic flux dependence
The output voltage of the frequency component that is twice the number and the predetermined frequency
Out, and obtaining an optimum value of the bias current, S
The optimum value of the obtained bias current is fed to the QUID.
Flowing a biased bias current .

Since the magnetic field is generated in the SQUID while the bias current is flowing and the optimum value of the bias current is obtained based on the output corresponding to the change of the magnetic field, the optimum value of the bias current can be automatically obtained. . Further, the accuracy of adjusting the bias current is improved.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a SQUID according to the present invention.
FIG. 3 is a block diagram showing the overall configuration of the bias current adjusting device of FIG. Referring to FIG. 1, the bias current adjusting device is SQ
UID10, a bias current supply circuit 11 that applies a bias current to the SQUID 10, a coil 12 that applies an AC magnetic field of frequency f to the SQUID 10, and a SQUI
A voltage amplifier 13 that extracts and amplifies the voltage from both ends of D10, a filter 14 that passes only a predetermined frequency component from the amplified voltage, a rectifier 15 that rectifies a signal of the predetermined frequency component, and a rectified voltage And a voltage monitor 16 for monitoring. In a device using an actual SQUID, the output of the voltage amplifier 13 is connected to the multiplier 84, the multiplier 30 corresponding to the integrator 85, and the integrator 31 shown in FIG. 7 to form a magnetic flux lock loop. Although the measurement of the magnetic field is performed by this method, the actual measurement system is omitted here, and only the initial adjustment will be described.

FIG. 2 shows a SQUID 1 to which the present invention is applied.
It is a schematic diagram which shows the whole 0 structure. Referring to FIG. 2, S
The QUID 10 is a washer type of 5 mm or less. A 7-mm square one-turn coil 12 is patterned around it. These are formed on the chip carrier 18. A 40 kHz sine wave current is passed through the coil 12 to generate a magnetic field. The amplitude of the magnetic field is a half cycle of the magnetic field-voltage characteristic. At this time, the voltage generated from the SQUID 10 is filtered by the above-described configuration and converted into a DC voltage, and the bias current is adjusted so as to obtain its maximum value.

FIG. 3 is a graph showing the magnetic field-voltage characteristics of the generated magnetic field. (A) shows a state of the generated magnetic field, and (B) shows a state in which a desired half cycle is taken out from the state. Here, it has been stated that a desired half cycle is taken out, but this means that even if the same current is applied, the cycle differs depending on the SQUID element. Therefore, by adjusting the strength of the magnetic field, the desired half cycle can be obtained. Generates a magnetic field equivalent to minutes.

In (A), the bias current is adjusted so that the maximum value Vpp of the amplitude becomes maximum.

Next, a method of adjusting the bias current will be described. FIG. 4 shows the magnetic field for the half period shown in FIG.
It is a figure which shows a voltage characteristic, (B) is a figure which shows the time change of the magnetic field based on the signal, (C) is a figure which shows the time change of a voltage.

In FIG. 4 (A) showing a state where a half cycle is extracted from the magnetic field amplitude, it is assumed that the half cycle indicated by A is extracted. Here, since the half cycle A is taken out from an arbitrary state, the maximum value and the minimum value are not necessarily included in the half cycle. In the extracted half cycle A, the voltage changes from the state shown by to the state shown by and then returns to the state shown by the arrow. This is repeated. A view showing the temporal change of the magnetic field from this state is shown in (B). As shown in (B),
In this case, one cycle of magnetic field fluctuation occurs, and this cycle is equal to the cycle f = 40 kHz of the sinusoidal current flowing through the coil 12.

FIG. 6 (C) shows the voltage change with time based on (A) and (B). In (C), the temporal change of the half cycle A is represented by A in the figure. That is, the frequency f1 is a mixture of the frequency f of the applied sine wave current and the frequency 2f which is twice the frequency f. This waveform is the waveform at point C in FIG. Only the signal having the waveform of the mixed frequency component is taken out by the filter 14. When this waveform is rectified by the rectifier 15, the high frequency component is removed and only the DC component is obtained, and the waveform shown in FIG. The bias current supply circuit 11 is manually adjusted while observing the voltage monitor 16 so that this DC component, that is, the output voltage V becomes maximum.

Next, a modified example of this embodiment will be described. FIG. 5 is a block diagram showing an SQUID bias current adjusting device according to a modification of the present invention. Referring to FIG. 5, it is basically similar to the SQUID bias current adjusting device shown in FIG. The difference is that the output from the rectifier 15 is converted into a digital signal by the A / D converter 21, and the personal computer 22 automatically feeds back the bias current supply circuit 11 so that the output voltage V becomes maximum.
The point is that the optimum bias current is made to flow. In this case, a monitor 23 is provided on the personal computer 22.

In this modification, the bias current can be automatically adjusted by the personal computer 22. Next, a further modified example of the present invention will be described. FIG. 6 is a block diagram showing an SQUID bias current adjusting device according to a further modification of the present invention. Basically, referring to FIG.
This is the same as the SQUID bias current adjusting device shown in FIG. The different points are that the bias current supply circuit 11 is composed of a power supply circuit 24 and a voltage holding circuit 25, and that the output from the rectifier 15 is a differentiation circuit 26 and a sign inversion detection circuit 27.
This is the point where feedback is performed to the voltage holding circuit 25 via.

That is, E (t) = E0 in the power supply circuit 24.
The voltage signal of t is applied. This signal has a waveform whose voltage rises over time. This voltage is applied to SQUID 10 through resistor 28. As the bias current increases, the voltage from the rectifier 15 increases, peaks at the optimum point, and then decreases. That is, the amount of change changes from positive to negative. If the sign of the voltage of the rectifier 15 passes through the differentiating circuit 26 and the sign becomes negative (the voltage decreases), the voltage holding circuit 25
A trigger signal is output to and the voltage value at that time is held.

In the case of this modification, unlike the previous embodiment, the bias current can be automatically set with the analog signal as it is.

The embodiments disclosed this time are to be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description but by the claims, and is intended to include meanings equivalent to the claims and all modifications within the scope.

[Brief description of drawings]

FIG. 1 is a block diagram showing a main part of an SQUID operating point adjustment device according to the present invention.

FIG. 2 is a schematic diagram showing an appearance of a SQUID to which the present invention is applied.

FIG. 3 is a graph showing a magnetic field-voltage characteristic.

FIG. 4 is a diagram for explaining an SQUID operating point adjustment method according to the present invention.

FIG. 5 is a block diagram showing a main part of an SQUID operating point adjustment device according to a modification of the present invention.

FIG. 6 is a block diagram showing a main part of an SQUID operating point adjustment device according to a further modification of the present invention.

FIG. 7 is a diagram for explaining a conventional SQUID operating point adjustment method.

[Explanation of symbols]

10 SQUID 11 Bias current supply circuit 12 coils 13 Voltage amplifier 14 filters 15 Rectifier 16 monitors 21 A / D converter 22 PC 23 monitors 24 power supply circuit 25 voltage holding circuit 26 Differentiating circuit 27 Sign inversion detection circuit 28 Resistance

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01R 33/035 ZAA

Claims (2)

(57) [Claims] 1. An SQUID, a means for supplying a bias current to the SQUID, a means for applying an alternating magnetic field of a predetermined frequency to the SQUID in a state where the bias current is supplied, and a modulation by the applied magnetic field. And a means for extracting a half cycle of the cycle in the magnetic flux dependence of the output power of the SQUID , and a means for obtaining the optimum value of the bias current based on the output corresponding to the change in the extracted magnetic field. The means for determining the optimum value of the bias current is
Half-cycle of the period in the magnetic flux dependence of the output of the generated SQUID
2 of the predetermined frequency and the predetermined frequency from the period
Includes filter means for extracting the output voltage of the double frequency component.
Look, the optimum value of the determined bias current, the bias
Further comprises means for feeding back current to the means
That, SQUID operating point adjustment device. 2. A step of applying a bias current to the SQUID.
And the bias current is applied to the SQUID
Applying an alternating magnetic field having a predetermined frequency, and outputting the SQUID modulated by the applied magnetic field.
A step for extracting a half period of the period in the magnetic flux dependence of the force
And the output corresponding to the change in the extracted magnetic field,
The frequency in the magnetic flux dependence of the output of the extracted SQUID
The specified frequency and the specified frequency from the half cycle
Extract the output voltage of the frequency component twice the wave number,
Determining the optimum value of the bias current, and calculating the optimum bias current for the SQUID.
The step of flowing the bias current whose value is fed back
A SQUID operating point adjusting method including: