JPH08327713A - Superconducting magnetic sensor - Google Patents

Abstract

PURPOSE: To realize the more highly sensitive magnetic sensor, because the more larger output can be obtained for an input magnetic flux and the positive and negative states of the input magnetic flux can be detected by the oscillating states of two ROS circuits. CONSTITUTION: A first relaxation-oscillating type SQUID circuit (first ROS circuit) 3 and a second ROS circuit 6 are connected in series, thereby making the constitution where circuits independently oscillate in accordance with the positive and negative states of the input magnetic flux. Outputs 11 and 12 are taken out of the respective ROS circuits 3 and 6. The bias currents of SQUIDs 1 and 4 of the respective ROS circuits 3 and 6 are supplied to a superconducting coil in the assymetrical pattern, and the threshold-value characteristics become assymetrical.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-sensitivity magnetic sensor using a superconducting oscillation circuit using Josephson elements.

[0002]

2. Description of the Related Art FIG. 2A shows a conventional relaxation oscillation type SQU.
A circuit diagram of a magnetic sensor using an ID circuit (RELAXATION OSCILLATORSQUID circuit, hereinafter referred to as ROS circuit) is shown. The circuit consists of one ROS circuit. The circuit configuration is shunt coil 14 and shunt resistor 1 connected in series.
DC SQUID16 in parallel with shunt circuit consisting of 5
Is connected. For circuit operation, set the shunt resistor 15 to D
By making it sufficiently smaller than the normal resistance value of C SQUID16 and applying a bias current (Ib) sufficiently larger than the critical current value to both ends of DC SQUID16,
Always oscillates.

Φo entered from the input coil 8 and the feedback coil 7
The critical current value of the DC SQUID 16 changes periodically due to the magnetic flux of the cycle, and the oscillation frequency also changes accordingly.
The oscillation frequency is the magnitude of the bias current (Ib), the shunt resistance 15 (Rs) and the shunt coil 14 (Ls).
The ratio (Rs / Ls) also changes. Oscillation waveform is DC
It is detected as a voltage change across the SQUID 16.

Operating point 1 when used as a magnetic sensor
In the threshold characteristic of the SQUID shown in FIG. 2B, points A and B, which are linear regions, are used as 7. Operating point 17
The setting is performed by applying a direct current to the offset coil 13. The input magnetic flux has a constant bias current Ib in the SQUID.
Is applied and is constantly oscillated, and is detected as a change in frequency.

FIG. 2 (c) shows an example of the frequency spectrum when operated at points A and B in FIG. 2 (b). When the input magnetic flux is 0, it oscillates at the center frequency fo. When the input magnetic flux enters and the operating point changes to A + and B +, the frequency becomes f +
Change to. When the operating point changes to A- and B-, the frequency reversely changes to f-. As a result, the magnitude of the input magnetic flux can be detected as a change in frequency.

[0006]

The conventional ROS always operates in an oscillating state and detects the magnitude of the input magnetic flux as a change in frequency. However, in a normal design, the frequency change is about three times that of the external magnetic flux, and sufficient sensitivity cannot be obtained as a magnetic sensor. Further, the offset coil 13 was required to set the operating point.

An object of the present invention is to obtain a high-sensitivity magnetic sensor which has a large output with respect to an input magnetic flux and has improved sensitivity.

[0008]

In order to solve the above problems, the present invention has a circuit configuration in which two ROS circuits are connected in series and oscillate separately depending on whether the input magnetic flux is positive or negative. It was taken out of the circuit. In addition, each SQU
The bias current of the ID is asymmetrically supplied to the first coil, which is a superconducting coil, so that the threshold characteristic becomes asymmetric.

[0009]

In the circuit configured as described above, different ROS circuits oscillate according to the positive and negative of the input magnetic flux and a pulsed output is obtained, so the positive and negative of the input magnetic flux can be detected with high sensitivity. Since the threshold characteristic of the SQUID is asymmetrical, the operating point is automatically determined and the offset coil becomes unnecessary.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 shows a circuit diagram of a first embodiment of the present invention. While the conventional circuit of FIG. 2 has only one ROS circuit, in the present invention, two first ROS circuits 3 and two second ROS circuits 6 are connected in series.

In operation, the connection portion of the two ROS circuits is set to the ground of a common potential, and the first bias current 9 (Ib1) and the second bias current 10 (Ib2) are applied to each ROS circuit. First SQUID1 and second SQUID of each ROS circuit
The input coil 7 and the feedback coil 8 are magnetically coupled to the ID 4, respectively. When the operating points of the bias current and SQUID are set to appropriate values, the ROS circuit does not oscillate when the input magnetic flux is 0, and when the magnetic flux enters the input, each ROS circuit oscillates according to the positive or negative of the input magnetic flux. The magnitude of the input magnetic flux can be known from the oscillation output of each ROS circuit.

There are several types of operation modes, R
The SQUID that constitutes the OS circuit, the shunt circuit and the bias current, and the method of detecting the output differ, and many combinations are possible. Each circuit configuration and operation method will be described. First, the structure of the SQUID will be described. FIG. 3A is an SQU composed of two Josephson devices 20 having almost the same characteristics.
Bias current with ID is the first coil 1 which is a superconducting coil
It flows from between 8 and shows a characteristic that is almost symmetrical with respect to the external magnetic flux. In order to prevent resonance caused by the first coil 18 and the Josephson element 20, a first damping resistor 19 is connected in parallel with the first coil 18. The first resistor 19 may be omitted if there is no particular problem of resonance.

FIG. 3B is an SQUID having the same circuit configuration as that of FIG. 3A, and a bias current is made to flow from the end portion of the first coil 18. The threshold characteristic of this SQUID becomes asymmetric. In FIG. 3C, in order to change the asymmetry of the SQUID characteristic, the second resistor 21 is connected to both ends of the first coil 18, and the bias current is divided by the resistance value of both ends.

FIG. 3D shows a three-junction SQUID composed of three Josephson devices, the critical current value of which is set to 1: 2: 1 from the left and the bias current is applied from the vicinity of the center of the first coil 18. Then, the characteristics become symmetrical. Since the three-junction SQUID has a larger change in the critical current value with respect to the input magnetic flux than the SQUID of the two Josephson elements in FIG. 3A, the magnetic field resolution of the SQUID is improved. Further, similarly to FIGS. 3B and 3C, when the bias current application is made asymmetric with respect to the first coil, an asymmetric characteristic is obtained.

In FIGS. 3B and 3C, since the operating point can be set at the point where the input magnetic flux is 0, the offset coil 13 is unnecessary and the operation can be set easily. FIG. 3 (a),
In (d), the threshold characteristic is symmetric with respect to the input magnetic flux, so that it is necessary to provide the offset coil 13 in each SQUID as shown in FIG. 2 in order to set the operating point.

Next, the shunt circuit section (the first shunt circuit 2 and the second shunt circuit 5 shown in FIG. 1) will be described. The basic configuration of the shunt circuit is shown in FIG. 4A, and the shunt coil 14 and the shunt resistor 15 are connected in series. The oscillation frequency is substantially determined by the inductance (Ls) of the shunt coil 14 and the resistance value (Rs) of the shunt resistor 15.

The shunt coil 14 depends on the stray capacitance in the circuit and the capacitance component of the SQUID Josephson element.
May cause resonance. When the oscillation frequency becomes unstable due to resonance, the third resistor 22 is connected in parallel with the shunt coil 14 to suppress the resonance, as shown in FIG.

When it is desired to increase the value of Ls, the value shown in FIG.
There is a method of coupling the shunt coil 14 to another coil as shown in (c). Second coil 2 for shunt coil
The fourth resistor 24 is connected in parallel with the second coil 23 in order to couple the third coil 3 and prevent resonance of the second coil 23.

Although the circuit configuration has been described above, there are several types of SQUIDs that can be used in the present invention, and various combinations are possible depending on the design. Typical threshold characteristics are shown in FIGS. 5 (a), (b), (c), and FIG.
(A), (b), (c), and each operating point in each characteristic is shown from A to L.

Next, various operation modes will be described using the threshold characteristics of SQUID and operating points. The operation mode is the direction of the bias current, the first SQUID1 and the second SQUID
There are many combinations depending on the seven types of parameters such as the combination of the threshold characteristics of No. 4, the direction of the input coil, the setting of the operating point, and the output detection method. Main combinations Figure 5 and 6
This will be described with reference to FIGS. First ROS circuit 3, second RO
When bias currents of different directions are applied to the S circuit 6,
The sum of the outputs of the ROSs becomes the input magnetic flux, and as shown in FIG.
The threshold characteristics of (b) and (c) correspond.

For example, in FIG. 5A, the first SQUID
Assuming that the operating points 17 of the first and second SQUIDs 2 are A1 and A2, respectively, the operating points move to the solid arrow for positive magnetic flux and the operating points move to the direction of dotted arrow for negative magnetic flux. As a result, the magnitude of the input magnetic flux can be detected by the sum of the first output 11 of the first ROS circuit 3 and the second output 12 of the second ROS circuit 6.

The threshold characteristics when the bias currents are passed in the same direction are shown in FIGS. 6 (a), 6 (b) and 6 (c).
In this case, unlike FIG. 5, the input magnetic flux can be detected by the difference between the first output 11 and the second output 12. Although FIG. 6 shows only one threshold characteristic at a time, actually, as in FIG. 5, the next thresholds overlap in the cycle of Φ o.

Next, the detailed operation will be described with reference to the representative point A of FIG. 5A in FIG. The outputs are the first output 11 and the second output, which are the outputs of the first ROS circuit and the second ROS circuit, respectively.
The output 12 is added to detect the magnetic field. First, the first bias current 9 and the second bias current 10 are respectively applied to the first SQUID.
1 and 2 are set to A1 and A2 near the threshold of SQUID4. When positive and negative currents flow through the input coil, the first SQUA
The operating points A1 and A2 of the ID and the second SQUID 4 change on the threshold characteristic.

When a positive current flows through the input coil 7, the operating point moves to A1 +, A2 + (solid line arrow) and the first SQU
ID1 is in the voltage state, and the second SQUID 4 is in the superconducting state. As a result, the first ROS circuit 3 enters an oscillating state and a positive oscillating waveform is output. When a negative current flows through the input coil 7, the operating point moves to A1- and A2-, and the first SQUA
ID1 is in a superconducting state, and the second SQUID 4 is in a voltage state. As a result, the second ROS circuit 6 enters an oscillating state, and a negative oscillating waveform is output. The oscillation frequency increases with the magnitude of the input. Therefore, the current flowing in the input coil can be detected by the positive and negative of oscillation and the frequency.

9 and 10 show the relationship between the operation modes of FIGS. 5 and 6 and the threshold characteristics. There are various combinations,
One of the operating characteristics is that one is in the voltage state and the other is in the superconducting state with respect to the positive / negative of the input. The arrows at the operating points from A to L show the changes in the operating points for positive and negative inputs, respectively, with a solid line and a dotted line.

In the operation mode where the offset is required in FIGS. 9 and 10, each SQU is required as in the case of FIG.
It is necessary to provide the offset coil 13 on the ID. Here, even if the output corresponding to the positive and negative of the input is reversed, the amplifier,
It can be used without problems if it is inverted by a feedback circuit. Further, the threshold characteristic shown here is an example in which the inductance is made asymmetric, but the threshold characteristic can be changed asymmetrically by further making the critical current value of the SQUID asymmetric.
A threshold characteristic suitable for the present invention can be obtained.

The configuration and operation when used as an actual magnetometer will be described with reference to FIG. In order to increase the dynamic range, it is necessary to input the outputs of the first and second ROS circuits to the amplifier 25 and feed them back through the D / A converter 26 and the feedback circuit 27. Amplifier 25
Has a function of amplifying the output of each ROS circuit and a function of adding or subtracting depending on the operation mode. Amplifier 2
As the output of 5, a positive and negative sawtooth wave oscillation signal corresponding to the input is output.

If it is desired to detect the output by the pulse density, a comparator and a pulse generating circuit are added and a pulse corresponding to the frequency is taken out from the digital output 28. This circuit can be configured by using a general-purpose analog IC such as a normal operational amplifier. Next, the D / A converter 26 uses the amplifier 2
It functions to convert the pulsed digital signal from 5 into a continuous analog signal. Circuit is D for logic circuit
The A / A converter and an integrator using an operational amplifier are used. The output of the D / A converter can be detected as an analog output and is further input to the feedback circuit 27.

The feedback circuit 27 outputs a signal corresponding to the input and takes it out as an analog output 29, and at the same time, causes a current to flow through the feedback coil 8 through the feedback resistor. By this feedback, the operating point 17 of the SQUID can be fixed. The feedback circuit 27 is an integrating circuit using an operational amplifier and can be easily constructed.

The detection of the input magnetic flux by this magnetic sensor is
Digital output measurement that counts pulse density,
Both analog methods of detecting the feedback amount are possible.

[0031]

As described above, according to the present invention, two ROS circuits are used, and one of the ROS circuits is used depending on the input magnetic flux.
The circuit enters an oscillating state, and positive and negative pulsed outputs are obtained according to the positive and negative of the input magnetic flux. Therefore, unlike the conventional operation method in which a small frequency change is constantly oscillated and a small frequency change is regarded as a change in the input magnetic flux, a large output can be obtained and an improvement in sensitivity can be expected.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a first embodiment of the present invention.

2A is a block diagram of a conventional ROS circuit, FIG.
(C) is a figure explaining the operation.

3 (a), (b), (c) and (d) are S respectively.
It is a figure which shows the circuit example of QUID.

4A, 4B, and 4C are diagrams showing examples of shunt circuits.

5A, 5B, and 5C are diagrams showing examples of threshold characteristics when the polarities of bias currents are different.

6A, 6B, and 6C are diagrams showing examples of threshold characteristics when the polarities of bias currents are the same.

FIG. 7 is a diagram for explaining the operation of the magnetic sensor of the present invention.

FIG. 8 is a diagram showing an example of a circuit for driving the magnetic sensor of the present invention.

FIG. 9 is a chart showing operation modes corresponding to the threshold characteristics of FIG.

10 is a table showing operation modes corresponding to the threshold characteristics of FIG.

[Explanation of symbols]

 1 1st SQUID 2 1st shunt circuit 3 1st ROS circuit 4 2nd SQUID 5 2nd shunt circuit 6 2nd ROS circuit 7 Input coil 8 Feedback coil 9 1st bias current 10 2nd bias current 11 1st output 12 2nd output 13 Offset Coil 14 Shunt Coil 15 Shunt resistor 16 DC SQUID 17 Operating point 18 First coil 19 First resistor 20 Josephson element 21 Second resistor 22 Third resistor 23 Second coil 24 Fourth resistor 25 Amplifier 26 D / A converter 27 Feedback circuit 28 Digital output 29 Analog output

Claims (2)

[Claims] 1. A SQUID that forms a superconducting loop by at least two Josephson elements and a superconducting coil.
An input coil and a feedback coil each magnetically coupled to the SQUID, a shunt circuit in which at least one coil and one resistor are connected in series, and a first relaxation oscillation in which the SQUID and the shunt circuit are connected in parallel. Type SQU
A superconducting device, comprising: an ID circuit; and a second relaxation oscillation type SQUID circuit configured in the same manner as the first relaxation oscillation type SQUID circuit and connected in series with the first relaxation oscillation type SQUID circuit. Magnetic sensor. 2. The superconducting magnetic sensor according to claim 1, wherein the bias current of the SQUID is supplied asymmetrically to the superconducting coil.