JPH0778527B2 - SQUID drive circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to a superconducting quantum interferometer (dc).
SQUID) drive circuit, especially pulse operation SQUI
SQU that improves detection sensitivity and SN ratio of ID magnetometer
The present invention relates to an ID drive circuit.

[0002]

2. Description of the Related Art Conventionally, a SQUID magnetometer operating in a superconducting state has been used to detect a weak magnetic field generated from a living body such as a lung, heart, brain or muscle. Figure 3 shows
IEEE'88 International Sol
id-State Circuit Conferen
ce Digest of Technical Pa
"Per", pages 40-41, or "IEEE Tra"
nsactionon Electron Device
es ”, Volume 5, Issue 12 (December 1988), 24
Conventional pulsed SQUI shown on page 12-2418
It is a principle diagram of a D magnetometer.

Usually, the tunnel junction type SQUID is I-Φ.
In order to have hysteresis in the characteristics (current-magnetic flux characteristics) ,
In order to use this as a magnetometer, it is necessary to shunt the Josephson junction of SQUID with a resistor to eliminate the hysteresis characteristic. However, pulse-operated SQU
The ID magnetometer operates as a switching element and requires a hysteresis characteristic to obtain a pulse output. Therefore, the two tunnel junction type Josephson junctions of the SQUID magnetometer of FIG. 3 have a structure having no shunt resistance in order to have a hysteresis characteristic of IV characteristic (current-voltage characteristic) . The critical current value of the two Josephson junctions equal, when asymmetrically to select the proper injection point of the bias current to one end of the inductance is asymmetric threshold characteristic as shown in FIG. 3 (a) is e. The injection point of the bias current is the midpoint of the inductance, that is, SQU.
When connected to the midpoint of the ID ring, the I-Φ characteristic becomes bilaterally symmetric, and positive and negative pulses corresponding to the input magnetic flux cannot be obtained.

An AC bias having a pulse waveform is applied to the SQUID magnetometer, and the AC bias amplitude value is close to the threshold value on the vertical axis (I axis) of the threshold curve of the I-Φ characteristic of FIG. 3 (a). To be elected. When the AC bias current crosses the threshold curve from inside to outside, the dcS of the SQUID magnetometer
QUID switches from superconducting state to finite voltage state,
As the output of dcSQUID, a positive pulse is obtained on the positive axis on the vertical axis (I axis) of the threshold curve of the I-Φ characteristic, and a negative pulse train is obtained on the negative axis. That is, as shown in FIG. 3B, the output of this pulse train is such that when the input magnetic flux Φ is negative, the operating point of the threshold curve is from the central point (point O) on the horizontal axis to (A).
Point), a negative pulse train is obtained, and in the case of a positive magnetic flux, the operating point of the threshold curve is from the center point (point O) on the horizontal axis to (B
Point) and become a positive pulse train. As shown in FIG. 3B, it is possible to know whether the input magnetic flux is positive or negative by detecting this positive or negative pulse.

The measurement of the magnitude of the input magnetic flux will be described below. FIG. 4 is a diagram showing a circuit configuration of a conventional SQUID magnetometer. In FIG. 4, dcSQUID3 and this dcSQ
A bias current source 6 for supplying an AC bias current to UID3, a counter 7 for integrating the pulse train output of dcSQUID3, and a D for converting the output of this counter 7 into a current value.
It is composed of a feedback circuit including an A / A converter 8, a feedback resistor 9 and a feedback coil 5. In FIG. 4, the magnitude of the external magnetic field is detected by the zero point detection method in which a current is passed through the feedback coil 5 so as to cancel the input magnetic flux. That is, when the external magnetic field changes, a feedback loop current in a direction that cancels the external magnetic field is passed through the feedback coil 5, so that the magnetic flux in the direction opposite to the change δΦ of the external magnetic flux becomes dcSQU.
The ID is created so that the operating point of the dcSQUID is always near the O point. In this state, the number of positive pulses (N
Since the appearance probabilities of (+) and the number of negative pulses (N-) are the same, the feedback magnetic flux amount that cancels the input magnetic flux at this time, that is, the output of the D / A converter 8 is measured to determine the magnitude of the input magnetic flux. You can measure.

[0006]

[Problems to be Solved by the Invention] However, SQU
The magnetic flux sensitivity of the ID magnetometer is the slope of the threshold curve (dI / dΦ)
However, since the conventional SQUID magnetometer uses the operating point on the side where the threshold curve of FIG. 3 (a) has a gentle slope, there is a problem that the magnetic flux sensitivity is deteriorated and the SN ratio is lowered. . An object of the present invention is to improve the magnetic flux sensitivity and SN ratio of a pulse operation SQUID magnetometer.

[0007]

SQUID driving circuit of the present invention In order to achieve the above object, according to a detection coil for detecting the external magnetic field (1), a superconducting quantum interferometer generates a pulse in response to a change in the external magnetic field (DcSQUID: 3 ) and the detection coil
The external magnetic field detected by (1) is applied to the dcSQUI
A first AC bias current is not applied to the input coil (2) for coupling with D (3) and the dcSQUID (3) described above.
A first bias current source (6) which is symmetrically supplied, and d
Feedback coil coupled with cSQUID (3)
(5), and a SQUID drive circuit for measuring an input magnetic flux by causing a feedback loop current to flow in the feedback coil (5) and creating a magnetic flux in the dc SQUID (3) in a direction opposite to the change of the external magnetic flux. At
Bias coil combined with the dcSQUID (3)
(4) is provided, and the dc is added to the bias coil (4).
With supplying a second AC bias current to the first AC bias current and the same frequency and synchronization flowing to SQUID (3), operation of the dcSQUID (3)
The point is on the steep side of the threshold curve of the current-magnetic flux characteristic.
So that the inductance of the bias coil (4)
Is configured to select .

[0008]

DETAILED DESCRIPTION OF THE INVENTION The present invention, to induce bias coils driven by alternating bias current coupled with DcSQUID, the AC bus <br/> bias current of the same frequency a and the bias coil an AC bias magnetic flux synchronized with DcSQUID Thus, the operating point of dcSQUID is set on the steep side of the slope of the threshold curve to improve the sensitivity and SN ratio of the SQUID magnetometer.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a first embodiment of the present invention. In FIG. 1, a bias coil 4 which is a feature of the present invention
It is the same as the conventional circuit (FIG. 4) except for. In Figure 1 the first
AC bias current output from the first is the bias current source AC bias current source 6 is supplied to the dcSQUID3 via the bias coil 4. At this time, the AC bias magnetic flux is synchronized with the magnetic flux generated in dcSQUID3. If the inductance of the bias coil 4 is selected to be an appropriate value, the operating point of dcSQUID can be set to the steep side of the slope of the threshold curve. This will be described in detail below.

FIG. 2 is a diagram showing the operating principle of the pulse operation SQUID magnetometer of the first embodiment of the present invention. Exchange in Figure 2
The flow bias magnetic flux is synchronized with the magnetic flux generated in dcSQUID3. For this reason, the operation curve becomes a downward-sloping diagonal line like a broken line on the I-Φ coordinate axis. Therefore, the point where this diagonal line and the threshold curve intersect is a steep portion on the threshold curve. This will be described in detail below. In FIG. 1, the coil in dcSQUID3 that opposes the bias coil 4 is an equivalent representation of the SQUID ring, and the number of turns is usually one. Magnetic flux induced in dcSQUID. Is represented as follows. Φ = MI = k × square root (L1 × L2) where M: mutual inductance between the bias coil 4 and the dcSQUID ring I: current flowing through the bias coil 4 L1: self-inductance of the dcSQUID ring L2: self of the bias coil 4 Inductance Here, the inductance of the dcSQUID ring is uniquely determined by its shape and area. Therefore, if the inductance of the bias coil is properly selected,
The magnitude Φ of the magnetic flux induced in dcSQUID3 can be brought to the steep portion of the threshold curve. That is, in the present invention, the slope of the threshold curve becomes steeper as compared with the circuit without the conventional bias coil 4, so that dI
The value of / dΦ increases, and a small change in the external magnetic field causes a transition from the voltage state to the superconducting state. Therefore, the measured magnetic flux sensitivity is improved.

The output pulse of the dcSQUID is digitally integrated by the counter 7 as in the prior art, and the output of the counter 7 is converted into an analog signal by the D / A converter 8. Then, the output of the D / A converter 8 supplies a current to the feedback circuit composed of the feedback resistor 9 and the feedback coil 5 to cancel the input magnetic flux input to the dcSQUID. As described in the conventional circuit, the feedback circuit is a so-called zero point detection circuit. Therefore, the input magnetic flux can be measured by measuring the feedback magnetic flux amount that cancels the input magnetic flux, that is, the output of the D / A converter 8. The waveform of the alternating bias current may be a triangular wave as shown in FIG. 1, a square wave, or a sine wave. The first fruit
In the embodiment, the AC bias current source 6 outputs
Current bias current corresponds to the first AC bias current
At the same time, it corresponds to the second AC bias current.

FIG. 5 is a block diagram showing a second embodiment of the present invention. In FIG. 5, the bias coil 4 has a second
A second bias current source that supplies an alternating bias current
That the bias current source 12, the first exchange to dcSQUID3
The bias current source 6 and the bias current source 12 are provided separately from the bias current source 6 for supplying a flowing bias current, and the synchronization circuit 13 synchronizes the bias current source 6 and the bias current source 12. In FIG. 5, the AC bias current output from the bias current source 6 is dcSQUID
3 is supplied. Exchange outputted from the bias current source 12
The current bias current is supplied to the bias coil 4. Transportation AC bias magnetic flux generated by the time bias coil occurring dcSQUID3 by the bias current source 6
Since the set by the synchronization circuit 13 to synchronize the flow flux, it is possible to the operating point of dcSQUID Selecting the inductance of the bias coil 4 to a suitable value to steep side of the slope of the threshold curve. Also in the second embodiment, as described in the first embodiment, the operating point comes to a point where the threshold curve has a steep slope (dI / dΦ) as compared with the circuit without the conventional bias coil 4. ) Becomes large, and a small external magnetic field change causes a transition from the superconducting state to the voltage state. Therefore, the measurement magnetic flux sensitivity is improved. The principle of operation is the same as that described in the first embodiment, so description will be omitted.

[0013]

As described above, according to the present invention,
In the pulse operation dcSQUID, by making the operating point on the steep side of the slope of the threshold curve, the magnetic flux sensitivity can be improved and therefore the SN ratio can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a pulse operation SQUID magnetometer according to an embodiment of the present invention.

FIG. 2 is an operation principle diagram of a pulse operation SQUID magnetometer according to an embodiment of the present invention.

FIG. 3 is a diagram showing an operating principle of a conventional SQUID magnetometer.

FIG. 4 is a block diagram of a conventional SQUID magnetometer.

FIG. 5 is a block diagram showing a configuration of a pulse operation SQUID magnetometer according to another embodiment of the present invention.

[Explanation of symbols]

 1 Detection coil 2 Input coil 3 dcSQUID 4 Bias coil 5 Feedback coil 6 Bias current source 7 Counter 8 D / A converter 9 Feedback resistance 10 Digital output 11 Analog output 12 Bias current source 13 Synchronous circuit

Claims (2)

[Claims] 1. A detection coil (1) for detecting an external magnetic field,
A superconducting quantum interferometer (dcSQUID : 3 ) that generates a pulse in response to a change in the external magnetic field, and an external magnetic field detected by the detection coil (1) as the dcSQU.
A first AC bias current is applied to the input coil (2) for coupling with ID (3) and the dcSQUID (3) described above.
A first bias current source (6) that is supplied asymmetrically and a feedback coil that is coupled to the dcSQUID (3)
(5), and a SQUID drive circuit for measuring an input magnetic flux by causing a feedback loop current to flow in the feedback coil (5) and creating a magnetic flux in the dc SQUID (3) in a direction opposite to the change of the external magnetic flux. A bias coil combined with the dcSQUID (3)
(4), and the dcSQUID is attached to the bias coil (4).
The same frequency as the first AC bias current flowing in (3)
Provides a second alternating bias current that is wavenumber and synchronized
At the same time, the operating point of the dcSQUID (3) is the current −
Set the magnetic flux characteristic threshold curve to have a steep slope.
An SQUID drive circuit, characterized in that the inductance of the bias coil (4) is selected . Wherein said second exchange to said bias coil (4)
An independent second bias current source (12) for supplying a flow bias current, and a synchronization circuit (13) for synchronizing the second AC bias current with the first AC bias current. SQUID driving circuit according to claim 1, wherein the a.