JPH04357482A - Squid driving circuit - Google Patents

Abstract

PURPOSE:To obtain a SQUID driving circuit which allows a flux sensitivity and an S/N ratio of a pulse operation SQUID flux meter to be improved. CONSTITUTION:In a SQUID driving circuit which measures an input flux, a bias coil 4 which is connected to a dc SQUID 3 is provided. A bias current which is synchronized with the bias current which flows at the dc SQUID 3 is supplied to this bias coil 4 so that an operation point of the dc SQUID 3 is set to a side where an inclination of a threshold curve is steep.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a superconducting quantum interferometer (dc
Regarding drive circuits for SQUIDs, especially for pulse-operated SQUIDs,
SQU improves detection sensitivity and S/N ratio of ID flux meter
This relates to an ID drive circuit.

0002

2. Description of the Related Art Conventionally, SQUID magnetometers operating in a superconducting state have been used to detect weak magnetic fields generated from living bodies such as the lungs, heart, brain, and muscles. Figure 3 shows “
IEEE'88 International Sol
id-State Circuit Conference
ce Digest of Technical Pa
per” pages 40-41, or “IEEE T
transactionon Electron Dev
ices” Volume 5, No. 12 (December 1988 issue)
FIG. 2 is a principle diagram of a conventional pulse-operated SQUID magnetometer shown on pages 2412-2418 of .

[0003] Usually, tunnel junction type SQUID is I-Φ
Because it has hysteresis in its characteristics, in order to use it as a magnetometer, it is necessary to shunt the Josephson junction of the SQUID with a resistor to eliminate the hysteresis characteristic. However, a pulse-operated SQUID magnetometer operates as a switching element and requires hysteresis characteristics to obtain a pulse output. Therefore, SQ in Figure 3
The two tunnel junction type Josephson junctions of the UID flux meter have a structure that does not have a shunt resistance in order to have a hysteresis characteristic of the IV characteristic. If the critical current values of these two Josephson junctions are made equal and the bias current injection point is selected at one end of the inductance, an asymmetric threshold characteristic as shown in FIG. 3(a) can be obtained. Note that the injection point of the bias current is the midpoint of the inductance, that is, the SQU
When connected to the midpoint of the ID ring, the I-Φ characteristic becomes symmetrical, and positive and negative pulses corresponding to the input magnetic flux cannot be obtained.

[0004] A pulse waveform alternating current bias (AC bias) is applied to the SQUID magnetometer, and the bias amplitude value is selected near the threshold on the vertical axis (I axis) of the threshold curve of the I-Φ characteristic shown in Fig. 3(a). It will be done. When the bias current crosses the threshold curve from inside to outside, the dcSQUID of the SQUID magnetometer
switches from the superconducting state to the finite voltage state, and dcSQ
As the output of the UID, a positive pulse is obtained on the positive axis of the vertical axis (I axis) of the threshold value curve of the I-Φ characteristic, and a negative pulse train is obtained on the negative axis. In other words, the output of this pulse train is shown in Figure 3.
As shown in (b), when the input magnetic flux Φ is negative, the operating point of the threshold curve moves from the center point of the horizontal axis (point O) to (point A), resulting in a negative pulse train, and when the input magnetic flux is positive, In this case, the operating point of the threshold curve moves from the horizontal axis center point (O point) to (B point), resulting in a positive pulse train. As shown in FIG. 3(b), by detecting this positive or negative pulse, it is possible to know whether the input magnetic flux is positive or negative.

Next, measurement of the magnitude of input magnetic flux will be explained. FIG. 4 is a diagram showing the circuit configuration of a conventional SQUID magnetometer. In Figure 4, dcSQUID3 and this dcSQ
A bias current source 6 that supplies current to the UID 3, and a dcS
A counter 7 that integrates the pulse train output of the QUID 3, and a D/A converter 8 that converts the output of this counter 7 into a current value.
, a feedback circuit consisting of a feedback resistor 9 and a feedback coil 5. Figure 4
The magnitude of the external magnetic field is detected by a 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, by flowing a feedback loop current in the direction of canceling the external magnetic field through the feedback coil 5, a magnetic flux in the opposite direction to the change in external magnetic flux δΦ is created in the dcSQUID.
The operating point of dcSQUID should always be around point O. In this state, the probability of appearance of positive pulse number (N+) and negative pulse number (N-) is the same, so the amount of feedback magnetic flux that cancels the input magnetic flux at this time, that is, D/A
By measuring the output of the converter 8, the magnitude of the input magnetic flux can be measured.

[0006]

[Problem to be solved by the invention] However, SQU
The magnetic flux sensitivity of the ID flux meter is the slope of the threshold curve (dI/dΦ)
However, because conventional SQUID magnetometers use an operating point on the gentler slope of the threshold curve shown in Figure 3(a), there is a problem in that the magnetic flux sensitivity deteriorates and the S/N ratio decreases. . The present invention aims to improve the magnetic flux sensitivity and signal-to-noise ratio of pulsed SQUID magnetometers.

[0007]

[Means for Solving the Problems] The SQUID drive circuit of the present invention includes: a detection coil that detects an external magnetic field; a dcSQUID that generates pulses in response to changes in the external magnetic field;
The external magnetic field detected by the detection coil is
an input coil for coupling with the SQUID; and the dcS
a bias current source that supplies current to the QUID;
a feedback coil coupled to the SQUID;
A feedback loop current is passed through the feedback coil, and the magnetic flux in the opposite direction to the change in external magnetic flux is applied to the SQUID.
A SQUID that measures the input magnetic flux by making
The drive circuit includes a bias coil coupled to the dcSQUID, and the bias coil is connected to the dcSQUID.
It is configured to supply a bias current that is synchronized with the bias current flowing through the QUID.

[0008]

[Operation] The present invention combines a bias coil driven by a bias current with a dcSQUID, and induces an AC bias magnetic flux synchronized with the AC bias current into the dcSQUID by the bias coil.
The operating point of the QUID is set on the steep side of the threshold curve to improve the sensitivity and S/N ratio of the SQUID magnetometer.

[0009]

Embodiments Hereinafter, embodiments of the present invention will be explained with reference to the drawings. FIG. 1 is a block diagram showing a first embodiment of the present invention. In FIG. 1, the circuit is the same as the conventional circuit (FIG. 4) except for the bias coil 4, which is a feature of the present invention. The AC bias current output from the bias current source 6 in FIG. 1 is supplied to the dcSQUID 3 via the bias coil 4. At this time, the AC bias magnetic flux is synchronized with the magnetic flux generated in the dcSQUID3. By selecting an appropriate value for the inductance of the bias coil 4, the operating point of the dcSQUID can be set to the steep side of the threshold curve. This will be explained in detail below.

FIG. 2 shows a pulse operation SQ according to an embodiment of the present invention.
It is a diagram of the operating principle of a UID flux meter. In FIG. 2, the AC bias magnetic flux is synchronized with the magnetic flux generated in the dcSQUID 3. Therefore, the operating curve becomes a diagonal line descending to the right like a broken line on the I-Φ coordinate axis. Therefore, the point where this diagonal line intersects with the threshold curve becomes a steeply sloped part of the threshold curve. This will be explained in detail below. In FIG. 1, the coil in the dcSQUID 3 that opposes the bias coil 4 is an equivalent representation of a SQUID ring, and the number of turns is usually one. The magnetic flux Φ induced in the dcSQUID is expressed as follows. Φ=MI=k×square root(L1×L2)
However, M: bias coil 4 and dcSQUI
Mutual inductance with D ring I: Current flowing through bias coil 4 L1: dcSQ
Self-inductance L2 of the UID ring: Self-inductance of the bias coil 4 Here, the inductance of the dcSQUID ring is uniquely determined by its shape and area. Therefore, by appropriately selecting the inductance of the bias coil, the magnitude Φ of the magnetic flux induced in the dcSQUID 3 can be brought to the steep part of the threshold curve. That is, compared to the conventional circuit without the bias coil 4, in the present invention, the slope of the threshold curve becomes steeper, so the value of dI/dΦ becomes larger, and a small change in external magnetic field changes the voltage state to the superconducting state. A transition will occur. Therefore, the measurement magnetic flux sensitivity is improved.

The output pulses of the dcSQUID are digitally integrated by a counter 7 as in the prior art, and the output of this counter 7 is converted into an analog signal by a D/A converter 8. Thereafter, the output of the D/A converter 8 supplies a current to a feedback circuit consisting of a feedback resistor 9 and a feedback coil 5, canceling out the input magnetic flux input to the dcSQUID. As explained 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 amount of feedback magnetic flux that cancels the input magnetic flux, that is, the output of the D/A converter 8. In addition, AC
The waveform of the bias current may be a triangular wave as shown in FIG. 1, or may be a square wave or a sine wave.

FIG. 5 is a block diagram showing a second embodiment of the present invention. In FIG. 5, a bias current source 12 that supplies current to the bias coil 4 is provided separately from a bias current source 6 that supplies current to the dcSQUID 3, and the bias current source 6 and the bias current source 12 are synchronized by a synchronization circuit 13. There is. The bias current output from the bias current source 6 in FIG. 5 is supplied to the dcSQUID 3. The bias current output from the bias current source 12 is supplied to the bias coil 4. At this time, the bias magnetic flux generated by the bias coil is dc by the bias current source 6.
Since it is set by the synchronization circuit 13 to be synchronized with the magnetic flux generated in the SQUID 3, if the inductance of the bias coil 4 is selected to an appropriate value, the operating point of the dcSQUID can be set to the steep side of the threshold curve. In the second embodiment, as described in the first embodiment, the operating point is at a point where the slope of the threshold curve is steeper than in the conventional circuit without the bias coil 4. ) increases, and a small change in external magnetic field 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 explained in the first embodiment, so the explanation will be omitted.

[0013]

[Effects of the Invention] As explained above, according to the present invention,
In a pulse-operated dcSQUID, by setting the operating point to the steep side of the threshold curve, it is possible to improve the magnetic flux sensitivity and, therefore, to improve the signal-to-noise ratio.

[Brief explanation of drawings]

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

FIG. 2 is a diagram illustrating the operating principle of a pulse-operated SQUID magnetometer according to an embodiment of the present invention.

FIG. 3 is a diagram illustrating the 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 the configuration of a pulse-operated 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 that detects an external magnetic field; a superconducting quantum interferometer (dcSQUID) that generates a pulse in response to a change in the external magnetic field; an input coil for coupling, a bias current source for supplying current to the dcSQUID, and a feedback coil for coupling to the dcSQUID, and a feedback loop current is passed through the feedback coil in a direction opposite to a change in external magnetic flux. A SQUID drive circuit that measures input magnetic flux by creating a magnetic flux in the SQUID includes a bias coil coupled to the dcSQUID, and supplies a bias current synchronized with a bias current flowing through the dcSQUID to the bias coil. Characteristic SQUID drive circuit. 2. An independent bias current source that supplies current to the bias coil, and a synchronization circuit that synchronizes the current of the bias current source that supplies current to the bias coil with the current of the bias current source that supplies current to the dcSQUID. 2. The SQUID drive circuit according to claim 1, further comprising: