JPH04143681A - Self-excitation oscillation type digital squid magnetic flux meter - Google Patents

Abstract

PURPOSE:To digitize output to strengthen the noiseproof so as to contrive the simplificity of the whole circuit mechanism by causing the output of a SQUID ring to self-excit and oscillate in accordance with the size of outside magnetic flux. CONSTITUTION:Since positive outside magnetic flux phix is applied to a SQUID ring 1 to exceed the threshold of bias current allowed to flow in the ring 1, it is changed to a voltage state and the current is not allowed to flow by an inductance L in its instant and the voltage state is maintained for a certain time to return to the zero voltage state after the time elapses. If the magnetic flux phix is always positive, the output of the ring 1 becomes a pulse train because the process is repeated again. The magnetic flux phix is in proportional to the difference of the count number C of the pulse train of the magnetic flux phix and the other count number C0 at the time of zero and feedback magnetic flux phi1 acting upon the direction for returning the magnetic flux phix to zero is equal to the magnetic flux phix in a normal state. Accordingly, if C-C0 which is a base for forming magnetic flux phi1 is measured, the outside magnetic flux phix for inputting into the ring 1 can be detected.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a SQUID magnetometer for detecting minute magnetism such as biomagnetism and terrestrial magnetism.

<Conventional technology> As a method for detecting minute magnetism, DC-3QU has conventionally been used.
ID and F L L (Flax Locked Loo)
p) Analog circuit systems combining circuits were the mainstream.

In such an analog circuit system, for example, “Masakazu Nakanishi et al., “All hard DC-SQUID” SCE 8
As shown in Figure 6-2J, a constant current is passed through the DC-3QUID, and the modulated magnetic flux at the frequency f is changed to the SQUID.
D, the output of the SQUID becomes an alternating current component with a frequency f having an amplitude proportional to the input magnetic flux φ8. This is detected by a subsequent phase detector, and the integrated result is converted into a current and fed back to the SQUID so as to cancel the input magnetic flux. By doing this, it is possible to remove 1/f noise in the circuit system, and also to correct the nonlinearity of the SQUID and make it linear.

<Problems to be Solved by the Invention> By the way, although the analog system has high sensitivity, it requires a noise matching transformer and the FLL circuit also has a complicated configuration.

Furthermore, since it essentially outputs an analog signal, it has the disadvantage of being susceptible to external noise.

To solve this problem, attempts have been made to digitize the SQUID output (for example, Norio
Fujimaki et al., “A sing
le-Chip SQUID Magnetometer
” IEEE Transaction on Ele
ctron Device,, vol, 35 NO
, 12, or Akira Fujimaki et al., “Pulse density modulated SQ
"Prototype production of UID", - Research report on advanced biomagnetic field measurement system, Mechanical Systems Promotion Association, 1-R
-3, March 1990), all of these require an external clock signal to be applied, similar to analog systems.
The circuit configuration remains complex.

The purpose of the present invention is to digitize the output of the SQUID to make it resistant to external noise, and to
The object of the present invention is to provide a SQUID magnetometer with a simpler circuit configuration than a QUID.

Means for Solving the Problems> In order to achieve the above object, the SQUID magnetometer of the present invention includes a SQUID ring in which a resistance element and an inductance element connected in series with each other are connected in parallel, and the SQUID
A current source that supplies bias current to the ring and this SQU
It has a feedback circuit means for an output signal from the ID ring, and is configured to cause the output of the SQUID ring to self-oscillate in accordance with the magnitude of external magnetic flux, and to measure the external magnetic flux from the oscillation frequency.

By connecting a resistance element and an inductance element in parallel to the SQUID ring, applying a bias current from the outside, and feeding back the output from the SQUID, the SQ
Since the UID self-oscillates and has a magnetic flux detection and comparison function, its oscillation frequency corresponds to the magnetic flux applied to the SQUID ring, and there is no need to supply a clock signal from the outside.

<Embodiment> FIG. 1 is a block diagram showing a circuit configuration of an embodiment of the present invention.

SQUID ring 1 is a DC-3 QUID ring with two Josephson junctions in the superconducting loop,
This SQUID ring 1 includes a pickup coil 2a.
The input coil 2b of the input circuit consisting of the input coil 2b and the input coil 2b are magnetically coupled. This results in
External magnetic flux picked up by pickup coil 2a or SQUID
transmitted to ring 1.

The SQUID ring 1 has a circuit configuration in which a resistor R and an inductance L are connected in parallel, and a bias current ゎ is supplied from a DC current source 3.

The output of the SQUID ring 1 becomes a pulse signal as described later, and the signal is sent to a pulse counter 5 via an amplifier 4.
has been introduced. A preset value C0 can be set in this pulse counter 5, and the counter 5 is connected to the amplifier 4.
A value obtained by subtracting this preset value C0 from the pulse count value C from 1 is output to the pulse integrator 6.

The output of the pulse integrator 6 is superimposed with a reference voltage V r *r, and then introduced into a V-I converter 7 where it is converted into a current signal and returned to the SQUID ring 1 via a feedback coil 8 . That is, the SQUID ring 1 receives the reference voltage V1. through the feedback coil 8 even when the external magnetic flux φ from the input coil 2b is zero. ．． A considerable magnetic flux φ7@ is applied.

In the above circuit configuration, the threshold curve (input magnetic flux φ - critical current 2) of the SQUID ring 1 is set as shown in FIG. 2. Also, the bias current value I, -φ1. ．． Adjust so that it is at the intersection of or on the threshold curve.

Next, the operation of the above embodiment of the present invention will be described.

FIG. 3 is a graph showing the IV characteristics and load curve of the SQUID ring 1.

External magnetic flux φ8 is applied to the SQUID ring 1 via the input circuit 2.
If positive, the bias current flowing through the SQUID ring 1 exceeds the threshold, so the SQUID ring 1 shifts to the voltage state. Also, if it is negative, it will not change to the voltage state. SQU
Since only a resistor R and an inductance are inserted into the ID ring 1, at the moment when the SQUID ring 1 changes to a voltage state, current does not suddenly flow due to the presence of L, and the voltage state is maintained for a certain period of time. This is V in Figure 3. The voltage shown in
In other words, it is the gap voltage of the SQUID.

After that, as time passes, the load curve settles to the one shown in FIG. At this time, the current flowing through the SQUID ring 1 is I m
111, the SQUID ring 1 returns to the zero voltage state again. The time required for this return is approximately L/R.

If the magnetic flux φ input to SQUID ring 1 is always positive,
Since the above process is repeated again, the output of the SQUID ring 1 becomes a pulse train with a period of L/R. In reality, there is fluctuation in the threshold curve due to thermal noise, so as shown in Figure 2,
Even if , is set on the threshold curve, a pulse train is generated in a state where the external input magnetic flux φ8 to the SQUID ring 1 is zero. S
If the number of counts by the pulse counter 5 of the pulse train generated when the magnetic flux φ input to the QUID ring 1 is zero is 00, if the measured count number C is C>CO, the input magnetic flux to the SQUID ring 1 is positive, C It can be seen that if <Co, it is negative. This relationship is shown in FIG. FIG. 4 shows the relationship between the external input magnetic flux φa to the SQUID ring 1 and the SQUID output when there is no feedback.

From the above, the feedback magnetic flux φ, which is proportional to CC, and in the direction of returning the external input magnetic flux φ8 to zero, is set to φ1.
．． Add it to SQUID ring 1 along with I. As a result, in a steady state, the total magnetic flux applied to the SQUID ring 1 is φkneeφ, +φ1. ．． In order to become φ, →φ, φ11. settle down.

Therefore, C-C is the basis for forming φ.

If we measure the external input magnetic flux φ to SQUID ring 1
You will understand 8.

<Effects of the Invention> As explained above, according to the present invention, since the SQUID output is digitized, it is resistant to external noise, and
By providing the SQUID with a self-oscillation function, there is no need for an external clock signal, so the overall circuit configuration is simpler than in conventional digital systems.

Eliminating the need for an external high-frequency clock means that similar SQ
When arranging many UIDs to create a multichannel system,
This simplifies the circuit system and eliminates crosstalk, which also leads to lower noise.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the circuit configuration of the embodiment of the present invention, Fig. 2 is a graph showing the threshold curve of the SQUID ring 1, and Fig. 3 shows the IV characteristic and load curve of the same SQUID ring 1. Graph, Figure 4 shows SQUID without feedback.
3 is a time chart showing the relationship between the external input magnetic flux φ8 to the ring 1 and the SQUID output. 1...SQUID ring 2...Input circuit 2a...Pickup coil 2b...Input coil 3...DC current source 4...Amplifier 5...Pulse counter 6 ... Pulse integrator 7 ... vr converter 8 ... Feedback coil R ... Resistance L ... Inductance

Claims (1)

[Claims] a SQUID ring in which a resistance element and an inductance element connected in series with each other are connected in parallel; a current source that supplies a bias current to the SQUID ring;
A self-excited oscillation type having a feedback circuit means for an output signal from the ring, and configured to self-oscillate the output of the SQUID ring according to the magnitude of external magnetic flux, and measure the external magnetic flux from the oscillation frequency. Digital SQUID magnetometer.