JPH028762A - Squid fluxmeter - Google Patents

Abstract

PURPOSE:To measure a low frequency magnetic field without receiving the effect of the I/f noise generated by an apparatus by taking out higher-order components by synchronous detection and calculating feedback quantity on the basis of said component. CONSTITUTION:AC bias modulated in amplitude at frequency fm is applied to a SQUID 1 having two Josephson junctions 2. A pickup coil 5 taking an external magnetic field is magnetically coupled with the SQUID 1 and the external magnetic field being an object to be measured is guided to the SQUID 1 as input magnetic flux and the function of each part of the output of a feedback circuit 6 is magnetically coupled with the SQUID 1. 2fm is formed from the output of a modulation wave generator 4 by a multiplier 7 and the product of 2fm and the output of the SQUID 1 is obtained and synchronous detection is performed to obtain fs from 2fm+ or -fs. On the basis of this fs-component, feedback output is calculated to be fed back to the SQUID 1 and the intensity of an external magnetic field is calculated from the feedback quantity when detection output is 0.

Description

[Detailed Description of the Invention] [1 Summary] The present invention relates to the configuration of a magnetometer using a superconducting quantum interference device (SQUID), and is directed to reducing the influence of l/r H sound in an amplitude modulation AC bias method. For this purpose, the pulse train of the SQUID output is subjected to synchronous detection with an amplitude modulation frequency or its harmonics, and the amount of magnetic feedback to the SQUID is obtained based on the synchronous detection output.

[Industrial application field]

The present invention relates to the configuration of a magnetometer using SQUIDs, and particularly relates to a SQUID magnetometer that applies an amplitude-modulated AC bias to 5QUIDs. In this specification, the term "SQUID" refers to a superconducting ring having a Josephson junction, and the term "SQUrD1 fluxmeter" refers to a magnetic field detection and measurement device composed of a SQUID, a feedback circuit, and the like.

SQUID magnetometers have the highest sensitivity among the various magnetometers currently in practical use, and are used for measurements that target weak magnetic flux, such as in biological measurements. In particular, in biomeasuring, there is a demand for knowing the spatial distribution of magnetic fields in a short time, and magnetometers called multi-channel 5QUID and the like have been proposed.

This is a system in which multiple SQUIDs are arranged so that the biomagnetic field of multiple sites can be measured at once. In that case, it is desirable to integrate not only the SQUID part but also necessary circuits such as a feed bank circuit, which will not only reduce the size of the device but also improve measurement performance. This is also advantageous in terms of improvement.

- In terms of integration, digital circuits are easier to integrate than analog circuits, so it is desirable that the SQUIDoff flux meter also operate digitally rather than analogously. As one method for this purpose, a method has been proposed in which the bias current of the SQUID is pulsed or alternating current, and voltage pulses generated by an external magnetic field are counted and processed. In that case, in the zero method method, the feedbank amount is set based on the counted number of pulses.

Regarding the AC bias operation of such SQUID,
For example, Drunk's paper “Digital Peed-
back Loops for d, c, 5QLIID
s' by D, Drung, Cry.

genetics, Vol. 26. pp, 623-62
7 (1986, Nov.). Further, an invention related to AC bias operation of SQUID by the present inventor is disclosed in Japanese Patent Application No. 61-296124 and No. 62-125469.
62-177509. It has been previously filed as No. 62-177515.

The SQUID (ff flux meter analog operation described above) is
In this method, a direct current bias current is applied to the SQUID, and the magnitude of the manual magnetic flux is detected from the voltage generated at the SQ[JID according to the input magnetic flux. Since this is a well-known method and does not directly relate to the problem or purpose of the present invention, a description thereof will be omitted.

On the other hand, in digital operation, a pulsed or alternating current bias current is applied to the SQUID.

If the peak current value is adjusted to the critical current of the Josephson junction (hereinafter referred to as threshold current) when the input magnetic flux is 0, when the input magnetic flux is induced in the SQUID,
Since a pulse voltage with a sign depending on the sign of the magnetic flux is generated,
A feedback output sufficient to cancel out the human magnetic flux is added to the SQUID, and the strength of the external magnetic field is detected from the magnitude of the output.

In this method, the signals taken are pulses and are digital quantities, and the feedback circuit is comprised of an up/down counter, a D/A converter, and the like. These circuits are easier to integrate than feedback circuits using DC bias type lock-in amplifiers, etc., and therefore are advantageous in terms of integration of SQUID magnetometers.

[Conventional technology]

In this way, the AC bias method of 5QUTD has advantages over the DC bias method, but when biasing with a constant amplitude pulse current, it is difficult to bring the current value exactly to the threshold current value. is necessary.

As an alternating current bias method that avoids this restriction, the present inventor has invented a method that uses an amplitude modulated alternating current as a bias current, and a patent application has been filed. Among the earlier applications, JP-A-62-177509 falls under this category.
Although its contents are explained in detail in the specification of the above-mentioned patent application, the main points thereof will be explained here.

FIG. 2 is a diagram showing the prior art of an amplitude modulation AC bias type 5QUTD magnetometer. In the device having the configuration shown in the figure, an AC bias whose amplitude is modulated at a frequency fm is applied to an SQ [JID (11) having two Josephson junctions (2).The frequency of the AC bias is fc, and (3 ) is an AC bias generator, and (4) is a modulated wave generator.Furthermore, a pickup coil (5) that picks up an external magnetic field is magnetically coupled to 5QLJID (11), and the external magnetic field to be measured is is induced into the SQUID as input magnetic flux.

FIG. 3 is a diagram explaining the present invention in detail, and by partially referring to this diagram, it becomes easier to understand the operation of the above-mentioned SQUID (ff magnetic flux).

Figure 3 (AC bias r of 5QLIID shown in al.
The modulation degree of c is, for example, as shown in the same figure (as is clear from the comparison with bl), when the magnetic field O is located between the maximum and minimum values of the bias current.
The threshold is set to exist. Diagram ib1 shows the relationship between the SQUID's manual magnetic flux and the current threshold.

S to which the amplitude modulated AC bias is applied in this way
When an input magnetic flux that changes as shown in the te1 diagram is induced in the QUID, a difference appears between the number of positive output pulses and the number of negative output pulses only when the bias current reaches the critical current. This is because in the range where the bias current is sufficiently larger than the critical current, the same number of voltage pulses as the bias frequency are generated for both positive and negative voltage pulses; does not occur either.

Therefore, if the pulse output is detected by an up/down counter or the like in such a state, a pulse output of one sign will be extracted only in the portion where the bias current matches the critical current. Figure (d) shows only the difference component of such pulse outputs.Based on this output, the feedback circuit (6) in Figure 2 is activated, and if a magnetic field is applied to the SQUID that cancels the external magnetic field, the external The feedback output indicates the strength of the external magnetic field when the magnetic field is accurately canceled.

[Problem to be solved by the invention] The measurement target of the SQUID magnetometer is 1 to 1, such as biomagnetism.
Although the magnetic field often changes at a low frequency ts of about 100 Hz, in the above amplitude modulation AC bias method, S
Since the signal component of the QUID output is mainly composed of this Is, the signal exists in a range where the influence of 1/l noise is large.

This situation is illustrated in FIG. 3+81. Note that other frequency components written in the figure will be explained later.

Therefore, the sensitivity of the device is limited by 1/[noise, and it is impossible to increase the sensitivity to the limit due to thermal noise or quantum noise.

The object of the present invention is to use SQUID in the measurement of low frequency magnetic fields.
By setting the operating frequency of the magnetometer higher than the frequency range of the magnetic field to be measured or the frequency range where l/f noise is large, it is not affected by the t/r noise generated by the device, and can be used for low frequency magnetic fields. An object of the present invention is to provide a highly sensitive SQUIDI fluxmeter capable of measuring.

means for applying an alternating current bias of a frequency "C" whose amplitude is modulated at a frequency fs to the superconducting quantum interference device; a SQUID magnetometer comprising means for generating a feedback signal that accurately cancels the external magnetic flux induced in the conductive quantum interference element, wherein the feed bank circuit is configured to generate a feedback signal output from the superconducting quantum interference element. The pulse train signal is subjected to synchronous detection with +ii modulation frequency f- or its harmonics, and the feedback amount is calculated based on the synchronous detection output.

[Means to solve the problem]

In order to achieve the above object, the SQUID magnetometer of the present invention includes two or more Josephson junctions and is a superconducting Keiko interference element magnetically coupled to an external magnetic flux. The reason why it operates in the 9n frequency range, which is not affected by noise, will be explained with reference to FIG. Please understand that for the sake of developing the argument, there may be some overlap with the content described in the explanation of the prior art.

The waveform of the amplitude modulated AC bias is shown in the fa1 diagram, and f
Figure b1 shows the relationship between the threshold current and the input magnetic flux of the SQUID. As is clear from these figures, the bias current is modulated so that the threshold current when there is no external magnetic field is at the center of the maximum and minimum values of the bias current.

(In the figure b1, the inclined rectangle indicates the critical line where the tunneling current flows in the Josephson junction, and the magnetic field inside the rectangle /
Under current conditions, the superconducting state is maintained and no voltage is generated in the SQUID, while the superconducting state is canceled and a voltage is generated externally.

In the same figure (C1), when the input magnetic flux changes at the frequency Is and is weak, in the range where the bias current is large, the same number of voltage pulses as the bias frequency are generated in both the positive and negative directions, so the difference between the two is calculated. The resulting output is 0, and no voltage is generated in a range where the bias current is small, and the difference between them is also 0.

When the bias current value is close to the threshold, a pulse voltage in one direction is generated according to the input magnetic flux, and a state occurs in which no pulse voltage is generated in the other direction, so the difference in the number of positive and negative pulses can be Then, as shown in the fdJ diagram, the number of positive or negative pulses remaining is proportional to the strength of the magnetic flux.

Considering that the modulation frequency ftm of the bias current is sufficiently large with respect to the magnetic field fluctuation frequency S, and the duration of each pulse group in Figure (d) is sufficiently short compared to the time Tm/2 between pulse groups. Therefore, each group can be approximated to a δ function having its waveform area. Therefore, n'-n #A Σ (sinω, t)xδ(t -n T+a/
2) −(1). The input magnetic flux is a component of frequency ω, /2π, and T m = January'rg
, A are constants.

Here, Σδ(t-nTm/2) = (2/T11) Σ exp(t2π(2L/Ts
+)n)=2fm From the formula of Σ exp(t2nωmt), the +11 formula becomes n'-n -exp(i(2nωm-ωt) t), and (C1
As shown in the figure, in addition to the frequency rs, 2fm±'s+4fg
It can be seen that frequency components of ±Is, . . . exist. This is equivalent to sampling the waveform of frequency Is at frequency 2fs+.

When the noise generated by the device is superimposed on such a spectrum, as shown by the dotted line in Figure 181, the SQUID noise appears strongly in the low frequency region, and in the higher frequency region Ha!Although a constant value of thermal noise appears, it can be seen that the frequency rs exists in a region where 1/f noise is strong.

In the prior art, the frequency rs acid component is extracted and the feed bank amount is calculated based on it, so 1/
Due to the limitation of ftI sound, it was not possible to increase the sensitivity, but in the present invention, the 2 fm+ rs acid component or higher-order components are extracted by synchronous detection, and the feedbank amount is calculated based on that. Therefore, there is no influence of l/f noise, and it is possible to increase the sensitivity until it is limited by thermal noise or injection amount noise.

Note that if the threshold value is not in the center between the maximum and minimum values of the modulated amplitude of the AC bias, nf is set as an integer.
Since the s±rs component exists, synchronous detection may be performed using the frequency fm.

, [Embodiment] FIG. 1 is a schematic diagram showing the configuration of an embodiment of the present invention. In the figure, (1) is the SQU I D, (2) is the Josephson junction, (3) is the AC bias generator, (4) is the modulated wave generator, and (5) is the SQ [magnetically coupled to the JID. The pin 7 assop coil (6) is a feedback circuit, and the functions of these parts and the output of the feed bank circuit are magnetically coupled to the 5QIJID, which is the same as in the prior art shown in FIG.

In this embodiment, 21- is generated by the multiplier (7) from the output of the modulated wave generator, and by multiplying this by the SQU r D output, that is, by performing synchronous detection with 2fa, 2cm:! : Calculate the feedback output based on the fs acid component obtained in this way by obtaining S from fS, feed it back to 5Q (JID, and calculate the external magnetic field from the feed bank amount when the detection output is O) The point of determining the strength of is the same as in the prior art described above.

Furthermore, in this embodiment, the frequency for performing synchronous detection is 2f+s, but this should be set appropriately according to the degree of modulation of the bias current, etc. - Generally speaking, rI
11 or its harmonics will be used.

〔Effect of the invention〕

As explained above, in the SQUID magnetometer of the present invention, 2
Since the feedback signal is calculated based on rs obtained from r-±rs, it is not limited by 1/fM sound. Therefore, by increasing the sensitivity until it is limited by thermal noise or quantum noise, It becomes possible to measure magnetic fields with weak frequencies.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the configuration of an embodiment of the present invention, FIG. 2 is a schematic diagram showing the configuration of a prior art, and FIG. 3 is a diagram explaining the present invention in detail. 1 is a SQUID, 2 is a Josephson junction, 3 is an AC bias generator, 4 is a modulated wave generator, 5 is a pick amplifier coil, 6 is a feedback circuit, 7 is a multiplier, and 8 is a synchronous detector. FIG. 1 is a schematic diagram showing the configuration of an embodiment of the present invention. FIG. 2 is a schematic diagram showing the configuration of the prior art. FIG. 3 is a diagram explaining the present invention in detail.

Claims (1)

[Claims] A superconducting quantum interference device comprising two or more Josephson junctions and magnetically coupled to an external magnetic flux; means for applying an alternating current bias; and means for generating a feedback signal that accurately cancels the external magnetic flux induced in the superconducting quantum interference device based on a pulse train-like voltage signal output from the superconducting quantum interference device. In the SQUID magnetometer, the feedback circuit performs synchronous detection with the modulation frequency f_m or its harmonics on the pulse train signal output from the superconducting quantum interference element, and A SQUID magnetometer, characterized in that the feedback amount is calculated based on a synchronous detection output.