JP2653916B2 - Multi-channel SQUID magnetometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Overview] Multi-channel SQ using multiple magnetic flux sensors using SQUID and detecting magnetic fields from many different areas at once
Regarding the UID magnetometer, a bias means for supplying the same bias current to a plurality of magnetic flux sensors in common with a view to accurately responding to each channel even if the bias frequency is low; First conversion means for converting each output into serial data;
Signal processing means provided corresponding to each of the plurality of magnetic flux sensors and outputting the magnetic flux signal; and converting the serial data into parallel data and supplying each to the corresponding one of the signal processing means. A second converter; and a controller for supplying a signal synchronized with the bias current to the first and second converters, wherein the bias current of the magnetic flux sensor is controlled by the bias current commonly supplied from the bias unit. The first converting means converts the pulse into the serial data by a signal from the control means while each outputs the pulse.

[Industrial applications]

The present invention relates to a multi-channel SQUID magnetometer that uses a plurality of magnetic flux sensors using SQUIDs and detects magnetic fields from many different regions at once.

In recent years, SQUID has been used to measure minute magnetic fields generated from living organisms.
A highly sensitive magnetometer using a (Superconducting Quantum Interference Device) is used. In particular, by measuring the magnetic field distribution of the brain and heart, it is possible to estimate the current source that is generating the magnetic field, which provides very meaningful information for diagnosis and elucidates neural activity in the living body. It has been pointed out that it is useful. Measurement of such a magnetic field distribution is based on a single-channel SQUI
It is possible to measure the time-series data for each part in order using a D magnetometer. However, this method has problems such as the fact that the measurement takes time and the subject becomes fatigued, and that the current source cannot be estimated with high accuracy because the magnetic fields in different places cannot be measured simultaneously. For this reason, a multi-channel SQUID magnetometer capable of arranging a plurality of magnetic flux sensors and simultaneously measuring the magnetic field of each part is required.

[Conventional technology]

As a conventional multi-channel SQUID magnetometer, one that has a processing circuit for converting an obtained detection signal into a magnetic field signal corresponding to a one-channel magnetic flux sensor and has a multi-channel structure has been proposed. (HE
Hoenig et al., Biomagnetic multichannel system with
integrated SQUIDs and first order gradiometers op
erating in a shielded room, Cryogenics, vol.29, Augus
t, pp809-813, 1989). Also, a method has been proposed in which each SQUID is driven at a different frequency and multiplexed to reduce the number of SQUID output lines to one to simplify the system (Furukawa et al., Japanese Journal of Apply).
Physics, vol. 28, No. 3, March, 1989, ppL456-L458). In these methods, as the SQUID, an SQUID of analog operation in which the SQUID output is a minute analog signal is used. On the other hand, as a digital SQUID capable of obtaining a pulse output, a SQUID comprising a two-junction quantum interference device is AC-biased and a pulse is output (Japanese Patent Laid-Open No. 63-290979)
It has been known. Applies a dc SQUID voltage output that operates in analog mode to a superconducting comparator or 1-bit A / D converter to obtain a pulse output (D. Drung, Cryogenics, vo
l.26, pp623-627, 1986). Digital SQU
ID has the feature that the output S / N ratio can be increased.

As a multi-channel method using digital SQUID, the output of the digital SQUID of each channel according to the application filed by the applicant of the present invention is switched by a multiplexer at regular intervals and converted into a magnetic flux signal by a single processing circuit. There is a method (Japanese Patent Application No. 1-178851). In addition, a one-chip SQUID (N.Fujimaki) that uses a superconducting feedback circuit, which is also related to the applicant's application, and integrates all functions necessary for converting magnetic flux into electrical signals on a single chip.
et al., “A Single-Chip SQUID Magnetometer”, IEEE T
rans.Electron Device, vol.35, No.12 pp2412-2418,198
8), using a selection circuit to switch a plurality of one-chip SQUIDs at regular intervals to obtain a magnetic flux signal
No. 340964). However, in both methods, the operation of the unselected SQUID is stopped (no pulse is output). Therefore, if a certain channel is selected again after scanning all the channels, it takes time to catch up with a new magnetic flux signal. This is specifically described as follows.

For example, the frequency band required for biomagnetism measurement is several hundred Hz in consideration of previous examples. So, temporarily, 300Hz
In order to have a frequency band of, all channels must be scanned every 1/600 Hz, that is, every 1.7 ms from the sampling theorem. Assuming now that the number of channels is 100, the allocated time per channel is 1.7m
s / 100 = 17 μS. On the other hand, when a digital SQUID is fed back to obtain a magnetic flux signal, the fluxmeter has the same transfer function as a first-order low-pass filter for small signals such as biomagnetism (Fujimaki et al., J.
Appl. Phys., 65 (4), pp1626-1630, 1989), the cutoff frequency ω ₀ is given by the following equation.

ω ₀ = 2f _B · ΔΦ · dP / dΦ f _B … Bias frequency given to SQUID ΔΦ… Feedback amount per pulse P… Switch probability Φ… Signal magnetic flux If bias frequency is 1MHz and ΔΦ is 1 × 10 ^-5 Φ ₀ , dP
/ When dΦ is to 100 / Φ _0, f ₀ is about 320 Hz. However,
In order to catch up with the input magnetic flux during the allocation time per channel of 17 μS, a band of at least 20 KHz (90%) is required, and therefore, the bias frequency must be increased by two digits or more. Further, in order to increase the sensitivity of the magnetometer, ΔΦ must be reduced in order to avoid the influence of quantization noise, but if so, it is necessary to further increase the bias frequency in order to compensate for the decrease.

By the way, one of the important things in multi-channeling is how to reduce the number of signal lines with SQUID. This is because the SQUID sensor is usually placed in a low-temperature environment such as liquid helium, and as a result, the number of channels increases, and as the number of cables connecting the circuit on the room temperature side increases, the consumption of cryogen such as expensive liquid helium increases. Because it also increases. In the conventional analog SQUID, a processing circuit (room temperature) is provided for each SQUID sensor as described above, so that output lines and feedback lines for the number of channels are required. Also, in the frequency multiplexing method, the number of output lines is one, but the number of feedback lines is required for the number of channels.

[Problems to be solved by the invention]

In order to realize a multi-channel SQUID magnetometer of 100 channels or more, it is necessary to operate at a bias frequency of several hundred MHz or more as described above. However, such operation in the frequency domain causes a problem that crosstalk between channels is likely to occur in the case of multi-channel. Further, in the conventional multi-channel SQUID magnetometer, there is a problem that as the number of channels is increased, the consumption of cryogen such as liquid helium is increased.

The present invention provides a multi-channel SQUI that can accurately respond to each channel even if the bias frequency is low.
It aims to provide a D magnetometer.

It is another object of the present invention to provide a multi-channel SQUID magnetometer that consumes less cryogen such as liquid helium even when the number of channels increases.

It is a further object of the present invention to provide a multi-channel SQUID magnetometer with accurate channel response and low cryogen consumption.

[Means for solving the problem]

FIG. 1 is a block diagram showing the principle of the present invention, and shows a multi-channel SQUID magnetometer according to the present invention.

Reference numeral 1 denotes a magnetic flux sensor using a SQUID, which converts a magnetic field from a living body into an electric signal, and is provided with a plurality of magnetic sensors. Reference numeral 2 denotes an oscillator, which supplies the same bias current to a plurality of magnetic flux sensors 1 in order to operate the SQUID in a pulsed manner. Reference numeral 3 denotes a control circuit, which is a signal for switching the switches to multiplexers 4 and 5. Is a multiplexer, which converts the outputs of a plurality of magnetic flux sensors 1 into serial data by switching the output of each magnetic flux sensor 1, and 5 is a multiplexer, which converts serial data into parallel data. It is sometimes called a demultiplexer because of its function. Reference numeral 6 denotes a signal processing circuit which extracts a magnetic flux signal from an electric signal (detection signal).
Is provided corresponding to each of.

[Operation] FIG. 2 is an explanatory diagram of the operation of the present invention, and shows a time sequence in the multi-channel SQUID magnetometer shown in FIG.

The oscillator 2 supplies a bias current (a) having a relatively low frequency (for example, 1 MHz) to each of the plurality of magnetic flux sensors 1. The pulse output (b1 to bN) from each of the magnetic flux sensors 1 (1 to N) corresponding to each part of the magnetic field measurement region of the living body is half of the bias current (a) by the control signal (c) of the control circuit 3. All are switched by the multiplexer 4 during a cycle (actually a quarter cycle), and are converted into serial data (d). Thereafter, the serial data (d) is transferred to the multiplexer 5 provided on the room temperature side with one output cable. Then, by switching the multiplexer 5 using the signal (c ') of the control circuit 3 synchronized with the control signal of the multiplexer 4, the serial data (d) is converted into parallel data, and each magnetic flux sensor 1
Is sent to the signal processing circuit 6 corresponding to. If this is repeated while the bias current is applied, time series data for each magnetic flux sensor 1 can be obtained in each signal processing circuit, so that it can be converted into a magnetic flux signal, and the magnetic flux signal corresponding to the measurement site 1 to N data are obtained.

As described above, in the present invention, each magnetic flux sensor 1 is constantly operating while the bias current (a) is applied (the on / off measurement of the pulse signal is performed every half cycle T / 2 of the bias current (a) every time). Therefore, each magnetic flux sensor 1 is equivalently operated in one channel. Therefore, no problem occurs in response. In addition, multiplexers 4 and 5
By using, the output lines are combined into one, so that the consumption of cryogen for SQUID such as liquid helium can be reduced.

〔Example〕

 FIG. 3 is a block diagram of one embodiment of the present invention.

This embodiment is an example in which a bias current (a) is supplied to a plurality of magnetic flux sensors 1 at the same timing, and a superconducting feedback circuit is built in each of the magnetic flux sensors 1.

In this embodiment, the magnetic flux sensor 1 and the multiplexer 4 are provided in the low-temperature bath 7 and are cooled by a cryogen such as liquid helium. This is the same in other embodiments described later.

In this embodiment, the magnetic flux sensor 1 according to the present invention is used.
In order to operate continuously during the measurement, the bias current (a) is continuously supplied during the measurement, and the feedback input is always inputted without being cut off by the selection circuit or the like.
The output of the magnetic flux sensor 1 is always supplied to the multiplexer 4. This is the same in other embodiments described later.

Looking at the cable extending from the low temperature chamber to the room temperature side,
Since the multiplexer 4 is provided, the number of output lines 8 is one for signal output. As for the bias current (a), there is one bias line 9 because the supply timing is set to one (common). As for the control signal (c) line 10 from the control circuit 3, if the number of magnetic flux sensors 1 is N, the number is log ₂ N. The number of feedback inputs is zero due to the built-in feedback circuit. Thereby, consumption of liquid helium can be reduced.

Hereinafter, the magnetic flux sensor 1 and the multiplexer 4 provided on the low temperature side will be described first.

Magnetic flux sensor 1 to incorporate feedback circuit
Has any of the configurations shown in FIGS. 4A to 4C. That is, the magnetic flux sensor 1 includes a sensor portion (hereinafter, referred to as a squid sensor) 21 using a SQUID and a superconducting feedback circuit 22 formed as a single chip.

FIG. 4A shows a configuration of a one-chip magnetic flux sensor. 4A
In the figure, reference numerals 31a and 31b denote a pair of pickup coils which are composed of a superconducting loop and take out a magnetic flux interlinked in the loop to pick up a magnetic flux to be measured (magnetic flux to be measured).
An input coil 2 forms a superconducting loop 33 together with the pickup coils 31a and 31b. The input coil 32 constitutes a part of the squid sensor 21 and applies the magnetic flux generated in the superconducting loop 33 to the squid sensor 21.

The squid sensor 21 is of a type in which a two-junction quantum interference device is AC-biased, and includes a superconducting loop 34 including Josephson junctions J ₁ and J ₂ and a superconducting inductance 35. Signal flux Φ _S
(Coupling coefficient is M ₁ ) and feedback magnetic flux Φ _FB , which is pulsed by AC bias current (a) and outputs a current pulse corresponding to input magnetic flux (Φ _S -Φ _FB ) to feedback circuit 22 .

As the feedback circuit 22, for example, a superconducting loop
37 includes Josephson junctions J ₃ and J ₄ and a first inductance 38, and has a second inductance 39 for magnetically coupling with the first inductance 38 (coupling coefficient is M _Z ). Gate 40 for converting a pulse to be applied to magnetic flux quanta, and a superconducting inductor 41 for converting a pulse passing through write gate 40 to magnetic quanta and storing the same
The one using a superconducting storage loop 42 consisting of Further, the magnetic flux quantum of the superconducting inductor 41 is returned to the input side of the squid sensor 21 as a feedback magnetic flux through a feedback loop 43 which is magnetically coupled. Therefore, the feedback circuit 22 measures a pulse output from the squid sensor 21 and feeds back a magnetic flux quantum corresponding to the measurement result to the input side of the squid sensor 21 through the magnetic field coupling by the feedback loop 43.

In the above configuration, the squid sensor 21 generates a pulse train depending on the applied magnetic flux, and this pulse train is sent to the multiplexer 4. The output of the squid sensor 21 is sent to the feedback circuit 22. Then, the output pulse of the squid sensor 21 is counted by the feedback circuit 22, and the magnetic flux proportional to the result is fed back to the squid sensor 21. Therefore, the feedback loop 43 operates so that the measured magnetic flux picked up by the pickup coils 31a and 31b is always canceled out by the feedback magnetic flux, and the magnitude of the measured magnetic flux can be understood from the feedback amount.

Next, FIG. 4B shows an example in which an analog output of a conventional dc-SQUID sensor is input to a superconducting comparator to obtain a pulse.

4A are assigned the same reference numerals as in the example shown in FIG. 4A. A dc-SQUID is used for the squid sensor 51, and the squid sensor 51 has a Josephson junction J ₁ ,
It is configured to include J ₂ , resistors 52 and 53, and superconducting inductance 35, and receives the difference between the measured signal magnetic flux Φ _S (coupling coefficient is M ₁ ) applied from the input coil 32 and the feedback magnetic flux Φ _FB. Change to voltage output by DC bias 54 and resistance
The signal is sent to the superconducting comparator 56 via 55. The superconducting comparator 56 includes Josephson junctions J ₅ , J ₆ and a first inductance 58 in a superconducting loop 57 and a second inductance 59 which is magnetically coupled to the first inductance 58 (coupling coefficient is M ₃ ). The output of the squid sensor 51 is compared in response to the AC bias current (a), and a digital current pulse is output to the superconducting digital feedback circuit 61. The superconducting digital feedback circuit 61 measures the pulse output from the superconducting comparator 56, and outputs a flux quantum according to the measurement result to the feedback loop 62.
Feeds back to the input side of the squid sensor 51 through magnetic field coupling by, for example, includes an up / down counter and a D / A converter composed of a Josephson circuit, etc., and further includes a circuit such as a filter as necessary. It is also possible.

FIG. 4C shows an example in which a superconducting digital feedback circuit 61 similar to the above is added to the pulsed squid sensor 21.

The superconducting digital feedback circuit 61 may be placed at room temperature,
The number of feedback lines can be reduced by the internal feedback system which incorporates.

FIG. 5 is an explanatory diagram of the operation of the squid sensor (digital SQUID) 21.

This figure shows the characteristics of the SQUID of the magnetic field current i _C with respect to the input magnetic flux [Phi _S of the SQUID (threshold characteristic). Φ ₀ is the flux quantum 2.
07 × 10 ^-15 wb. The curve A shows the threshold characteristic, and the SQUID is in a superconducting state in an area inside the area (indicated by hatching). In the area outside the SQUID, the SQUID is in a voltage state and outputs a pulse.

Here, the magnitude of the bias current (a) depends on the input magnetic flux Φ
It is made equal to the magnitude of the critical current i _C when _S = 0. Under such a bias condition, a positive pulse and a negative pulse are output according to the positive and negative of the input magnetic flux. Also,
Since the threshold characteristics are asymmetric, the bias current
In the period T, one positive or negative pulse is output.

It should be noted that the magnitude of the input magnetic flux Φ _S is, for example, as shown by a dashed line in the figure. The pulse width of the outputted pulses is dependent on the magnitude of the input magnetic flux [Phi _S,
It has a pulse width of at least T / 4 or more.

The multiplexer 4 is configured using a Josephson logic circuit to operate in a refrigerant such as liquid helium.

6A to 6C show the configuration of the multiplexer 4. FIG.

As shown in Figure 6A, the output b ₁ to b _N of the plurality of magnetic flux sensors 1, in parallel, it is input to the Josephson AND gate 71, each corresponding. The AND gate 71 has a selection circuit
The selection signal from 70 is input. As a result, the outputs b _{1 to} b _N are selectively output from the AND gate 71 in this order,
It is a multiplexer output composed of serial data.

Here, the details of the selection circuit 70 are shown as in FIG. 6B. Selection circuit 70 has a 256 selection gate Q ₁ to Q _256, an 8-bit signal consisting of A ₁ to A _m are input to each select gate Q ₁ to Q _256, these 8-bit signal a ₁ to a _m 256 pieces from the selectable control signal 1 to n (logical product of a ₁ to a _m) is generated.

As an example, the configuration of the one selection gate Q ₁ is shown as FIG. 6C, the selection gate Q ₁ is 4-bit signals A ₁ to A ₄ (Originally 8 bits, indicates only 4 bits) along with receiving Josephson oR gate 72-75, respectively, which incorporates a Josephson oR gate 79 to 81 ... and Josephson aND gate 76 to 78 ..., the logical product of a ₁ to a ₄ Generate a control signal. This control signal is supplied to the AND gate 71 shown in FIG. 6A.

The multiplexer 4 may have the configuration shown in FIG. 7A or 7B.

In FIG. 7A, a multiplexer 4 outputs a three-phase clock Φ.
_1, a shift register driven by the [Phi ₂ and [Phi _3. Once the load signal L and the shift signal S are both _"1", Φ 1 driving the OR gate 82 and AND gate 84
Outputs “1”. Next, [Phi ₂ OR gate 85 of the drive shifts the signal L, also 3J logical OR gate 86 is "1"
Outputs, also, 3J logical OR gate 87 outputs a signal b _1. Then, further AND gate 88 of the [Phi ₂ drive, the signal b ₁
Is output to the OUTPUT terminal. Next, the 和₃ driven OR gate 89 shifts the signal L to the next stage (second stage). here,
When the shift signal S is input, the same operation is performed, the signal b ₂ is output. Here, the 3J OR gate is SQUI
The page pulse from D is converted into a positive pulse of positive polarity to facilitate subsequent processing means.

FIG. 7B shows an output pulse train b ₁ , b ₂ ,... B _n by omitting the input of the shift signal S in FIG. 7A and shifting the load signal in synchronization with the clocks Φ _{1 to} Φ _3. 3 shows a shift register.

FIG. 7C shows that in FIG. 7B, three-phase clocks Φ ₁ , Φ ₂ and Φ
₃ shows an example in which three OR gates driven by 3 are connected in series. In this case, prior to the output pulse b _1, the output pulse always "1" (hereinafter, MARK signal) is obtained. Therefore, the timing can be easily set by extracting the MARK signal in the multiplexer 5 or the preceding stage and using this to trigger the control signal C '. Thus, in response to the time delay caused by the transmission of the serial data d via the cable, the control signal C '
Can be easily delayed.

The circuit corresponding to the 0th stage (including the output resistance)
It is not always necessary to provide it before the first stage. That is, the first
One or more may be provided in a predetermined stage between the stage and the Nth stage. In this way, by setting one or more "1" high-level bits to be included in the output pulse train, the operation timing of the control signal C 'of the multiplexer 5 can be determined based on the information of these bits. Can be.
Alternatively, a plurality of circuits corresponding to the 0th stage may be provided, and the logical product of the outputs thereof may be obtained, and this may be used as a trigger.
By taking the logical product, the reliability of the operation of the multiplexer 5 can be further improved in terms of determining the timing.

The description of the magnetic flux sensor 1 and the multiplexer 4 provided on the low temperature side in FIG. 3 has been described above. Next, each circuit provided on the room temperature side will be described. FIG. 8 is referred to in the following description. FIG. 8 is a time sequence showing the operation of each circuit including the magnetic flux sensor 1 and the multiplexer 4.

The oscillator 2 has a sine wave bias current a having a period T, for example.
And a synchronizing pulse e having the same phase as this is supplied to the 90 ° phase shifter 12. The bias current is not necessarily a sine wave, while the switching by the multiplexer 4, the output pulse of the magnetic flux sensors 1 of all output by the input magnetic flux [Phi _S may if waveform to last.

The 90 ° phase shifter 12 shifts the phase of the synchronization pulse e by 90 ° to form a 90 ° synchronization pulse f. This pulse f
Is supplied to the control circuit 3 and the positive / negative discriminating circuit 14.

The control circuit 3 sends signals A _{1 to} A ₁ to the multiplexers 4 and 5 in synchronization with the bias current a using the pulse f.
Multiplexer control signal consisting of A _m for supplying the C and C '. The signal C 'has a timing later than the signal C by a transmission delay time τ in a cable or the like.

Here, the operation timing of the multiplexer 4 is always a timing delayed by 90 ° from the bias current a. That is, the operation of the multiplexer 4 is started from the time when the pulse output of all the magnetic flux sensors 1 ends. Thus, even if the multiplexer 4 is switched at high speed, it is possible to prevent the SQUID from malfunctioning due to the influence of crosstalk or the like.

On the other hand, the output pulses of the magnetic flux sensor 1 will as of b ₁ to b _n. That is, this pulse has a pulse width of 1/4 cycle or more of the bias current a. Therefore, within the period of the pulse width, the output pulses b ₁ to b _n of the total magnetic flux sensor 1, the multiplexer 4, sampled at high speed. As a result, serial data d is obtained. This sampling is
It starts with the 90 ° cycle pulse f as a trigger and ends within T / 4 cycle.

The serial data d is amplified by the preamplifier 13 and sent to the multiplexer 5. This can improve the S / N ratio.

Multiplexer 5, the control signal C ', to form a parallel data g ₁ to g _N from the serial data d, and inputs the corresponding signal processing circuit 6. The parallel data g ₁ to g _N is obtained by mixing the positive and negative pulse output. Note that the control signal C ′ may have the same timing as the signal C. Also, instead of obtaining from the control circuit 3, the synchronous clock of the pulse train of the serial
(Locked Loop) or the like, and this may be used as the signal C ′.

The signal processing circuit 6 includes a positive / negative discriminating circuit 14 and an up / down counter 15.

The positive / negative discriminating circuit 14 uses the 90 ° synchronizing pulse f to discriminate the sign of the corresponding parallel data g. That is, 9
The data g when the 0 ° synchronization pulse f is “1” is a positive pulse P ⁺
And then, data g a period of "0" is negative pulse P ^- and. Positive pulse P ⁺ and negative pulse P ^-, respectively, up-down counter
There are 15 up inputs and down inputs. From the above,
For each magnetic flux sensor 1, its magnetic flux signal can be obtained as digital data. That is, the pulse output b ₁ to b _n is because a pulse that is obtained as a result of the feedback magnetic flux sensor within 1, by counting this, the digital data of the magnetic flux signal is obtained immediately.

It should be noted that an analog integrator may be used instead of the positive / negative discriminating circuit 14 and the up / down counter 15, and if a D / A converter is used after the up / down counter 15, it goes without saying that analog data can be directly obtained.

FIG. 9 is a block diagram of another embodiment of the present invention, and FIG. 10 is a time sequence showing the operation.

This embodiment is an example in which a plurality of (2N) magnetic flux sensors 1 are divided into two sets, and bias currents a and a 'at different timings are supplied to each set.

The same number (N) of the magnetic flux sensors 1 ′ as the magnetic flux sensors 1 are prepared. That is, each of the magnetic flux sensors 1 and 1 'is a set including N magnetic flux sensors. A bias current a is supplied to the magnetic flux sensor 1 as in the above-described embodiment. The magnetic flux sensor 1 'is supplied with a bias current a' having a phase different from that of the bias current a by 90 °. 90 ° for this
A phase shifter 12 'is newly provided. Bias current a
', The magnetic flux sensor 1 and 1' and a output b ₁ to b _N of
And b _'1 to _{b' N} will as in Figure 10 shown.

The control circuit 3 includes a signal f similar to that of the above-described embodiment,
A signal f 'obtained by inverting the synchronization pulse e is supplied. The signal f 'results in a 90 corresponding to the bias current a'.
° Synchronization pulse. The control circuit 3 forms a control signal for sampling the output pulses b ₁ to b _N using the signal f, a control signal for the sampling of ₁ to b _'N' output pulse b with a 'signal f To form

Therefore, the multiplexer 4 operates during a half cycle that has not been operated in the above-described embodiment. Thus, the serial data d shown in FIG. 10 is obtained.

A signal f 'is supplied to a positive / negative discriminating circuit 14' corresponding to the magnetic flux sensor 1 'and used for positive / negative discrimination.

In this embodiment, the control signal is not supplied from the control circuit 3 to the multiplexer 5 and the serial data d
A synchronous clock is extracted from the data by a PLL or the like, and is used.

As described above, even if the operating frequency of the multiplexer 4 is limited, the number of the magnetic flux sensors 1 can be doubled without increasing the frequency. That is, the multiplexer 4 can be operated efficiently, and one output line 8 can be left.

FIG. 11 is a configuration diagram of still another embodiment of the present invention.
FIG. 12A is a time sequence showing the operation.

This embodiment has a plurality each of the flux sensors 1 of (N), time series to the bias current a ₁ to at different timings
It is an example of supplying the a _N.

The bias currents a _{1 to} a _N are supplied in this order with the timing shifted as shown in the figure, and the repetition period is T
(Equal to the period T of the above-described embodiment). Each of the bias currents a _{1 to} a _N is a sine wave for one cycle of the pulse width t. Therefore, the number of operable magnetic flux sensors 1 is T / t.

As for the output of one magnetic flux sensor 1, a positive output and a negative output are continuously obtained within the pulse width t. To detect this, the synchronization pulse e is supplied with the bias currents a _{1 to} a _N
(Not shown), and the 90 ° synchronization pulse f rises and falls in synchronization with the positive and negative peaks of the bias currents a _{1 to} a _N. Multiplexer 4, in synchronization with the rising edge of the 90 ° sync pulse f, for example, the output b ₁ selects and outputs, in synchronization with the falling edge of the 90 ° sync pulse, and outputs again selects the output b _1. In addition,
A delay corresponding to the operation time of the control circuit 3 and the multiplexer 4 is considered.

Here, the bias current may be as shown in FIG. 12B. That is, in the first T / 2 cycle, the pulse width t / 2
Is supplied to each magnetic flux sensor sequentially, and the subsequent T /
Similarly, a negative pulse is supplied in two cycles. In other words, in the cycle T, positive and negative pulses may be given to the magnetic flux sensors 1 one by one with a time phase shifted.

 FIG. 13 is a configuration diagram of still another embodiment of the present invention.

This embodiment is an example in which a feedback circuit is provided on the room temperature side.

The digital output of the up / down counter 15 is input to the corresponding D / A converter 11, and is converted into an analog signal. This signal is input to the corresponding magnetic flux sensor 1 via the feedback resistor 90.

The magnetic flux sensor 1 has the configuration shown in FIG. 14A or FIG. 15B. The magnetic flux sensor 1 of FIG. 14A and FIG.
It has the same configuration as the squid sensor 21 in FIG. 4 and the squid sensor 51 in FIG. 4B. As shown in FIG. 14A, a feedback coil is used to input the feedback signal FB.
91 is added. The same applies to the case of FIG. 14B, although not shown.

According to this embodiment, the number of feedback lines 92 (N
), The number of connections between the cooling side and the room temperature side increases. However, the feedback amount is reduced to 90
Can be changed easily. Further, since the dynamic range of the entire magnetometer is determined by the dynamic range of the up / down counter 15 and the D / A converter 11, it is easy to change the dynamic range.

As described above, the present invention has been described with reference to the embodiments. However, the present invention is not limited to these, and various modifications can be made within the scope of the gist.

For example, as shown in FIG. 15, in any of the embodiments, the ground (ground potential or ground potential wiring) may be separated between the magnetic flux sensor 1 and another circuit. This is for the following reasons. Normally, a bias current several tens times the bias current of the SQUID flows through the multiplexer 4 formed by a Josephson circuit. This bias current is called TT
It corresponds to the DC current at L, and is the AC current in the Josephson circuit. Now, it is assumed that the ground of the SQUID is common to the grounds of other circuits. In this case, since it is inevitable that the ground has impedance, its potential fluctuates due to the return current of the bias current. Here, as described above, since the bias current of the multiplexer 4 is larger than that of the SQUID, the fluctuation of the ground level has a great influence on the operation of the SQUID and cannot be ignored. In particular, when the bias current has noise, the current value becomes large, which causes a serious problem.
Therefore, it is preferable to separate the grounds in order to eliminate such effects.

That is, in FIG. 15, a transformer T by magnetic field coupling is provided between each of the magnetic flux sensors 1 and the multiplexer 4, and the grounds thereof are separated from each other. In the drawing, different ground symbols are used to indicate separation. The same applies between the magnetic flux sensor 1 and the bias line 9 (that is, the oscillator 2). By doing so, the bias current of other circuits can be reduced by SQUI
It can be prevented from affecting the ground of D as noise,
The reliability of the operation of the magnetic flux sensor 1 can be improved.

Further, the bias current does not necessarily have to be a sine wave, but gives a positive or negative bias, and can maintain the output of the magnetic flux sensor 1 for a predetermined period (that is, while the multiplexer 4 is sampling). I just need.

Also, the control circuit 3 can be provided on the cooling side. In this case, the number of cables extending from the cooling side can be further reduced.

Further, the above embodiments can be used in combination. That is, in the embodiment of FIG. 13, the bias current may be supplied as in the embodiment of FIG. 9 or FIG. In each of the embodiments shown in FIGS. 3, 9, and 11, a part (for example, half) of the N magnetic flux sensors 1
It may be configured as in the embodiment of FIG. In this case, the feedback amount can be appropriately changed by providing the feedback circuit on the room temperature side. Further, a plurality of sets of the embodiment shown in FIGS. 3, 9, and 11 and the embodiment shown in FIG. 13 may be provided in parallel. For example, N magnetic flux sensors 1 according to the embodiment of FIG. 3 and N magnetic flux sensors 1 according to the embodiment of FIG. 13 may be used together. Further, a plurality of the embodiments shown in FIG. 3 may be provided in parallel. This is the same for the embodiments of FIGS. 9, 11 and 13. In this case, it is not necessary to consider the limitation of the number of the magnetic flux sensors 1 due to the operating frequency of the multiplexer 4, but the advantage of each embodiment is not lost.

[Effects of the Invention] As described above, according to the present invention, in a multi-channel SQUID magnetometer, a plurality of SQUID flux sensors are always operating, and the number of output lines is reduced by using a multiplexer. Therefore, there is no problem of the response to the input magnetic flux, and it is possible to reduce the consumption of the cryogen of SQUID such as liquid helium.

[Brief description of the drawings]

1 is a diagram illustrating the principle of the present invention, FIG. 2 is a diagram illustrating the operation of the present invention, FIG. 3 is a diagram illustrating the configuration of one embodiment, FIG. 4 is a diagram illustrating the configuration of a magnetic flux sensor, FIG. 6 is a diagram of a multiplexer, FIG. 7 is another diagram of a multiplexer, FIG. 8 is a diagram showing a time sequence, FIG. 9 is a diagram of another embodiment, FIG. 10 is a diagram showing a time sequence, FIG. 11 is a block diagram of still another embodiment, FIG. 12 is a diagram showing a time sequence, FIG. 13 is a block diagram of still another embodiment, and FIG. 14 is a block diagram of a magnetic flux sensor. Fig. 15 is an explanatory diagram of magnetic flux sensor separation, where 1: magnetic flux sensor, 2: oscillator, 3: control circuit, 4, 5: multiplexer, 6: signal processing circuit, 11: D / A converter, 12: phase shifter, 13: Preamplifier, 14: Positive / negative discrimination circuit, 15: Up / down counter.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Norio Fujimaki 1015 Uedanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (72) Inventor Osamu Hayashi 1015 Ueodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Fujitsu Limited ( 72) Inventor Kotaro Goto 1015 Uedanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (56) References JP-A-2-74882 (JP, A) JP-A-64-21379 (JP, A) 2-10181 (JP, A) JP-A-63-313082 (JP, A) JP-A-63-290979 (JP, A) JP-A-62-226072 (JP, A) U) Shokai 60-114960 (JP, U) Heikai 98662 (JP, U) "Practical noise reduction technique" 93 JATEK Publishing, issued on June 15, 1978, Ishida, Yanagawa, Yoshiyoshi "Superconducting Integrated Circuits" PP. 71-74 IEICE March 25, 1983

Claims (10)

(57) [Claims] 1. A multi-channel SQUID magnetometer for arranging a plurality of magnetic flux sensors (1) each comprising a digital SQUID capable of obtaining a pulse output and detecting a magnetic flux signal, wherein the same bias current is applied to the plurality of magnetic flux sensors (1). And a bias means (2) for supplying the same in common, and an on / off measurement for each output of the plurality of magnetic flux sensors (1) is performed every half cycle T / 2 of the bias current, so that each output is First conversion means (4) for converting the serial data into serial data; signal processing means (6) provided for each of the plurality of magnetic flux sensors (1) for outputting the magnetic flux signal; After converting to
A second conversion means (5) for supplying each of them to a corresponding one of the signal processing means (6), and a signal synchronized with the bias current to the first and second conversion means (4, 5) Control means (3), and while each of the magnetic flux sensors (1) outputs a pulse by the bias current commonly supplied from the bias means (2), the control means (3) Wherein the first conversion means (4) converts the pulse into the serial data according to the signal of (1). 2. A pulse is sequentially applied from the biasing means (2) to each of the magnetic flux sensors (1) in a time-series manner, and in synchronism therewith, the first converting means (4) applies the corresponding magnetic flux. The multi-channel according to claim 1, wherein the output of the sensor (1) is sampled and converted into serial data.
SQUID magnetometer. 3. The plurality of magnetic flux sensors (1), bias means (2), control means (3), first and second conversion means (4,
5) and the signal processing means (6) as one set, a plurality of sets are provided, the plurality of magnetic flux sensors (1) are divided into two sets, and the phase of the bias current applied to each set is 90 °. The multi-channel according to claim 1, wherein the pulses output at different timings by 90 ° for each set are converted into serial data by the first conversion means during the output period.
SQUID magnetometer. 4. The multi-channel device according to claim 1, wherein said biasing means is an oscillator.
SQUID magnetometer. 5. The multi-channel device according to claim 1, wherein said first conversion means is a multiplexer.
SQUID magnetometer. 6. The multiplexer (4) outputs one or more high-level bits, and the second conversion means (5) uses the high-level bits as a trigger for the conversion. The multi-channel according to claim (5)
SQUID magnetometer. 7. The multi-channel device according to claim 5, wherein said multiplexer is a shift register comprising a Josephson integrated circuit.
SQUID magnetometer. 8. The multi-channel device according to claim 1, wherein each of the plurality of magnetic flux sensors includes a feedback circuit formed of a superconducting circuit.
SQUID magnetometer. 9. A feedback circuit comprising the superconducting circuit comprises a superconducting inductance and a superconducting gate for storing magnetic flux quanta therein, and the signal processing means (6) comprises a counter (15). The multi-channel SQUID magnetometer according to claim 8, wherein 10. Each of the signal processing means (6) comprises a counter (15) and a D / A converter (11), and applies its output to each of the corresponding magnetic flux sensors (1) in a direction to cancel the input magnetic flux. The multi-channel according to claim 1, wherein:
SQUID magnetometer.