JPH05312929A - Magnetic-flux measuring apparatus using digital superconducting quantum interferrometer - Google Patents

Abstract

PURPOSE:To adjust the apparatus in the obtimum state by judging magnetic traps, the presence or absence of a feedback lock and the counting errors in a counter circuit with regard to a magnetic-flux measuring apparatus using a digital superconducting quantum interferometer (SQUID), which is used in the high-sensitivity measurement of minute magnetic flux. CONSTITUTION:A pulse-generating-frequency measuring circuit 5, which measures the positive or negative pulse generating frequency P outputted from a digital SQUID 1 in response to a bias current 1b, is provided. A judging circuit 6 judges the trapped state of the magnetic flux in the digital SQUID 1, the presence or absence of a feedback lock and the counting errors in a counter circuit 4 based on the measured frequency P from the pulse-generating frequency measuring circuit 5. A control circuit 7 performs the adjustment based on the result of the judgment of the judging circuit 6. There circuits 6 and 7 are further provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital superconducting quantum interferometer (SQUI) used for highly sensitive measurement of minute magnetic flux.
The magnetic flux measuring device using D). In recent years, superconducting quantum interferometers (hereinafter referred to as "SQUIDs") have been used to measure minute magnetic fields generated by living organisms.
A high-sensitivity magnetometer is used.

In particular, it is possible to estimate the current source generating a magnetic field by measuring the magnetic field distribution in the brain and heart, which provides very meaningful information for diagnosis and neural activity in the living body. It has been pointed out that it is useful for elucidating. A magnetometer using such a SQUID is conventionally
It is driven by an analog circuit that requires subtle adjustments. In addition, the measurement of the magnetic field distribution can be performed by SQUID of one channel.
Using a magnetometer, time series data for each part is measured in sequence. However, in the one-channel SQUID magnetometer, there are problems that the measurement takes time and the subject is fatigued, and the magnetic fields at different locations cannot be measured simultaneously, so that the current source cannot be estimated with high accuracy.

For this reason, a multi-channel SQUID capable of arranging a plurality of SQUIDs and simultaneously measuring the magnetic fields of respective parts
Flux meters are being used. However, since the number of channels is increased, the complexity of adjustment increases by the number of channels. Therefore, there is a need for a multi-channel SQUID magnetometer that is easy to adjust.

[0004]

2. Description of the Related Art Conventionally, as a digital SQUID capable of obtaining a pulse output according to a magnetic field to be measured, an SQUID composed of a two-junction quantum interference device is AC biased to output a pulse as shown in FIG. 12546
No. 9) magnetometer and DC SQ that operates in analog
A magnetometer (D.D.) that obtains a pulse output by adding the UID voltage output to a superconducting comparator or 1-bit A / D converter.
Drung, Cryogenics, vol.26, pp623-627, 1986) are known.

In the digital SQUID of FIG. 6, 1
Is an SQUI containing one or more Josephson junctions 11 and 12
D and 2 are feedback circuits, which are a write gate 12 and a superconducting inductance 13 for obtaining a pulse output proportional to the difference between the positive pulse and the negative pulse from the SQUID 1.
It is composed of a superconducting feedback circuit 2 provided with.

3 is an AC bias current (I
b) a pulse generator for supplying the signal, 17 a register for storing the pre-measured optimum bias current Ibo of SQUID 1, 18 a DA converter for converting the digital value of the register 17 into an analog quantity, 19 a multiplier And the bias current Ib according to the output of the DA converter 18
Control the amplitude value of.

Reference numeral 16 is a pickup coil for picking up an external magnetic flux, and 17 is SQ for the magnetic flux picked up by the pickup coil 16.
An input coil 15 for transmitting to the UID1 is a feedback coil, which generates a magnetic flux proportional to the output of the superconducting feedback circuit 2 and magnetically couples to the SQUID1 to enable a feedback operation. Further, 4 is a counter circuit, which includes a comparator 20 and an up / down counter 21.
Equipped with. The comparator 20 obtains two output pulse trains of only the positive pulse and only the negative pulse from the output pulse train of the mixed positive and negative pulses from the SQUID 1 to which the feedback is applied.

The up-down counter 21 counts up with a positive pulse from the comparator 20 and down-counts with a negative pulse, and feeds back SQU.
A digital output (the number of bits is determined by the number of bits of the counter) proportional to the input magnetic flux Φi is obtained based on the output pulse train of ID1. For the AC bias current Ib for SQUID1, the optimum value Ibo is measured and found in advance,
If the value is set in the register 17 so as to be the optimum value Ibo, the adjustment only needs to be performed once.

[0009]

However, in the magnetic flux measuring device using such a conventional digital SQUID, for example, the Josephson junction 1 of SQUID1 is used.
If the trap magnetic flux is linked to 1 and 12, SQUID
The critical current value Ibo of the AC bias current flowing in 1 changes, and it does not become the optimum value measured in advance, and it is necessary to remove the trap magnetic flux.

The effect of the trap magnetic flux will be described in detail below. FIG. 7A shows a digital SQ.
It shows the operating characteristics by the magnetic flux Φ of the UID and the bias current Ib, and in the state of Φ = 0 without a magnetic flux trap, the amplitude value of the alternating bias current Ib of both polarities is positive and negative ± symmetrical.
It is set to be Ibo, and FIG.
FIG. 7 synchronized with the amplitude of the AC bias current shown in (c).
The occurrence frequencies of the positive pulse and the negative pulse in (d) are completely the same and are in the zero point state.

On the other hand, when the digital SQUID has a magnetic flux trap, as shown in the characteristic of the solid line in FIG. 7B, the trap magnetic flux shifts on the horizontal axis, and the positive pulse and the negative pulse when the magnetic flux Φ = 0. The frequency of occurrence of does not match, resulting in a detection error. More specifically, the operation characteristics of the digital SQUID are blurred due to noise,
A pulsed output is not completely obtained with respect to the amplitude of the AC bias current, and the generation of pulses is determined with a certain probability as shown in FIG.

Therefore, by setting the bias current amplitude values to be symmetric when Φ = 0, it is possible to keep the occurrence frequency of the positive pulse and the negative pulse, that is, the probability the same. However, if this symmetry is broken by the magnetic flux trap, the frequency of generation of the positive pulse and the frequency of the negative pulse are different, and an error occurs in the count value according to the measured magnetic flux as the difference between the two. Further, if a magnetic flux trap is generated in the write gate 13 of the superconducting feedback circuit 2 or the like, feedback lock is not applied and the trap magnetic flux must be removed.

FIG. 9 shows the feedback characteristic in the digital SQUID. When the magnetic flux trap is shifted within the range shown by the shaded portion of the operating characteristic, the feedback is effective and it can be locked to the symmetrical characteristic of the solid line. .. However, when the magnetic flux is trapped and shifts beyond the feedback possible area as shown by the broken line, the feed hack does not occur and the symmetrical characteristic of the solid line cannot be locked, resulting in a measurement error.

In the conventional digital SUQID against the occurrence of such a magnetic flux trap, the feedback lock can be confirmed only by observing the signal waveform with a synchroscope or the like, which is confirmed at the time of inspection and adjustment. It is not possible to confirm when using, and the presence or absence of feedback lock is unknown. Even if the feedback lock is applied, the positive pulse or the negative pulse may not be reproduced correctly due to the temperature drift of the comparator 20 provided in the counter circuit 4, and a counting error may occur in the up-down counter 21. Regarding this point, there has been a problem that no measures have been taken conventionally.

The present invention has been made in view of the above conventional problems, and a digital SUQUI capable of adjusting to an optimum state by judging the presence or absence of a magnetic trap, a feedback lock, and a count error in a counter circuit.
An object is to provide a magnetic flux habitat device using D.

[0016]

FIG. 1 is a diagram for explaining the principle of the present invention. First, according to the present invention, the output characteristics of a digital SUQUID (digital superconducting quantum interferometer) 1 and a digital SUQID 1 that receive a magnetic flux input according to a magnetic field to be measured Φi and generate a pulsed output according to an AC bias current Ib are linear. A feedback circuit (2) for converting the digital SUQID1 into a bias current source 3 for supplying an AC bias current Ib having a predetermined amplitude and a predetermined amplitude to the digital SUQID1; and counting pulse signals obtained through the feedback circuit 2 of the digital SUQID1. A magnetic flux measuring device using a digital SUQID provided with an output counter circuit 4 is targeted.

According to the present invention for such a magnetic flux measuring device, the bias current Ib is calculated from the digital SUQID1.
Pulse generation frequency measuring circuit 5 for measuring the generation frequency P of the pulse output according to the pulse generation frequency, and the magnetic flux trap state of the digital SUQID 1 based on the measurement frequency (P) of the pulse generation frequency measuring circuit 5 and the presence or absence of feedback lock, And a judgment circuit 6 for judging a count error in the counter circuit 4.
And a control circuit 7 that performs a corresponding process based on the determination result of the determination circuit 6.

Here, the digital SUQID 1 outputs a positive pulse and a negative pulse by the alternating bias current Ib of both polarities, and the pulse generation frequency measuring circuit 5 is provided with a positive pulse counter 8 and a negative pulse counter 9. Judgment circuit 6
Is a magnetic flux trap state of the digital SUQID1 by comparing the positive pulse generation frequency (P +) and the negative pulse generation frequency (P-) obtained by the pulse generation frequency measuring circuit 5 in a predetermined amplitude adjustment state of the AC bias current Ib. ,
It is characterized in that the presence / absence of feedback lock and the count error in the counter circuit 4 are determined.

Further, the pulse generation frequency measuring circuit 5 is provided with a counter 30 for counting the output pulses synchronized with the AC frequency of the bias current source 3, and the count value of the counter 30 is a specified total number used for measuring the pulse generation frequency. When the number of pulses N is reached, the positive pulse counter 8 and the negative pulse counter 9 are reset, and the counter count value C +,
From C- and the total number of pulses N, the pulse generation frequencies P + and P- are calculated as P + = C + / N and P- = C- / N.

Specifically, the control circuit 7 adjusts the amplitude value of the AC bias current Ib from the bias current source 3 to an amplitude smaller than the critical current value Ibo of the digital SUQID1 measured in advance, and in this amplitude adjustment state. The determination circuit 6 uses the digital SUQID when the positive pulse occurrence frequency (P +) or the negative pulse occurrence frequency (P-) becomes high.
It is determined that a magnetic flux trap has occurred in the control circuit 1 and the control circuit 7 supplies a current for canceling the magnetic flux trap to the heaters 10 and 11 provided in the vicinity of the digital SUQID 1 based on the determination result.

Further, the control circuit 7 adjusts the amplitude value of the AC bias current Ib from the bias current source 3 to an amplitude near the critical current value Ibo of the digital SUQID1 measured in advance, and in this amplitude adjustment state, the judgment circuit 6 is positive. When the pulse generation frequency (P +) and the negative pulse generation frequency (P-) do not match, it is determined that the feedback lock is not applied, and the control circuit 7 determines the digital SU based on the determination result.
A current is applied to the heaters 10 and 11 provided in the vicinity of QID1 in order to perform feedback lock.

Further, if the control circuit 7 repeats the operation of supplying the current for canceling the state of the magnetic flux trap or the state where the feedback lock is not applied to the heaters 10 and 11 a plurality of times, the pulse generation frequency does not change from the original state. ,
Stops the device when it judges that it is not recoverable. Furthermore, after confirming the feedback lock and canceling the magnetic flux trap by the control circuit 7, the pulse generation frequency measurement circuit 5 outputs the positive pulse generation frequency (P +) and the negative pulse generation frequency (P +).
-) Is measured at regular intervals, and if the average value of positive pulse occurrence frequencies (Pa +) is larger than the average value of negative pulse occurrence frequencies (Pa-), it is determined that a positive pulse count error has occurred. Then, the count value in the counter circuit 4 is decreased.

On the contrary, the average value of negative pulse occurrence frequency (Pa-)
Is larger than the average value (P +) of the positive pulse generation frequencies, it is determined that a negative pulse count error has occurred, and the count value of the counter circuit 4 is increased.

[0024]

The digital S of the present invention having such a structure
According to the magnetic flux measuring device using the QUID, the positive pulse generation frequency (+ P) and the negative pulse generation frequency (P) are gradually increased while gradually increasing the amplitude value of the AC bias current Ib toward the optimum value Ibo.
-) Is measured and compared, and based on the result, digital SQ
UID and circuit system abnormalities are detected and S is heated by the heater.
The initial start of the QUID will be forced.

For example, a digital SQUA measured in advance
When the amplitude of the bias current is smaller than the critical current value Ibo of the ID, the frequency of occurrence of the positive pulse or the negative pulse is high, so that it is determined that the magnetic flux trap is generated in the digital SQUID, and the magnetic flux trap is generated for the digital SQUID. The heater is heated by passing an electric current for canceling.

Pre-measured digital SQUID
The positive pulse generation frequency and the negative pulse generation frequency are measured in the adjustment state near the critical current value Ibo of 1., and when the two do not match, it is determined that the feedback lock is not applied, and the magnetic flux trap is canceled. A current is supplied to heat with a heater. After confirming the feedback lock and canceling the magnetic flux trap for the digital SQUID, the positive pulse occurrence frequency and the negative pulse occurrence frequency are measured at regular intervals, and the average value of the positive pulse occurrence frequencies is the negative pulse. If it is larger than the average value of the occurrence frequency of the positive pulse, it is determined that a positive pulse count error has occurred, the counter value of the up-down counter is decreased, and conversely, the average value of the negative pulse occurrence frequency is If the occurrence frequency is larger than the average value, it is determined that a negative pulse counting error has occurred, and the counter value is increased.

By such processing, the digital SQUI
The magnetic flux trap of D, the presence / absence of feedback lock, and the count error of the counter in the feedback circuit 2 are determined, and the SQUID and the abnormality of the circuit system are recovered and corrected, so that the optimum operating state can always be created.

[0028]

2 is a block diagram of an embodiment showing a basic embodiment of the present invention. In FIG. 2, the digital SQUID
1 includes two Josephson junctions 12 as a two-junction quantum interference device. An input coil 17 is provided for the digital SQUID 1. The pickup coil 16 is connected in series to the input coil 17, and the measured magnetic flux Φi picked up by the pickup coil 16 is digital SQUA.
Tell ID1.

A feedback coil 15 is provided for the digital SQUID 1. The feedback coil 15 is connected to the superconducting feedback circuit 2, and the superconducting feedback circuit 2 is provided with a write gate 13 and a superconducting inductance 14. Digital S
A pulse generator 3 is provided as a bias current source for supplying a bipolar AC bias current to the QUID 1. The output of the pulse generator 3 is input to the multiplier 19. The output of the DA converter 18 is also input to the multiplier 19,
By multiplying the output value of the DA converter 18 and the AC bias current of the pulse generator 3, the digital SQUI
The amplitude value of the AC bias current Ib flowing through D1 via the resistor R1 can be adjusted.

In the steady operation state, the DA converter 1
Reference numeral 8 receives the data setting for obtaining the optimum bias current (critical current value) Ibo for the digital SQUID 1 measured in advance and produces an analog output. The mixed pulse output of the positive pulse and the negative pulse generated by the supply of the AC bias current to the digital SQUID 1 is given to the comparator 20 and separated into the positive pulse and the negative pulse,
It is supplied to the up / down counter 21. The up / down counter 21 counts up with a positive pulse separated by the comparator 20, and counts down with a negative pulse. The coefficient value of the up / down counter 21 becomes a count value proportional to the measured magnetic flux Φi picked up by the pickup coil 16.

In addition to the circuit system for the digital SQUID 1, the generation of magnetic flux traps and the presence / absence of feedback lock when the power is turned on to start the apparatus,
Also, in order to determine a counter mistake in a state where the feedback lock is effectively applied and take a countermeasure, a pulse generation frequency measuring circuit 5, a determining circuit 6 and a control circuit 7 are provided.

The pulse generation frequency measuring circuit 5 is provided with a positive pulse counter 8 for counting the positive pulses output from the comparator 20 and a negative pulse counter 9 for counting the negative pulses output from the comparator 20. The positive pulse counter 8 and the negative pulse counter 9 are reset at regular intervals corresponding to the total number N of pulses that gives a pulse generation probability 1 from the digital SQUID 1 depending on the frequency of the AC bias current. The numerical value is output to the determination circuit 6.

The determination circuit 6 divides the count value of the positive pulse counter 8 by the total number N of pulses having a pulse generation frequency of 1 to obtain the positive pulse generation frequency P +, and also the count value of the negative pulse counter 9 with the total pulse number N. Divide negative pulse frequency P
Based on the positive pulse generation frequency P + and the negative pulse generation frequency P-, magnetic trap determination, presence / absence of feedback lock, and counting of the up / down counter 21 provided in the counter circuit 4 with feedback lock applied Make a mistake.

The control circuit 7 uses the judgment circuit 6 to
Based on the determination result of 1., control is performed to take countermeasures for the digital SQUID 1 and its circuit system. Further, the control circuit 7 is D
The amplitude value of the AC bias current Ib for the digital SQUID 1 by the multiplier 19 is controlled by adjusting the data for the A converter 18. Since the optimum value Ibo of the AC bias current Ib measured in advance is known in the control of the amplitude value of the AC bias current Ib, the AC bias current Ib is set in the calibration process immediately after the power of the apparatus is turned on. The determination circuit 6 determines the relationship between the positive pulse generation frequency P + and the negative pulse generation frequency P- based on the positive pulse and negative pulse counter count values obtained by the pulse generation frequency measurement circuit 5 while gradually increasing toward the optimum value Ibo. First, the presence or absence of the magnetic flux trap is determined.

The determination of the magnetic flux trap is that either one of the positive pulse generation frequency P + and the negative pulse generation frequency P- is high at the amplitude of the AC bias current smaller than the AC bias current optimum value Ib. Digital SQU
It is determined that the ID has a magnetic flux trap. When the determination result of the magnetic flux trap by the determination circuit 6 is obtained, the control circuit 7 controls the heater 10 provided near the write gate 13 provided in the feedback circuit 2 and the heater 11 provided near the digital SQUID 1. On the other hand, the heater drive circuit 22 is driven to pass an electric current, and the heater 1
The feedback circuit 2 and the digital SQUID are temporarily heated by turning 0 and 11 on and off.

The temporary heating of the feedback circuit 2 and the digital SQUID 1 by the heaters 10 and 11 restores both of them to the initial state before starting the apparatus, and carries out a temperature initial reset to bring them into the starting state again. That is, in an actual device, the digital SQUI
The D1, the input coil 17, the pickup coil 16, the feedback coil 15, and the feedback circuit 2 are put in a liquid helium container and used in a cooled state. Therefore, temporarily heating with the heaters 10 and 11 means returning the digital SQUID and the feedback circuit 2 to the state before the start of cooling, and by returning the heaters 10 and 11 that have been turned on once again to the original state, It returns to the cooling state and adjustably releases the magnetic flux trap generated at the first cooling start.

This is because the generation of the magnetic flux trap at the time of starting the apparatus happens because the magnetic flux to be trapped is activated at the timing when it acts on the digital SQUID 1 and the feedback circuit 2. Since there is no magnetic flux trap if there is no timing, the magnetic flux trap is eliminated by retrying the initial start by turning on / off the heaters 10 and 11.

Next, the presence or absence of feedback lock in the determination circuit 6 determines whether the AC bias current Ib is the optimum value Ib after the magnetic flux trap is eliminated or after the magnetic flux trap is eliminated.
If the positive pulse occurrence frequency P + and the negative pulse occurrence frequency P- measured with the amplitude value adjusted to o match, it is determined that the feedback lock is applied. If they do not match, the feedback lock is applied. Determine not.

When the determination circuit 6 determines that the feedback lock is not applied, the control circuit 7 drives the heater drive circuit 22 to turn on / off the heaters 10 and 11 to turn on the current, as in the case of determining the magnetic flux trap. Sink, digital SQUID 1 and feedback circuit 2
Is heated temporarily to perform an initial start and create a state where feedback lock is applied.

Further, the count error of the up / down counter 21 caused by the determination circuit 6 is to measure the positive pulse generation frequency P + and the negative pulse generation frequency P- in a constant cycle after the magnetic flux trap is eliminated and the feedback lock is determined. At the same time, the average value Pa + of the positive pulse generation frequency and the average value Pa− of the negative pulse generation frequency are obtained each time a predetermined number of measurement results are obtained.

The average value Pa + of the positive pulse generation frequency and the average value Pa− of the negative pulse generation frequency thus obtained are compared, and the average value Pa + of the positive pulse generation frequency is calculated from the average value Pa− of the negative pulse generation frequency. If the average value Pa− of the positive pulse generation frequency is larger than the average value Pa + of the positive pulse generation frequency, a negative pulse count error occurs. When the positive pulse counting error is determined, the control circuit 7 corrects the up-down counter 21 by subtracting a predetermined correction value ΔC to reduce the correction value. When the negative pulse counting error is determined, the control circuit 7 corrects. Correction is performed by adding a predetermined correction value ΔC to the counter value of the up / down counter 21 and increasing the count value.

Specifically, since the up-down counter 21 is reset at every predetermined measurement cycle, the reset value + ΔC or −ΔC corresponding to the determination result of the positive pulse or negative pulse count error is preloaded after the reset. Then, the counter operation may be performed. FIG. 3 is a block diagram of a more specific embodiment in which the judgment circuit 6 and the control circuit 7 shown in the basic embodiment of FIG. 2 are realized by a CPU.

In FIG. 3, the judgment circuit 6 and the control circuit 7 of FIG. 2 are realized by the functions of the judgment unit 6a and the control unit 7a realized by the program control of the CPU 27. I / O to the CPU 27 via the internal bus 26
The device 25 is connected. The count value of the positive pulse counter 8 is input to the I / O device 25 via the latch circuit 23, and the count value of the negative pulse counter 9 is input to the I / O device 25 via the latch circuit 24.

The setting data for the DA converter 18 which determines the amplitude value of the AC bias current is the latch circuit 2.
Given through 8. Further, the heater drive circuit 22 that drives the heaters 10 and 11 is driven via the DA converter 29, and the value of the current flowing through the heaters 10 and 11 can be adjusted by the setting data for the DA converter 29.

The counter 30 resets the positive pulse counter 8 and the negative pulse counter 9 provided in the pulse generation frequency measuring circuit 5. The counter 30 counts the pulse output synchronized with the frequency of the AC bias current generated from the pulse generator 3 and counts a predetermined number of pulses giving a generation frequency of 1, that is, the total number of pulses N. The pulse counter 9 is reset.

Therefore, as for the count values of the positive pulse counter 8 and the negative pulse counter 9, the digital SQUID 1 produces pulse outputs in synchronization with all positive half cycles and all negative half cycles of the AC bias current. In this case, the counter is reset every time the count value reaches N, and since the pulse generation frequency is actually 1 or less, a value of the total pulse number N or less is counted.

Further, the I / O device 25 can load the up / down counter 21 provided in the counter circuit 4 with a correction value for correcting a count error. The write gate 13 provided in the feedback circuit 2 for the digital SQUID 1 is the digital SQUID 1
Similarly to the above, two Josephson junctions 12 are provided.

FIG. 4 is a flow chart showing a calibration process by the CPU 27 shown in the embodiment of FIG. 3 immediately after power-on of the apparatus. In FIG. 4, when the power of the apparatus is turned on and the apparatus is in the operating state, in step S1, the digital SQUA
The AC bias current Ib for ID is increased by a predetermined constant value ΔIb. The initial value of the AC bias current Ib is Ib = 0.

Then, in step S2, the positive pulse generation frequency P measured in the adjustment state of the AC bias current Ib at this time
The + and the negative pulse occurrence frequency P- are fetched, and it is determined in step S3 whether or not they match. If they match, the magnetic flux trap has not occurred, so the process proceeds to step S4, the AC bias current is further increased, and the current is increased until the AC bias current Ib reaches the optimum value Ibo in step S5.

When the AC bias current reaches the optimum value Ibo in step S5, the process proceeds to step S6 again, and the positive pulse generation frequency P is generated in the amplitude state of the optimum value Ibo of the AC bias current.
The + and the negative pulse occurrence frequency P- are fetched, and it is determined in step S7 whether or not they match. This determines the presence or absence of feedback lock. If they match in step S7, it is determined that the feedback lock is applied, and the process proceeds to step S8 to end the bias setting of the n channel that is currently the calibration target.

Subsequently, the channel number is incremented by 1 in step S9, it is determined in step S10 whether or not the channel is the final channel, and steps S1 to S are executed until the final channel is reached.
The process of 10 is repeated. If it is the final channel, the process proceeds to step S11, the bias setting of all channels is completed, and the normal biomagnetic measurement operation is started. On the other hand, if the positive pulse generation frequency P + and the negative pulse generation frequency P- do not match in the state where the AC bias current Ib in step S3 is smaller than the optimum value Ibo, it is determined that the magnetic flux trap is generated, and the step Go to S12 and digital SQU
Heater 1 provided in ID1 and feedback circuit 2
0 and 11 are turned on and off, and an initial reset is forcibly performed to eliminate the magnetic flux trap.

Subsequently, in step S13, the positive pulse generation frequency P + after the heater is turned on and off and the negative pulse generation frequency P- are fetched and compared in step S14. If they match, it is determined that the magnetic flux trap has been eliminated. The processing after step S4 is performed. If they do not match each other in step S14, it is determined in step S15 whether or not a predetermined number of retries have been performed, and the positive / negative pulse generation frequency after the forced reset by the heater on / off in steps S12 to S14. The matching judgment of is repeated.

If the positive and negative pulse generation frequencies do not match even after retrying the magnetic flux trap elimination processing M times, the current n channel bias cannot be set because the magnetic flux trap cannot be started in step S16.
After outputting an alarm in 13, the system is stopped in step S18. If the positive pulse generation frequency P + and the negative pulse generation frequency P- do not match in the state where the AC bias current is set to the optimum value Ibo in step S7, it is determined that the feedback lock is not applied,
Similar to the case of the magnetic flux trap determination, the process proceeds to step S12, and the recovery measure for the state in which the feedback lock is applied by turning the heater on and off is performed.

If the positive and negative pulse generation frequencies do not match even after M times of retrying the recovery measures to the state in which the feedback lock is applied, it is determined that the feedback lock is impossible, and the system is stopped in step S18. Become. FIG. 5 is a flow chart showing the counting error determination and correction processing performed in the operating state of the apparatus in which the calibration processing for eliminating the magnetic trap and recovering the feedback lock state shown in FIG. 4 has been completed.

In FIG. 5, first, in step S1, the positive pulse generation frequency P + and the negative pulse generation frequency P- are fetched,
After holding in the memory or the register, it is determined in step S2 whether or not a predetermined correction cycle has been reached. Step S2
When it is determined that the predetermined correction cycle has been reached in step S3, the process proceeds to step S3, and from the plurality of positive and negative pulse generation frequencies P +, P- obtained in one correction cycle, the average generation frequency Pa +, P- is generated.
Calculate a-.

Subsequently, the average occurrence frequency Pa + of the positive and negative pulses calculated in step S4 is compared with Pa-, and if the positive pulse average occurrence frequency Pa is larger than the negative pulse average occurrence frequency Pa-, it is determined that the positive pulse is missed. Then step S
At 5, the counter count value C is reduced to C = C−ΔC by setting the correction value −ΔC by the initial load immediately after the reset of the up / down counter.

On the other hand, if the average pulse generation frequency Pa- is larger than the positive pulse average generation frequency Pa + in step S4, it is determined that a negative pulse count error has occurred, and in step S6, the up-down counter 21 counts. Set + ΔC with preset load immediately after reset. As a result, the up / down counter 21 corrects the count error of the negative pulse by setting the counter count value C = C + ΔC.

Further, if the average occurrence frequency of the positive pulse and the negative pulse coincide with each other in step S4, it is natural that there is no counting error, so no correction is performed. In the count error determination and correction process of FIG. 5, a simple average of a plurality of pulse generation frequencies calculated for each fixed correction cycle is calculated and compared. It is also possible to perform a moving average calculation for obtaining an average value of a predetermined number of pulse generation frequencies in the past, and perform a counter correction by determining a count error each time the pulse generation frequency is captured, which can further improve the correction accuracy. ..

Since the count error of the up / down counter 21 is due to the influence of the temperature change of the device, the count error determination and correction processing are performed in a short cycle in a temperature unstable state immediately after the power supply of the device is turned on, and the time is counted. Since the temperature state stabilizes with the passage of, the correction cycle may be lengthened.

[0060]

As described above, according to the present invention, the counter meter corresponding to the generation of the magnetic flux trap, the presence / absence of the feedback, and the measured magnetic flux based on the generation frequency of the positive pulse and the negative pulse from the digital SQUID. A system configuration that can determine the numerical count error, eliminate the magnetic trap, recover the feedback lock state, correct the count error, and requires almost no artificial adjustment even for a multi-channel SQUID with multiple channels. This can greatly improve the performance of the magnetic flux measuring device using the digital SQUID.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of the principle of the present invention.

FIG. 2 is a configuration diagram of a basic embodiment of the present invention.

FIG. 3 is a configuration diagram of a specific embodiment of the present invention.

FIG. 4 is a flowchart showing a magnetic flux trap determination and cancellation process, a feedback lock presence / absence determination, and a recovery process according to the present invention.

FIG. 5 is a flowchart showing a count mistake determination and correction process according to the present invention.

FIG. 6 is an explanatory diagram of a conventional digital SQUID.

[Fig. 7] Operating characteristic diagram of digital SQUID

FIG. 8 is a characteristic diagram of pulse generation probability with respect to bias current of digital SQUID.

FIG. 9 is a characteristic diagram showing feedback characteristics of a digital SQUID.

[Explanation of symbols]

1: Digital SQUID (digital superconducting quantum interferometer) 2: Feedback circuit (superconducting feedback circuit) 3: Bias current source (pulse generator) 4: Counter circuit 5: Pulse frequency measuring circuit 6: Judgment circuit 6a: Judgment unit 7: Control circuit 7a: Control part 8: Positive pulse counter 9: Negative pulse counter 10, 11: Heater 12: Josephson junction 13: Write gate 14: Superconducting inductance 15: Feedback coil 16: Pickup coil 17: Input coil 18, 29: DA converter 19: Multiplier 20: Comparator 21: Up / down counter 22: Heater drive circuit 23, 24, 28: Latch circuit 25: I / O device 26: Internal bus 27: CPU 30: Counter

Claims (7)

[Claims] 1. A digital superconducting quantum interferometer (1) which receives a magnetic flux input corresponding to a magnetic field to be measured (Φi) and produces a pulsed output corresponding to an alternating bias current, and the digital superconducting quantum interferometer ( A feedback circuit (2) for linearizing the output characteristic of (1) and an AC bias current (I) of a predetermined amplitude at a predetermined frequency in the digital superconducting quantum interferometer (1).
a bias current source (3) for supplying b), and a counter circuit (4) for counting and outputting pulse signals obtained via the feedback circuit (2) of the digital superconducting quantum interferometer (1). In a magnetic flux measuring device using a digital superconducting quantum interferometer, the generation frequency (P) of pulses output from the digital superconducting quantum interferometer (1) according to the bias current (Ib) is A pulse generation frequency measuring circuit (5) to be measured, and based on the measurement frequency (P) of the pulse generation frequency measuring circuit (5), the magnetic flux trap state of the digital superconducting quantum interferometer (1) and the presence or absence of feedback lock. And a determination circuit (6) for determining a count error in the counter circuit (4), and a control circuit (7) for performing a corresponding process based on the determination result of the determination circuit (6). Flux measurement apparatus using a digital superconducting quantum interferometer, characterized in that. 2. A magnetic flux measuring apparatus using a digital superconducting quantum interferometer according to claim 1, wherein the digital superconducting quantum interferometer (1) is a bipolar alternating current bias current (I).
The pulse generation frequency measuring circuit (5) includes a positive pulse counter (8) and a negative pulse counter (9), and outputs the positive pulse and the negative pulse in accordance with the determination circuit (6). ) Compares the positive pulse generation frequency (P +) and the negative pulse generation frequency (P-) obtained by the pulse generation frequency measurement circuit (5) in a state where the amplitude of a specific AC bias current (Ib) is adjusted, Magnetic flux measurement using a digital superconducting quantum interferometer, characterized by determining a magnetic flux trapping state of the digital superconducting quantum interferometer (1), presence / absence of feedback lock, and counting error in the counter circuit (4). apparatus. 3. A magnetic flux measuring device using a digital superconducting quantum interferometer according to claim 2, wherein the pulse generation frequency measuring circuit (5) is synchronized with the AC frequency of the bias current source (3). A counter (3) for counting output pulses, the positive pulse counter (8) and the negative pulse when the count value of the counter (30) reaches a prescribed total number of pulses used for measuring the pulse generation frequency. Counter (9)
And a pulse generation frequency (P +, P-) is calculated from the counter count value immediately before the reset and the total number of pulses. A magnetic flux measuring apparatus using a digital superconducting quantum interferometer. 4. A magnetic flux measuring device using a digital superconducting quantum interferometer according to claim 1, wherein the control circuit (7) has an amplitude of an alternating bias current (Ib) from the bias current source (3). The critical current value (Ib) of the digital superconducting quantum interferometer (1) whose value is measured in advance
The amplitude is adjusted to be smaller than that of o), and in the amplitude adjustment state, the determination circuit (6) outputs the digital signal when one of the positive pulse generation frequency (P +) and the negative pulse generation frequency (P-) becomes high. It is determined that a magnetic flux trap is generated in the superconducting quantum interferometer (1), and the control circuit (7) determines the heater (10) provided near the digital superconducting quantum interferometer (1) based on the determination result. , 11), a magnetic flux measuring apparatus using a digital superconducting quantum interferometer, wherein a current for canceling a magnetic flux trap is passed. 5. A magnetic flux measuring apparatus using a digital superconducting quantum interferometer according to claim 1, wherein the control circuit (7) has an amplitude of an alternating bias current (Ib) from the bias current source (3). The critical current value (Ib) of the digital superconducting quantum interferometer (1) whose value is measured in advance
o) Amplitude is adjusted to the vicinity, and in the amplitude adjustment state, the determination circuit (6) is in feedback lock when the positive pulse occurrence frequency (P +) and the negative pulse occurrence frequency (P-) do not match. It is determined that there is not, and the control circuit (7) determines the digital superconducting quantum interferometer (1) based on the determination result.
A magnetic flux measuring device using a digital superconducting quantum interferometer, characterized in that an electric current is caused to flow in order to apply a feedback lock to the heaters (10, 11) provided in the vicinity of. 6. A magnetic flux measuring device using a digital superconducting quantum interferometer according to claim 4, wherein said control circuit (7) is a current for canceling a state in which a magnetic flux trap or a feedback lock is not applied. The heater (1
(0, 11) A magnetic flux measurement apparatus using a digital superconducting quantum interferometer, which is characterized in that the apparatus is stopped when the pulse generation frequency does not change from the original state even if the operation is repeated a plurality of times. 7. A magnetic flux measuring device using a digital superconducting quantum interferometer according to claim 4, wherein after confirmation of feedback lock and cancellation of magnetic flux trap by the control circuit (7), The pulse generation frequency measuring circuit (5) measures the positive pulse generation frequency (P +) and the negative pulse generation frequency (P-) at regular intervals, and the average value (Pa +) of the positive pulse generation frequency is the negative pulse generation frequency. Average value (Pa
If it is larger than −, it is determined that a positive pulse count error has occurred, the count value in the counter circuit (4) is decreased, and conversely, the average value of negative pulse occurrence frequencies (Pa
(-) Is larger than the average value (P +) of the positive pulse generation frequency, it is determined that a negative pulse count error has occurred, and the count value of the counter circuit (4) is increased. Magnetic flux measurement device using a superconducting quantum interferometer.