JPH0690267B2 - Digital squid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] Regarding a digital squid used for measuring a minute magnetic flux or current, the number of bits of an up / down counter is set so that positive and negative output pulses of the digital squid appear almost alternately. For the purpose of realizing a highly sensitive and highly sensitive measuring device in terms of mounting, a squid consisting of a superconducting loop including a superconducting inductance and two or more Josephson junctions, and a current to be measured is passed through the squid. A magnetic field coupling line and an amplitude modulation waveform generator for applying a bias current of an amplitude modulation waveform to the squid, the amplitude modulation wave is applied to at least one injection terminal of the squid, and the output is extracted as a pulse train. To configure.

[Industrial application field]

The present invention relates to a digital squid used for highly sensitive magnetic flux and current measurement.

In the narrow sense, a superconducting loop including two or more Josephson junctions and a superconducting inductance is squeezed (SQUID, Supercondu
cting Quantum Interference Device), and it is also called a two-junction quantum interferometer when it has two Josephson junctions. At present, this is an element that can detect magnetic flux (current) with the highest sensitivity. On the other hand, squid in the title is a general term that has a function as a magnetometer including circuits such as detection system and feedback system. The system.

Therefore, the squid in the narrow sense is indicated as SQUID here.

The analog squid, which is commercially available, has already been used in a medical device for obtaining a magnetocardiogram, a magnetoencephalogram, etc. by detecting a magnetic flux generated by a minute electric current flowing in the nervous system of the body.

In place of the electrocardiogram etc. which have been used to detect voltage, these obtain similar or finer body information by detecting magnetic flux.

Moreover, SQUID has not been successfully detected yet, but it has been applied to the detection of gravitational waves.

The analog squid applies a dc bias to the SQUID and obtains an output at a voltage or current proportional to the input signal. On the other hand, the digital squid proposed by the present inventor is such that the bias current is set to ac so that it can be output with the number of pulses that is proportional to the input signal and has the same period as the bias ac.

[Conventional technology]

3 (1) to 3 (3) are a circuit diagram, an operation explanatory diagram, and an output pulse diagram of a digital squid according to a conventional example, respectively.

This digital squeeze is proposed by the present inventor, and seeks the measured magnetic flux Φ _c by feeding back a current I _f proportional to the difference between the number of positive output pulses and the number of negative output pulses. It is the one.

The measured magnetic flux Φ _c is Φ _c = (L ₁ + L ₂ ) × I _c . It is represented by. Where I _c is the current to be measured, L ₁ is the pickup coil, and L ₂ is the magnetic coupling to the SQUID, and both are superconducting inductances that form the superconducting loop to be measured.

More generally, without using the pickup coil L ₁ ,
It can be used as an ammeter if the measured current I _c is directly applied to the superconducting inductance L ₂ .

In this method, the amount of feedback is the difference between the positive and negative pulse numbers, and the absolute value of each is not necessary.

Next, the operation of this digital squeeze will be described.

In the figure, a superconducting loop including _two Josephson junctions J ₁ and J ₂ and a superconducting inductance L is called a two-junction SQUID.

For a pickup coil with superconducting inductance L ₁ ,
When the measured magnetic flux Φ _c crosses over, the measured current I _c flows in the measured superconducting loop so as to cancel this magnetic flux, and this creates a magnetic field in the two-junction SQUID, changing the characteristics of the two-junction SQUID and changing it. Find more magnetic flux.

A first ac waveform generator 1 injects a first ac current into the injection (bias) terminal of the two-junction SQUID.

Superconducting inductance L magnetically coupled to a two-junction SQUID
_A second ac current having a frequency lower than that of the first ac current is supplied to the _third AC waveform generator 2.

FIG. 3 (2) is a diagram showing the threshold characteristics of the two-junction SQUID, in which the vertical axis represents the bias current I _G and the horizontal axis represents the measured current I _c .

When the operating point is inside the curve, the voltage of the SQUID is 0, and when it is outside the curve, the Josephson junction is in the voltage state, and the voltage of the SQUID is a finite voltage.

Now, make the waveform of the second ac current a triangular wave as shown in the figure,
Its positive half-cycle is t ₁ to t ₂ , and its negative half-cycle is t ₂ to t _3.
And

First, in the vicinity of t ₁ , I _c is in the vicinity of point A (OA corresponds to the difference between the current obtained by converting the measured magnetic flux Φ _c and the feedback current I _f ), and even if the bias I _G is entered here, the operating point is SQUID does not switch without exceeding the threshold.

As time passes from t _1, I _c increases, and from a certain point, the operating point exceeds the threshold value when bias I _G enters, and SQUID switches to output a voltage. When the bias I _G drops, its output voltage becomes zero. That is, one output pulse is output for each bias pulse.

The threshold characteristic is a left-right asymmetry, when I _c is in the positive, the output is switch to the positive side, it does not switch on the negative side.

On the contrary, in the half cycle of t ₁ to t ₃ , the output switches to the negative side,
Do not switch to the positive side.

When the center of the triangular wave is at the origin and I _c = 0, the threshold characteristic is symmetrical with respect to the origin, so that the number of outputs that are switched to the positive side is equal to the number that is switched to the negative side. However, I _c ≠ 0
In the case of, the threshold characteristic is left-right unbalanced and I _c
When is positive, there are many positive output pulses, and when I _c is negative, there are many negative output pulses.

Thus, the difference between the positive and negative of the output pulse is representative of the input current I _c.

The measurement system actually uses a null method with a feedback circuit added as shown in FIG.

The output is taken out from the injection terminal B of the two-junction SQUID, and the output pulse shown in Fig. 3 (3) appears here.

This is put in the up / down counter 3, integrated by the integrator 4, and fed back by the D / A converter 5 which is an analog amount of feedback current
It is converted to I _f and returned to the superconducting inductance L _f that is magnetically coupled to the two-junction SQUID.

When there are many positive output pulses, the sign of return is such that the current flows in the negative direction on the I _c axis. If the opposite is the case, reverse.

The output may take I _f , or may take the digital pulse number corresponding to I _c from the middle of the feedback circuit.

[Problems to be solved by the invention]

The difference between the number of positive and negative pulses is obtained by the up / down counter, but, for example, a positive pulse appears in succession in the counter, and a negative pulse comes out after that. These numbers are large numbers such as 10 ⁴ (13 to 14 bits), but when the dynamic range of the measured magnetic flux is not extremely large, this difference is usually a much smaller number. For example, a small number such as 0 to 10 ² , but the counter must temporarily count the number of pulses of about 10 ⁴ .

Therefore, the number of bits of the up / down counter has been increased more than necessary. In addition, the D / A converter in the next stage also had an increased number of bits, which placed a burden on the feedback circuit.

[Means for solving problems]

To solve the above problems, a squid consisting of a superconducting loop including a superconducting inductance and two or more Josephson junctions, a loop for passing a measured current and magnetically coupling to the squid, and a bias current for the squid are used. A digital squid having means for injecting an AC wave and extracting the output of the squid as a pulse train, and means for counting the difference between the positive output pulse and the negative output pulse of the pulse train, which is injected into the squid It has an amplitude modulation waveform generator that gives an amplitude modulation wave as a bias current.
Injection terminals providing bias current are provided at asymmetrical locations on the squid, or each Josephson junction is achieved by a digital squid having different critical current values.

[Action]

According to the present invention, the injection terminals of two or more SQUIDs are placed at asymmetrical positions, or each Josephson junction is configured to have different critical currents, the threshold characteristics are modified, and the SQUIDs are amplitude-modulated. By applying a wave, the output pulse train alternates between positive and negative, so that the number of counts does not temporarily increase.

1 (1) to (5) are a circuit diagram of a digital squeeze for explaining the principle of the present invention, an operation explanatory diagram and an output pulse.

In the figure, the device is composed of two Josephson junctions J ₁ and J ₂ .
A two-junction SQUID composed of a superconducting loop formed of a superconducting inductance L, and a superconducting inductance L ₂ that is magnetically coupled to the superconducting loop and allows a current I _c to be measured to flow therethrough. The junction SQUID is provided at an asymmetric position, and the amplitude-modulated waveform generator 1A injects the amplitude-modulated bias current I _G.

FIGS. 1 (2) and 1 (4) are diagrams showing threshold characteristics of a two-junction SQUID, where the vertical axis represents the bias current I _G and the horizontal axis represents the measured current I _c.
Represents

Threshold characteristics by providing the injection terminal positioned asymmetrically, the peak value of I _G axis direction compared with the conventional example in which the position of symmetry is shifted to the left and right.

FIG. 1 (2) shows the case where the magnetic field coupling current (current to be measured) I _c = 0, and the operating point moves on the vertical axis of the threshold characteristic diagram. In this case, the positive and negative threshold values are + I ₀ and −I ₀ , respectively, and their absolute values are equal. When the bias I _G exceeds the threshold value, that is, when it becomes + I ₀ or more or −I ₀ or less, the SQUID switches to generate a positive pulse or a negative pulse, respectively.
This output pulse train alternately outputs positive and negative as shown in FIG.

Therefore, if I _c = 0, the number of positive and negative output pulses is equal,
The difference is 0.

FIG. 1 (4) shows the case where I _c ≠ 0, for example I _c > 0, the positive threshold I ₁ is smaller than I ₀ (I ₁ <I ₀ ), and the negative threshold − I ₂ is smaller than −I ₀ (I ₂ > I ₀ ).

Therefore, as shown in FIG. 1 (5), the output pulse train has more positive pulses than negative pulses.

For example, the envelope (modulation wave) of the bias current I _G of the amplitude modulation waveform is a triangular wave, the maximum and minimum values of the waveform are I _g2 and I _g1, and the AC waveform of M cycles in one cycle of the modulation triangular wave ( For example, if there is a sine wave, the difference between the number of positive and negative output pulses is represented by [(I ₂ −I ₁ ) / (I _g2 −I _g1 )] × M. Knowing the threshold characteristics can be obtained I _c from I ₁ -I _1.

In practice, feedback is applied and the difference in the number of pulses is 0.
Since the feedback current I _{f is made} to flow so that I ₂ = I ₁ = I ₀ , it is not necessary to know the shape of the threshold characteristic, and the measured current can be obtained from I _f. To be

〔Example〕

FIG. 2 is a circuit diagram of a digital squeeze for explaining an embodiment of the present invention.

The output is taken out from the injection terminal B of the two-junction SQUID, put into the up / down counter 3, the difference in the number of output pulses is obtained, integrated by the integrator 4, and converted into the feedback current _If by the D / A converter 5, It returns to the superconducting inductance L _f and is magnetically coupled to the two-junction SQUID.

The output is obtained as the number of integrated pulses.

The actual circuit has a response control circuit that is used in a normal feedback circuit, and a latch circuit that prevents abnormal feedback current from flowing due to the transient operation of the D / A converter.

In the embodiment, the two-junction SQUID may change the position of the injection terminal, and may change the ratio of the critical currents of the two Josephson junctions.

Furthermore, the amplitude modulation waveform may be shunted and added to the injection terminal and the magnetic field coupling line.

Also, the modulating waveform may be other waveforms such as a sine wave instead of a triangular wave, and the waveform to be modulated is also another waveform, such as a rectangular wave or a JJ pulser (pulse generation using two Josephson junctions. Pulse) having a short half-value width or the like.

Further, in the embodiment, the two-junction SQUID is used, but the three-junction or more SQUID may be used.

〔The invention's effect〕

As described above in detail, according to the present invention, the positive and negative output pulses of the digital squid appear alternately, so that the number of bits of the up / down counter need not be increased more than necessary.

Moreover, in the conventional example, it was necessary to add two alternating-current waveforms having greatly different frequencies, but in this method, only one amplitude-modulated wave needs to be added, which is advantageous in mounting the apparatus.

Furthermore, since this measurement is a minute measurement, it may cause malfunction if there is guidance from the outside or wraparound. Therefore, when two waveforms are inserted as in the conventional example, they cause mutual wraparound and the like, and the method of the present invention with less input waveforms is more suitable for high sensitivity measurement.

[Brief description of drawings]

1 (1) to (5) are circuit diagrams of a digital squeeze for explaining the principle of the present invention, operation explanatory diagrams and output pulse diagrams, and FIG. 2 is a digital squeeze for explaining an embodiment of the present invention. A circuit diagram of the squid, and FIGS. 3 (1) to 3 (3) are a circuit diagram, an operation explanatory diagram and an output pulse diagram of a digital squid according to a conventional example, respectively. In the figure, J ₁ and J ₂ are Josephson junctions, L is the superconducting inductance, I _c is the measured current, Φ _c is the measured magnetic flux, L ₁ is the pickup coil superconducting inductance, and L ₂ is the measured superconducting. Loop superconducting inductance, L _f is feedback superconducting inductance, B is injection terminal, I _G is bias current, I _f is feedback current, 1A is amplitude modulation waveform generator, 3 is up-down counter, 4 is integrator , 5 are D / A converters.

Claims (1)

[Claims] 1. A squid consisting of a superconducting loop including a superconducting inductance and two or more Josephson junctions, a loop for passing a current to be measured and magnetically coupling to the squid, and an AC wave as a bias current for the squid. A bias current to be injected into the squeeze, the digital squeeze having a means for injecting the squid and extracting the output of the squid as a pulse train and a means for counting the difference between the positive output pulse and the negative output pulse of the pulse train As the squid, an injection terminal for applying a bias current is provided at an asymmetrical position of the squid, or each of the Josephson junctions has a different critical current value. A digital squid characterized by having.