JPH05126925A - Digital Squid - Google Patents

Abstract

(57) [Summary] [Purpose] Even if the frequency of the AC bias is increased to improve the measurement sensitivity, it is possible to avoid changes in the switch probability,
The objective is to realize a digital squid that does not reduce the measurement accuracy. [Arrangement] An AC bias generating circuit for generating an AC bias current whose polarity alternates, and a current containing the bias current and a current to be measured is flown into a loop, and the flow current is a positive polarity side or a negative polarity side. A squid sensor that outputs a pulse with the polarity on the exceeded side when the threshold value is exceeded,
A digital squid comprising: a feedback circuit for determining a difference between the number of positive polarity pulses and the number of negative polarity pulses to generate a feedback current corresponding to an integrated value of the difference and feeding back the feedback current to the squid sensor. Holding means for holding the output pulse for one cycle or a half cycle; and a logic circuit for taking a logic between the pulse held by the holding means and the pulse output from the squeeze sensor, the output of the logic circuit It is characterized by being applied to a feedback circuit.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to digital squids suitable for measuring small magnetic flux. The SQUID, which is a representative of high-sensitivity magnetometers, has attracted attention as being applicable to the measurement of biomagnetism and gravitational waves. For example, it can accurately and stably measure extremely small magnetic flux generated from the inside of a living body. Therefore, further improvement in sensitivity is required. In addition, in order to measure the distribution of the minute magnetic field, multichannelization and squid are required.

[0002]

2. Description of the Related Art A digital squid having an improved S / N ratio and increased sensitivity by operating the squid digitally is disclosed in, for example, Japanese Patent Laid-Open No. 63-290979.
JP-A-64-21378 and JP-A-6-21678
The thing described in the 4-21379 gazette is known.

FIG. 5 is a principle diagram of these digital squids. In the figure, 10 is a squid (two-junction SQUID) including _two joints J ₁ and J ₂ , and squid 1
0 is the AC bias current I from the AC bias circuit 11.
_G , the measured current I _C from the pickup coil 12 that is magnetic flux coupled, and the feedback current I _F from the feedback coil 13 that is also magnetic flux coupled are fed.

The squid 10 is a pickup coil 12
It receives an input magnetic flux through and outputs a pulse train. When the AC bias I _G is supplied to the end (asymmetrically) of the inductor as shown in FIG. 5A, the threshold characteristic of the squid 10 can be made asymmetrical. Can output negative pulses. That is, Squid 1
0 operates as a comparator that determines whether the input is positive or negative.

The feedback circuit is configured to have the functions of the up / down counter 14a and the D / A converter 14b. For example, although not shown, a one-chip device including a superconducting write gate including two Josephson junctions that are transiently switched by a pulse from the squeeze 10 and a superconducting storage loop in which magnetic flux is taken in and out by the write gate. SQUID magnetometers are known.

The interlinkage magnetic flux of the superconducting storage loop is quantized, and only an integral multiple of the magnetic flux quantum Φ ₀ (2.07 × 10 ^-15 Wb) is allowed. When the write gate receives one positive pulse, it adds one positive flux quantum to the superconducting loop. Alternatively, when one negative pulse is received, one negative flux quantum is added, which is the same as extracting one positive flux quantum. Therefore, the number of magnetic flux quanta corresponding to the difference between the number of positive pulses and the number of negative pulses are stored in the superconducting storage loop, and the same operation as the up / down counter is performed. Furthermore, the accumulated magnetic flux is directly fed back to the squid 10 through magnetic field coupling without performing D / A conversion.

According to such a conventional digital squeeze, a pulse having a number and polarity according to the magnitude of the magnetic flux to be measured is extracted from the squid 10 and is inputted to the feedback circuit 14 to obtain the number of positive and negative output pulses. A feedback current I _F corresponding to the integrated value of the difference between is generated. The number and polarity of the output pulses from the squid 10 (note that their absolute value is not important) corresponds to the measured current I _C , so that the minute magnetic flux can be taken out as a “digital quantity”, which is digital. The interface with the processing system and the connectivity with the Josephson integrated circuit can be improved. As a result, processing accuracy and processing speed can be improved.

[0008]

However, in the above-described conventional digital squeeze, the output pulse from the squeeze 10 is directly applied to the feedback circuit 14, a digital processing system (not shown), or a Josephson integrated circuit. Therefore, there is a problem that there is a limit when trying to further improve the measurement sensitivity while optimizing the switching probability of the squid 10.

Since the sensitivity of the squid 10 depends on the frequency of the AC bias I _G , the frequency is, for example, 1M.
If the frequency is 10 Hz to 10 MHz or higher, a considerably high sensitivity can be obtained. However, the presence of such a high frequency bias becomes a factor that destabilizes the operation of the squid 10 due to the influence of crosstalk between wirings. As a representative of the unstable operation, the switching probability of the Josephson junction (P
_O ) change. In order to obtain the highest sensitivity, the switch probability is generally set to 50% (P _O = 0.5), that is, the ratio of whether or not to switch is set to 50% (in other words, neither can be said).

When crosstalk occurs, it is often
The switch probability P _O becomes larger than 0.5, which makes it easy to mis-switch. FIG. 6 is a diagram illustrating an example of changes in the switch probability when crosstalk occurs. In this figure, when the switch is normally performed at a certain time point, the switch probability P _{O (1)} at that time point is 1.00.
(100%). If the next switching probability P _{O ( 2)} changes to, for example, 0.80 due to the influence of this normal switching operation, the change ΔP in the switching probability during this period is P
Given by _{O (1)} -P _O , P _{O (1)} = 1.00, P _O = 0.5
Therefore, ΔP is 0.3. That is, 0.3
(30%) is more likely to cause a misswitch. The switch probability P _{O (i)} at each time point is set to the switch probability P _O ,
The previous switch probability P _{O (i-1)} and the change Δ in the switch probability
It can be obtained by adding the products of P, and can be expressed as the following equation (1).

[0011] According to _{P O (i) = P O} + P O (i-1) × ΔP ...... (1) the above equation (1), P _{O (3)} or _{later, P O (3) = 0} . 5 + 0.80 × 0.3 = 0.74 P _{O (4)} = 0.5 + 0.74 × 0.3 = 0.722 P _{O (5)} = 0.5 + 0.722 × 0.3 = 0.7166 ( It will be understood that the following will be omitted), and the transition will be about 0.7. Therefore, since a normal switch operation induces a continuous or sporadic miss switch, that is, a so-called continuous switch phenomenon, an increase in the output pulse extracted from the SQUID due to such a continuous switch phenomenon is completely different from the measured magnetic flux. Since it is irrelevant, the measurement accuracy will be significantly reduced.

Therefore, it is an object of the present invention to realize a digital squid which can avoid a change in the switch probability even when the frequency of the AC bias is increased in order to improve the measurement sensitivity and does not cause a decrease in the measurement accuracy. ..

[0013]

In order to achieve the above object, the present invention provides an AC bias generating circuit for generating an AC bias current whose polarity alternates, and a current including the bias current and a current to be measured. A squeeze sensor that outputs a pulse having a polarity on the side that has flowed into the loop when the flow-in current exceeds the threshold value on the positive polarity side or the negative polarity side, and the number of positive polarity pulses and the number of negative polarity pulses In a digital squid, a feedback circuit, which calculates a difference and generates a feedback current corresponding to an integrated value of the difference and feeds it back to the squid sensor, holds a pulse output from the squid sensor for one cycle or a half cycle. Means and a logic circuit for taking the logic of the pulse held by the holding means and the pulse output from the squeeze sensor Comprises, characterized in providing the output of the logic circuit to the feedback circuit.

Preferably, the logic circuit takes an AND logic of the pulse one cycle before held in the holding means and the pulse output from the squid sensor, and preferably the logic circuit is in the holding means. It is characterized by taking the NOR logic of the pulse held in the half cycle before and the pulse output from the squeeze sensor.

[0015]

In the present invention, the output pulse from the squeeze sensor is delayed by a time corresponding to one cycle or a half cycle, and the logical result of the delayed pulse and the non-delayed pulse is taken out as the squeeze sensor output. For example, in the case of AND logic, the switch probability at a certain point (for example, P _{O (4)} = 0.72 ₎
2) and the switch probability (P _{O (2)}
= 0.80) AND (P _{O (2)} × P _{O (4)} = 0.80 ×
0.722) is taken and the logical result (≈ 0.57)
Is replaced by the correct switch probability P _{O (4)} at some point. Therefore, the setting switch probability (P _O =
The value can be close to 0.5), and as a result, the continuous switching phenomenon when the frequency of the AC bias is increased can be avoided, and the measurement sensitivity can be improved without lowering the measurement accuracy.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. 1 to 4 are diagrams showing an embodiment of a digital squeeze according to the present invention. In FIG. 1, 20 is an AC bias generation circuit that generates a high frequency AC bias current I _G , and 21 is a voltage pulse having a number and polarity according to the direction and magnitude of the measured magnetic flux that is converted into the measured current I _C. A squid sensor (see the pickup coil 12 and the squid 10 in FIG. 5 (a) for a detailed configuration), 22 is a holding means for holding the pulse from the squid sensor for a time corresponding to one cycle or a half cycle, and 23 is a holding means. Pulse (hereinafter, delayed pulse) held by the squeeze sensor 2
A logic circuit that takes the logic of the pulse from 1 (hereinafter, non-delayed pulse) and outputs the result, 24 is a feedback circuit (detailed configuration is the feedback circuit 14 of FIG. 5A).
The is a reference), along with obtaining the difference between the number of positive and negative pulses from the logic circuit 23, and generates a feedback current I _F of the analog amount corresponding to the integral value of the difference.

FIG. 2 shows an example of the configuration of the holding means and the logic circuit when the superconducting logic circuit is used. In this example, an AND gate Gd is provided via three OR gates Ga to Gc (delay means) that operate in synchronization with the multiphase clocks φa to φc.
A pulse is applied to one input of the (logic circuit) and a pulse is applied to the other input of the AND gate Gd. Each of the multiphase clocks φa to φc has a phase difference of 120 degrees, and this phase difference of 120 degrees corresponds to 1/3 of one operation cycle of the squid sensor 21. That is, three OR gates G
The delay time of one cycle (360 degrees) can be given to the pulse by a to Gc, and the AND logic of the delayed pulse and the non-delayed pulse can be taken by the AND gate Gd.

FIG. 3 is a diagram showing the transition of the switch probability according to the configuration example of FIG. According to this figure, assuming that the normal switch probability P _{O (1)} at a certain time point is 1.00, the next switch probability P _{O (2)} is 0.80 and the next switch probability P _{O (3)} is The switch probabilities P _{O (4)} , P _{O (5)} , and so on are 0.57 and 0.5, respectively.
Change to 3, ... That is, the setting switch probability P _O
It is improved to a value close to (P _O = 0.5).

This is because the product (AND logic) of the switch probabilities for each cycle is taken.
Focusing on a, the switch probability (P _{O (2)} = 0.80) one cycle before that time ta and the original switch probability of the squeeze sensor 21 at that time ta ₍ P _{O (4) in} FIG. 6 _).
= 0.722) AND logical result (0.80 x 0.7
22.apprxeq.0.57) is replaced by the legal switch probability _{P.sub.O (4)} at that time ta.

Therefore, the switch probability P after 1.5 cycles from the start of the operation of the squeeze sensor 21
_{O (i)} can be set to a value close to the setting switch probability P _O (P _O = 0.5), and the continuous switching phenomenon can be avoided. As a result, it is possible to solve the problem of deterioration in accuracy when the frequency of the AC bias is increased to improve the measurement sensitivity. FIG. 4 is another example of the configuration of the logic circuit. A delay pulse is applied to one input of the AND gate Gf via the inverter gate Ge and a non-delay pulse is applied to the other input of the AND gate Gf. This is an example in which the NOR logic is constituted by the inverter gate Ge and the AND gate Gf.

That is, to the AND gate Gf, a delayed pulse half-cycle before the polarity inverted by the inverter gate Ge and a non-delayed pulse with the same polarity are added, and when the polarities of both pulses match, the AND gate Gf outputs the same. Pulses of the same polarity are output. Therefore, in this example, since the NOR of the switch probabilities for each half cycle is taken, for example, focusing on the time point ta in FIG. 3, the switch probability (P
_{O (3)} = 0.74) and the original switch probability of the squeeze sensor 21 at that time point ta ₍ P _{O (4)} = 0.7 in FIG. 6 _).
22) and the logical result (0.74 × 0.722≈0.4
2) is the legal switch probability P at that time ta
_{Since it} is replaced as _{O (4)} , the continuous switching phenomenon can be almost completely eliminated.

[0022]

According to the present invention, the pulse output from the squeeze sensor is delayed by one cycle or a half cycle, and the logical result of the delayed pulse and the non-delayed pulse is taken out, so that the measurement sensitivity is improved. Therefore, even if the frequency of the AC bias is increased, it is possible to avoid a change in the switch probability, and it is possible to realize a digital squeeze that does not reduce the measurement accuracy.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment.

FIG. 2 is a configuration diagram of a holding unit and a logic circuit according to an embodiment.

FIG. 3 is a transition diagram of a switch probability according to an embodiment.

FIG. 4 is another configuration diagram of a logic circuit according to an embodiment.

FIG. 5 is a configuration diagram and an operation waveform diagram of a conventional example.

FIG. 6 is a transition diagram of a switch probability of a conventional example.

[Explanation of symbols]

 20: AC bias generation circuit 21: Squeeze sensor 24: Feedback circuit 22: Holding means 23: Logic circuit

Claims (3)

[Claims] 1. An alternating current bias generation circuit for generating an alternating current bias current whose polarity alternates, a current including the bias current and a current to be measured is flown into a loop, and the flowing current is a positive polarity side or a negative polarity side. If a squeeze sensor that outputs a pulse with a polarity on the exceeded side when the threshold value is exceeded and the difference between the number of positive polarity pulses and the number of negative polarity pulses is obtained, a feedback current corresponding to the integral value of the difference is generated. And a feedback circuit for feeding back to the squid sensor, holding means for holding the pulse output from the squid sensor for one cycle or a half cycle, and the pulse held in the holding means and the output from the squid sensor A logic circuit that takes a logic with the pulse, and the output of the logic circuit is the feedback circuit. A digital squid characterized by being applied to. 2. The logic circuit is held by a holding means.
2. The digital squeeze according to claim 1, wherein an AND logic of the pulse before the cycle and the pulse output from the squeeze sensor is taken. 3. The digital squeeze according to claim 1, wherein the logic circuit takes a NOR logic of the pulse held by the holding means before the half cycle and the pulse output from the squeeze sensor.