• Progress in Superconductivity
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• 제4권1호
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• pp.42-47
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• 2002

Measuring magnetic fields with a SQUID sensor always requires preliminary adjustments such as optimum bas current determination and flux-locking point search. A conventional magnetoencephalography (MEG) system consists of several dozens of sensors and we should condition each sensor one by one for an experiment. This timeconsuming job is not only cumbersome but also impractical for the common use in hospital. We had developed a serial port communication protocol between SQUID sensor controllers and a personal computer in order to control the sensors. However, theserial-bus-based control is too slow for adjusting all the sensors with a sufficient accuracy in a reasonable time. In this work, we introduce programmatic control sequence that saves the number of the control pulse arrays. The sequence separates into two stages. The first stage is a function for searching flux-locking points of the sensors and the other stage is for determining the optimum bias current that operates a sensor in a minimum noise level Generally, the optimum bias current for a SQUID sensor depends on the manufactured structure, so that it will not easily change about. Therefore, we can reduce the time for the optimum bias current determination by using the saved values that have been measured once by the second stage sequence. Applying the first stage sequence to a practical use, it has taken about 2-3 minutes to perform the flux-locking for our 37-channel SQUID magnetometer system.

• Progress in Superconductivity
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• 제3권1호
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• pp.78-82
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• 2001

The spatial resolution of $high-T_{c}$ scanning SQUID microscope is limited by the washer size of SQUID and the gap distance between SQUID sensor and the sample. In this work, we tried to improve the spatial resolution of scanning SQUID microscope by reducing the size of SQUID sensor fabricated with $YBa_2$$Cu_3$$O_{7}$ thin film. Outer dimensions of the SQUiDs we tested are 24 $\mu\textrm{m}$ $\times$ $ 28\mu\textrm{m}$, $12 \mu\textrm{m}$ $\times$ $16\mu\textrm{m}$, $12\mu\textrm{m}$ x $12\mu\textrm{m}$, $10 \mu\textrm{m}$ $\times$ $10 \mu\textrm{m}$ each. To operate them in the flux-locked loop scheme, we used a direct-coupled electronics instead of using conventional electronics involving a modulation scheme. Since the direct-coupled feedback scheme does not require modulation current adjustment that poses as a practical difficulty in the SQUID operation in modulation-scheme, the direct feedback operation is rather simpler than the conventional modulation method. The resulting noise features were dominated by the noise of preamp in FLL electronics except that of the largest SQUID. The noise levels of SQUIDs are expected below 1$\times$$10^{-5}$ $\Phi_{0}$H $z^{1}$2/ (at 300 Hz), that is a typical noise level for SQUID made of $YBa_2$C $u_3$$O_{7}$ thin film. The data acquisition and motion-controlling parts were also improved, resulting in faster data acquisition rate and less vibration of the system.m.

• Progress in Superconductivity
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• 제5권2호
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• pp.98-104
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• 2004

We analyzed a noise-sensitivity profile of a specific SQUID sensor system for the localization of brain activity. The location of a neuromagnetic current source is estimated from the recording of spatially distributed SQUID sensors. According to the specific arrangement of the sensors, each site in the source space has different sensitivity, that is, the difference in the lead field vectors. Conversely, channel noises on each sensor will give a different amount of the estimation error to each of the source sites. e.g., a distant source site from the sensor system has a small lead-field vector in magnitude and low sensitivity. However, when we solve the inverse problem from the recorded sensor data, we use the inverse of the lead-field vector that is rather large, which results in an overestimated noise power on the site. Especially, the spatial sensitivity profile of a gradiometer system measuring tangential fields is much more complex than a radial magnetometer system. This is one of the causes to make the solutions of inverse problems unstable on intervening of the sensor noise. In this study, in order to improve the localization accuracy, we calculated the noise-sensitivity profile of our 40-channel planar SQUID gradiometer system, and applied it as a normalization weight factor to the source localization using synthetic aperture magnetometry.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2006년도 제37회 하계학술대회 논문집 D
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• pp.2149-2150
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• 2006

FLL 회로는 측정된 신호를 voltage to current converter를 거쳐 feedbak coil에 인가함으로써 외부 자장을 상쇄하여 SQUID의 동작점을 원점으로 회귀시켜 선형 구간을 유지하도록 하는 역할을 한다. FLL회로의 동자 범위와 특성을 분석하기 위해서는 일반적인 time-delayed feedback 회로와 사용된 OP amp의 slew rate, filter 의 amplitude 및 위상 특성, SQUID의 critical current, pickup coil 및 SQUID의 inductance 등 다양한 파라미터를 고려하여야 한다. 이러한 SQUID 회로의 복합적인 특성을 SQUID 에뮬레이터를 사용함으로써 FLL 회로를 손쉽게 설계할 수 있고, 또한 회로의 최적화도 쉽게 이를 수 있다. 또한 초전도에서 동작하는 SQUID 나 자기 차폐실이 없어도 FLL 회로 등을 개발할 수 있기 때문에 생체자기시스템의 개발 초기 단계에 널리 활용될 수 있다. 따라서 이 논문의 목적은 FLL을 포함한 SQUID 제어 회로를 SQUID 센서와 분리하기 위한 방법을 제안하는 것으로 자기적으로 coupling되어 있는 feedback 회로를 회로적으로 addition을 수행하게 함으로써 SQUID와 분리하여 회로의 동작 및 특성을 측정할 수 있다.

• Progress in Superconductivity
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• 제10권2호
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• pp.139-143
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• 2009

We have fabricated a low-noise 61-channel axial-type first-order gradiometer system for measuring fetal magnetocardiography(MCG) signals. Superconducting quantum interference device(SQUID) sensor was based on double relaxation oscillation SQUID(DROS) for detecting biomagnetic signal, such as MCG, magnetoencphalogram(MEG) and fetal-MCG. The SQUID sensor detected axial component of fetal MCG signal. The pickup coil of SQUID sensor was wound with 120 ${\mu}m$ NbTi wire on bobbin(20 mm diameter) and was a first-order gradiometer to reject the environment noise. The sensors have low white noise of 3 $fT/Hz^{1/2}$ at 100 Hz on average. The fetal MCG was measured from $24{\sim}36$ weeks fetus in a magnetically shielded room(MSR) with shielding factor of 35 dB at 0.1 Hz and 80 dB at 100 Hz(comparatively mild shielding). The MCG signal contained maternal and fetal MCG. Fetal MCG could be distinguished relatively easily from maternal MCG by using independent component analysis(ICA) filter. In addition, we could observe T peak as well as QRS wave, respectively. It will be useful in detecting fetal cardiac diseases.

• Progress in Superconductivity
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• 제11권1호
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• pp.78-82
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• 2009

We have fabricated a helmet type magnetoencephalogrphy(MEG) with a $1^{st}$ order gradiometer in vacuum to improve the signal-to-noise ratio(SNR) and the boil-off rate of liquid helium(LHe). The axial type first-order gradiometer was fabricated with a double relaxation oscillation SQUID(DROS) sensor which was directly connected with a pickup coil. The neck space of LHe dewar was made to be smaller than that of a conventional dewar, but the LHe boil-off ratio appeared to increase. To reduce the temperature of low Tc SQUID sensor and pickup coil to 9 K, a metal shield made of, such as copper, brass or aluminum, have been usually used for thermal transmission. But the metal shield exhibited high thermal noise and eddy current fluctuation. We quantified the thermal noise and the eddy current fluctuation of metal. In this experiment, we used the bobbin which was made of an alumina to wind Nb superconductive wire for pickup coil and the average noise of coil-in-vacuum type MEG system was $3.5fT/Hz^{1/2}$. Finally, we measured the auditory evoked signal to prove the reliability of coil-in-vacuum type MEG system.

• 대한의용생체공학회:의공학회지
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• 제18권4호
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• pp.421-428
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• 1997

초전도양자간섭소자(SQUID)를 이용한 자장센서는 현재 개발된 자장센서중에서 감도가 가장 우수한 소자로서 인체의 두뇌에서 발생하는 매우 미약한 자장에 측정이 가능하다. 뇌자도측정은 현재 많이 사용되고 있는 전기적인 측정(뇌파, 뇌유발전위) 에 비해 공간분해능이 우수하고, fMRI나 PET에 비해서는 시간분해능이 우수하므로 뇌기능연구에 유용하게 사용될 수 있다. 본 연구에서는 뇌자도 측정을 위하여 4-채널 SQUID시스템을 개발하였다. 개발된 시스템의 주요 특징은 새로운 방식의 SQUID센서를 채택함으로서 간단한 회로로써 SQUID구동이 가능하도록 하였으며, 검출코일의 신뢰성을 향상시키기 위하여 집적화된 평면형 코일을 사용하였다. 외부 환경잡음을 소거하기 위하여 자기차폐실을 설치하였고, 개발된 SQUID 시스템을 이용하여 뇌의 청각령으로부터 발생하는 자기신호를 측정하였다.

• Progress in Superconductivity
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• 제13권3호
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• pp.139-145
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• 2012

Electric activity of cardiac muscles generates magnetic fields. Magnetocardiography (or MCG) technology, measuring these magnetic signals, can provide useful information for the diagnosis of heart diseases. It is already about 40 years ago that the first measurement of MCG signals was done by D. Cohen using SQUID (superconducting quantum interference device) sensor inside a magnetically shielded room. In the early period of MCG history, bulky point-contact RF-SQUID was used as the magnetic sensor. Thanks to the development of Nb-based Josephson junction technology in mid 1980s and new design of tightly-coupled DC-SQUID, low-noise SQUID sensors could be developed in late 1980s. In around 1990, several groups developed multi-channel MCG systems and started clinical study. However, it is quite recent years that the true usefulness of MCG was verified in clinical practice, for example, in the diagnosis of coronary artery disease. For the practical MCG system, technical elements of MCG system should be optimized in terms of performance, fabrication cost and operation cost. In this review, development history, technical issue, and future development direction of MCG technology are described.

• Progress in Superconductivity
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• 제5권1호
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• pp.31-37
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• 2003

Wide-bandwidth SQUID current amplifier and its control electronics have been constructed for detecting pulse outputs of a superconducting microcalorimeter. The current amplifier made of a double relaxation oscillation SQUID (DROS) has a bandwidth of 1.2 MHz and typical white noise level of about 6 pA/(equation omitted) Hz. To increase the dynamic range of the current amplifier, the flux-locked loop (FLL) has additional circuits to reset the integrator and to count reset numbers which present the number of passed flux quanta. In this system, dynamic range covers from -65 mA to +65 mA. SQUID electronics are controlled by software to get the optimum FLL condition, and to control the current to bias the transition edge sensor (TES). The electronics are shielded from the outside electromagnetic noises by using an aluminum case of 66 mm ${\times}$ 25 mm ${\times}$ 100 mm, and consist of 2 separate printed-circuit-boards for the current amplifier and the control electronics, respectively. The SQUID current amplifier and its control electronics will be used in TESs for detecting photons such as UV and X-ray with high energy resolution.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2004년도 학술대회 논문집 정보 및 제어부문
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• pp.538-540
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• 2004

Electrical currents generated by human heart activities create magnetic fields represented by MCG(MagnetoCardioGram). Since an MCG signal acquisition system requires precise and stable operation, the system adopts hundreds of SQUID(Superconducting QUantum Interface Device) sensors for signal acquisition. Such a system requires fast real-time data acquisition in a required sampling interval, i.e., 1 mili-second for each sensor. This paper presents designed hardware to acquire data from 256-channel analog signal with 1 ksamples/sec speed, using 12-bit 8-channel ADC devices, SPI interfaces, parallel interfaces, 8-bit microprocessors, and a DSP processor. We implemented SPI interface between ADCs and a microprocessor, parallel interfaces between microprocessors. Our result concludes that the data collection can be done in $168{\mu}sec$ time-interval for 256 SQUID sensors, which can be interpreted to 6 ksamples/sec speed.