Operation of Orthogonal Fluxgate of Fundamental Mode in Null Detection Method

Ichiro Sasada,Masanori Murakami 한국자기학회 2008 한국자기학회 학술연구발표회 논문개요집 Vol.- No.-

Optimization of a Preamplifier with Low Input-Bias Current for Operating Double Relaxation Oscillation SQUIDs (DROSs)

Jin-Mok Kim,Hyukchan Kwon,Ichiro Sasada,Ki-Dam Kim,김기웅,Yong-Ho Lee 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.48 No.5I

A double relaxation oscillation SQUID (DROS) provides a large flux-to-voltage transfer coefficient, and a room-temperature preamplifier can detect the DROS output directly. When the DROS is connected to a preamplifier consisting of several bipolar transistors, SSM2210s or MAT02s, the operation of DROS can be disturbed because the input-bias current of the preamplifier can flow through the DROS and exceed the critical current of the reference junction in the DROS. In order to know how the DROS acts when an input-bias current goes through the reference junction of DROS, we made a current controller to regulate the input-bias current of the preamplifier. The controller regulates an input-bias current of 0 . 12.5 μA with a resolution of 3 nA, which can cover preamplifiers made up of 10 pairs of SSM2210s in parallel. We have determined the input-bias current range for stable operation of the DROS when using Flux-Locked-Loop electronics with a preamplifier consisting of 4 SSM2210s. The higher the critical current of the reference junction in DROS is, the wider the operation input-bias range is. If the DROS has a low critical-current of the reference junction below 3 μA, however, a fixed input bias for reducing the input-bias current of the preamplifier may disturb the operation of the DROS. When the output of the DROS is detected by a preamplifier with an input-bias current controller, the noise of the DROS attains the same level of 10 fT/pHz at 1 Hz and 5 fT/pHz at 100 Hz, as long as the preamplifier has an input-bias current within the operation range, ever though it flows through the reference junction of the DROS.

Noise Characteristics of 64-channel 2nd-order DROS Gradiometer System inside a Poorly Magnetically-shielded Room

김진목,이용호,김기웅,박용기,권혁찬,유권규,Ichiro Sasada 한국초전도학회 2006 Progress in superconductivity Vol.8 No.1

Effect of Additional Reference Current in a Reference Junction-Double Relaxation Oscillation SQUID (RJ-DROS)

Kim, Jin-Mok,Lee, Yong-Ho,Kwon, Hyukchan,Kim, Kiwoong,Sasada, Ichiro IEEE 2007 IEEE transactions on applied superconductivity Vol.17 No.1

<P>A double relaxation oscillation SQUID with a reference junction (RJ-DROS) already has a fixed reference current, so that it holds a fixed flux-to-voltage characteristics when fabricated. When an additional current flows through the reference junction but not through the signal junction in the RJ-DROS, we have obtained the variation of its characteristics. The additional reference current has an effect on reducing the fixed reference current, which results in changing averaged relaxation voltage and transfer coefficient. In particular, while the critical current of the signal SQUID exceeds the reference current, the averaged relaxation voltage at the reference junction is enlarged by the added bias current combined with an additional current and an initial bias current. Therefore the additional reference current can provide the RJ-DROS with variable modulation depth and width within the operating range. Although the characteristics of RJ-DROSs are fixed differently each to each when they are fabricated, the additional current can adjust their characteristics to the same one, which is useful for controlling DROSs in a multichannel DROS system. However, an input-bias current in the preamplifier detecting the outputs at the reference junction, acts as an additional current, and then makes the reference current shift from the optimum current or deviate from the operating range, the input-bias current needs to be regulated within the operating range</P>

열린 자기차폐실의 심자도 시스템

김진목,이용호,권혁찬,유권규,김기웅,박용기,Kim, J.M.,Lee, Y.H.,Kwon, H.,Yu, K.K.,Kim, K.,Park, Y.K.,Sasada, Ichiro 한국초전도학회 2007 Progress in superconductivity Vol.9 No.1

We have installed a 61-channel magnetocardiography (MCG) system inside a magnetically shielded room (MSR) with a size of $2.4\;m\;{\times}2.4\;m\;{\times}2.4\;m$. The MCG system consists of 1st-order axial gradiometers containing double relaxation oscillation SQUIDs (DROSs) with pick-up coils of a base line of 70 mm. The MSR holds a shielding factor of 50 at 0.1 Hz and 10000 at 100 Hz, when its door in the middle on a front wall is closed. On opening the MSR door, we have obtained the characteristics of the MCG system with a 2.9 Hz noise generated from an air conditioning unit at 13 m distance off the MSR. In an open-door MSR ($140^{\circ}$ opening), a noise at the center channel increases up to $700\;fT/Hz^{l/2}$ at 2.9 Hz and $1.7\;pT/Hz^{1/2}$ at 60 Hz. MCG signals for a healthy human do not show distortion until the door opens to $45^{\circ}$, but show the effect of noise when the door opens further at $90^{\circ}$ and $140^{\circ}$. With the door opens to $45^{\circ}$, MCG measurement can be performed with ease of door operation and without creating claustrophobia for the patient.

Noise Characteristics of Readout Electronics for 64-Channel DROS Magnetocardiography System

김진목,김기담,이용호,유권규,김기웅,권혁찬,Kim J. M.,Kim K. D.,Lee Y. H.,Yu K. K.,Kim K. W.,Kwon H. C.,Sasada Ichiro The Korean Superconductivity Society 2005 Progress in superconductivity Vol.7 No.1

We have developed control electronics to operate flux-locked loop (FLL), and analog signal filters to process FLL outputs for 64-channel Double Relaxation Oscillation SQUID (DROS) magnetocardiography (MCG) system. Control electronics consisting of a preamplifier, an integrator, and a feedback, is compact and low-cost due to larger swing voltage and flux-to-voltage transfer coefficients of DROS than those of dc SQUIDs. Analog signal filter (ASF) serially chained with a high-pass filter having a cut-off frequency of 0.1 Hz, an amplifier having a gain of 100, a low-pass filter of 100 Hz, and a notch filter of 60 Hz makes FLL output suitable for MCG. The noise of a preamplifier in FLL control electronics is $7\;nV/{\surd}\;Hz$ at 1 Hz, $1.5\;nV/{\surd}\;Hz$ at 100 Hz that contributes $6\;fT/{\surd}\;Hz$ at 1 Hz, $1.3\;fT/{\surd}\;Hz$ at 100 Hz in readout electronics, and the noise of ASF electronics is $150\;{\mu}V/{\surd}\;Hz$ equivalent to $0.13\;fT/{\surd}\;Hz$ within the range of $1{\sim}100\;Hz$. When DROSs are connected to readout electronics inside a magnetically shielded room, the noise of 64-channel DROS system is $10\;fT/{\surd}\;Hz$ at 1 Hz, $5\;fT/{\surd}\;Hz$ at 100 Hz on the average, low enough to measure human MCG.

저성능 자기차폐실에서 64채널 DROS 2차 미분계 시스템의 잡음 특성

김진목,이용호,유권규,김기웅,권혁찬,박용기,Kim, J.M.,Lee, Y.H.,Yu, K.K.,Kim, K.,Kwon, H.,Park, Y.K.,Sasada, Ichiro 한국초전도학회 2006 Progress in superconductivity Vol.8 No.1

We have developed a second-order double relaxation oscillation SQUID(DROS) gradiometer with a baseline of 35 mm, and constructed a poorly magnetically-shielded room(MSR) with an aluminum layer and permalloy layers for magnetocardiography(MCG). The 2nd-order DROS gradiometer has a noise level of 20 $fT/{\surd}Hz$ at 1 Hz and 8 $fT/{\surd}Hz$ at 200 Hz inside the heavily-shielded MSR with a shielding factor of $10^3$ at 1 Hz and $10^4-10^5$ at 100 Hz. The poorly-shielded MSR, built of a 12-mm-thick aluminum layer and 4-6 permalloy layers of 0.35 mm thickness, is 2.4mx2.4mx2.4m in size, and has a shielding factor of 40 at 1 Hz, $10^4$ at 100 Hz. Our 64-channel second-order gradiometer MCG system consists of 64 2nd-order DROS gradiometers, flux-locked loop electronics, and analog signal processors. With the 2nd-order DROS gradiometers and flux-locked loop electronics installed inside the poorly-shielded MSR, and with the analog signal processor installed outside it, the noise level was measured to be 20 $fT/{\surd}Hz$ at 1 Hz and 8 $fT/{\surd}Hz$ at 200 Hz on the average even though the MSR door is open. This result leads to a low noise level, low enough to obtain a human MCG at the same level as that measured in the heavily-shielded MSR. However, filters or active shielding is needed fur clear MCG when there is large low-frequency noise from heavy air conditioning or large ac power consumption near the poorly-shielded MSR.