Relaxation Oscillation SQUID Research Articles

Comparison of Double Relaxation Oscillation SQUIDs and DC-SQUIDs of Large Stewart-McCumber Parameter

Increasing the flux-to-voltage transfer coefficient of the SQUID output voltage is necessary to release the constraint on the preamplifier input noise. As one of the approaches to increase the transfer coefficient, we have been using a double relaxation oscillation SQUID (DROS) having a transfer coefficient of about 10 times larger than those of DC-SQUIDs. Traditionally, DC-SQUIDs were operated in the non-hysteretic condition with the Stewart-McCumber parameter (β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) less than 1. In this study, we fabricated hysteretic DC-SQUIDs with the β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ranging from 2.5 and compared the transfer coefficient and the system noise of DC-SQUIDs and DROSs. Both DROS and DC-SQUID have the same structure and design parameters for the SQUID loop inductance, resonance-damping circuits in the SQUID loop, and input coil. All the SQUIDs were fabricated in the same wafer using the Nb/AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb junction process. The transfer coefficients of DROSs were about 1 mV/Φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , regardless of SQUID parameters. For DC-SQUIDs, the transfer coefficients were sensitive to the β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and the operating margin was narrower than that of DROS. The white noise of both SQUIDs in the flux-locked loop mode was around 1.5 μΦ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /√Hz at 100 Hz, measured with a preamplifier of input voltage noise of 1 nV/√Hz. DC-SQUIDs had mostly a narrower operating margin against the bias current, and the modulation voltage was smaller than DROS. In a near-optimized operating condition, both DROS and hysteretic DC-SQUID showed comparable noise performance.

Noise Properties of Digital SQUID Using Double Relaxation Oscillation SQUID Comparator With Relaxation Oscillation Resonant Circuit

We designed a digital superconducting quantum interference device (SQUID) system that contains a double relaxation oscillation SQUID (DROS) as a magnetic flux comparator that is connected to an additional relaxation oscillation (RO) circuit with mutually coupled inductances. In the designed digital SQUID, the RO circuit works as a resonator and increases the relaxation oscillation frequency without decreasing the input current of dc/SFQ converters. Numerical simulation shows that the digital SQUID system can operate up to 7 GHz. A fabricated digital SQUID using a DROS comparator with an RO circuit provides correct digital SQUID operation. For a magnetometer using the digital SQUID system, we can realize a magnetic field noise less than 1 pT/ $\sqrt{\mathrm{Hz}}$ with a small signal bandwidth of more than 10 MHz.

Design of Digital DROS for Wide Dynamic Operation Range with Sub-Single Flux Quantum Resolution

Abstract We designed a digital double relaxation oscillation SQUID (DROS) with an single flux quantum (SFQ) feedback circuit for digital SQUID magnetometers. The DROS is magneticall coupled to a feedback SQUID loop and a signal magnetic flux is input to the feedback SQUID loop. The DROS generates two kinds of voltage pulses corresponding to the total magnetic flux in the feedback SQUID loop. Those voltage pulses are converted to SFQ and anti-SFQ pulses and input to the feedback SQUID loop to keep the magnetic flux in the feedback SQUID loop constant. A read-out dc SQUID (R/O SQUID) is also magnetically coupled to the feedback SQUID loop for monitoring the actual magnetic flux in the feedback SQUID loop. The number of the generated SFQ pulses is estimated by SFQ feedback steps on output voltage curves generated from the R/O SQUID. Wide dynamic operation range is realized by the SFQ feedback and sub-Φ0 magnetic flux resolution is realized by the R/O SQUID, where Φ0(= h /2 e ) is the single magnetic flux quantum. Also one digital DROS magnetometer can be operated with less than 10 mA. We fabricated a designed digital DROS using ISTEC SRL 2.5 kA/cm 2 Nb standard process II and are trying to confirm the correct operation of the digital DROS.

Design of Digital DROS With SFQ Up/Down Counter for Wide Dynamic Operation Range

We designed a digital double relaxation oscillation SQUID (DROS) with an SFQ up/down counter. The DROS is connected to a feedback SQUID loop and a signal magnetic flux is input to the feedback SQUID loop. The DROS generates two kinds of voltage pulses corresponding to the total magnetic flux in the feedback SQUID loop. Two kind of voltage pulses are converted to SFQ and anti-SFQ pulses and input to the feedback SQUID loop to keep the magnetic flux in the SQUID loop constant. The number of the generated SFQ pulses is counted using the SFQ up/down counter. Thus, the count corresponds to the input magnetic flux. Because the SFQ up/down counter is operated in the pulse driven mode, the digital DROS needs only dc bias current for operation. We designed and fabricated a digital DROS with a relaxation oscillation frequency of 3 GHz and an 8-bit SFQ up/down counter. Numerical simulation suggested the designed digital DROS has a dynamic range exceeding ±100 Φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and magnetic flux slew rate of 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> Φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /s.

A High-sensitivity Magnetocardiography System with 64 Axial Gradiometers Inside a Low Boil-off Dewar

Event Abstract Back to Event A High-sensitivity Magnetocardiography System with 64 Axial Gradiometers Inside a Low Boil-off Dewar Yong-Ho Lee1*, Kwon-Kyu Yu1, Hyukchan Kwon1, Jin-Mok Kim1 and Kiwoong Kim1 1 Korea Research Institute of Standards and Science, South Korea For accurate diagnosis and high-throughput screening of ischemic heart disease or arrhythmia, high-sensitivity economic MCG system is needed. We fabricated an MCG system having axial gradiometers with low stray pickup area and dewar with low boil-off rate of liquid He. The axial gradiometers have very simple structure for the superconductive connection. The two ends of pickup coil wires were placed near the input coil pads of SQUID, and annealed Nb wires were bonded directly between pickup coil wires and input coil pads. In this way, the stray pickup area (intrinsic balance) made by the connection structure is about 0.1 % of the pickup coil area, and the gradiometers can be operated inside a thin magnetically shielded room. The baseline of the first-order gradiometer is 70 mm, and diameter of pickup coil is 20 mm. The neck diameter of the dewar is smaller than the diameter of gradiometer array (dewar inner bottom). By dividing the gradiometer array into 4 plates, each plate can be installed separately into the dewar bottom. In this way, the boil-off rate of liquid He can be reduced to 3 L/day, for the inner bottom diameter of 280 mm. The SQUIDs are double relaxation oscillation SQUIDs with large flux-to-voltage transfer coefficients, enabling compact flux-locked loop electronics. The developed 64-channel system was installed inside a thin (80 mm thick) shielded room, and showed field noise level of 3 fT/sqrt(Hz) at 100 Hz. Measurements of adult and fetal MCG signals were done successfully using the developed system. Analysis software allows parameter analysis for the screening of ischemic heart disease and 3-dimensional imaging of myocardium current (reentry current source) on the realistic image of the heart. Conference: Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010. Presentation Type: Poster Presentation Topic: MCG: Instrumentation, Modeling, Basic and Clinical Studies Citation: Lee Y, Yu K, Kwon H, Kim J and Kim K (2010). A High-sensitivity Magnetocardiography System with 64 Axial Gradiometers Inside a Low Boil-off Dewar. Front. Neurosci. Conference Abstract: Biomag 2010 - 17th International Conference on Biomagnetism . doi: 10.3389/conf.fnins.2010.06.00367 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 08 Apr 2010; Published Online: 08 Apr 2010. * Correspondence: Yong-Ho Lee, Korea Research Institute of Standards and Science, Daejeon, South Korea, yhlee@kriss.re.kr Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Yong-Ho Lee Kwon-Kyu Yu Hyukchan Kwon Jin-Mok Kim Kiwoong Kim Google Yong-Ho Lee Kwon-Kyu Yu Hyukchan Kwon Jin-Mok Kim Kiwoong Kim Google Scholar Yong-Ho Lee Kwon-Kyu Yu Hyukchan Kwon Jin-Mok Kim Kiwoong Kim PubMed Yong-Ho Lee Kwon-Kyu Yu Hyukchan Kwon Jin-Mok Kim Kiwoong Kim Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

A high-sensitivity magnetocardiography system with a divided gradiometer array inside alow boil-off Dewar

We fabricated a low-noise 64-channel first-order axial gradiometer system for measuringmagnetocardiography (MCG) signals. The key technical features of the system are thecompact structure of the gradiometer, division of the sensor array plate, direct mounting ofthe sensor plates into the Dewar bottom, reduced neck diameter of the liquid He Dewar,and compact readout electronics. To make the refill interval of liquid He longer, thedistance between the compensation coil of the gradiometer and the input coil pads of thesuperconducting quantum interference device (SQUID) was reduced to 20 mm. By usingdirect ultrasonic bonding of Nb wires between the pickup coil wires and input coil pads,the superconductive connection structure became simple. The baseline of thefirst-order gradiometer is 70 mm, a little longer than for typical conventional axialgradiometers, to provide a larger signal amplitude for deep sources. The 64-channelgradiometer array consists of four blocks, and each block is fixed separately onto thebottom of the Dewar. The neck diameter of the He Dewar (192 mm) is smaller thanthe bottom diameter (280 mm) in which the gradiometers are distributed. Theaverage boil-off rate of the Dewar is 3 l per day when the 64-channel system is inoperation every day. Double relaxation oscillation SQUIDs (DROSs) having largeflux-to-voltage transfer coefficients were used to operate SQUIDs via compactelectronics. The magnetically shielded room (MSR) has a wall thickness of 80 mm, andconsists of two layers of permalloy and one layer of aluminum. When the 64-channelsystem was installed inside the MSR, the field noise level of the system was about3.5 fTrms Hz−1/2 at 100 Hz. MCG measurements with high signal quality were done successfullyusing the developed system. In addition to the parameter analysis method, wedeveloped software for the three-dimensional imaging of the myocardial currenton a realistic image of the heart based on the anatomical image of the torso.

AIN ceramic thermal guide for cooling a SQUID and its effect on thermal magnetic noise

†We evaluated the thermal conductivity of AlN ceramic plates as a potential material of a thermal guide for cooling SQUID in cryocooler cooled SQUID systems. We found that the AlN ceramic plate does not show a good thermal conductivity at very low temperature. We tried to enhance its thermal conductivity by attaching a lot of thin insulated copper wires in parallel with each other to cover a back plane of the plate. With this enhancement we succeeded to operate a double relaxation oscillation SQUID (DROS) first-order gradiometer. Comparison of measurement results on white noise level between two cases where DROS was attached to a copper plate thermal guide and where it was attached to a modified AlN thermal guide shows almost 5 time reduction in noise floor with the latter plate.

Characterization of a low frequency magnetic noise from a two-stage pulse tube cryocooler

Magnetic noise of a two-stage pulse tube cryocooler (PT) was measured by a fundamental mode orthogonal fluxgate magnetometer and by a LTS Double Relaxation Oscillation SQUID (DROS) first-order planar gradiometer. The magnetometer was installed in a dewar made of aluminum at 12 cm distance from a section containing magnetic regenerative materials of the second pulse tube. The magnetic noise spectrum showed a clear peak at 1.8 Hz, which is the fundamental frequency of the He gas pumping rate. The 1.8 Hz magnetic noise registered a peak, during the cooling down process, when the second cold-stage temperature was around 12 K, which is well correlated with the 1.8 Hz variation of the temperature of the second cold stage. Hence, we attributed the main source of this magnetic noise to the temperature variation of the magnetic moments resulting from magnetic regenerative materials, Er 3Ni and HoCu 2, in the presence of background static magnetic fields. We have also pointed out that the superconducting magnetic shield of lead sheets reduced the low frequency magnetic noise generated from the magnetic regenerative materials. With this arrangement, the magnetic noise amplitude measured with the LTS DROS gradiometer, mounted at 7 cm horizontal distance from the magnetic regenerative materials, in the optimum condition, was lower than 500 pT peak-to-peak, whereas the noise level without lead shielding was higher than the dynamic range of DROS instrumentations which was around ± 10 nT .

Effects of Electromagnetic Radiation From Digital Wireless Telephone Frequencies on a Double Relaxation Oscillation SQUID

In this paper, the effects of electromagnetic fields from the frequency band of digital wireless telephones on a superconducting quantum interference device (SQUID) were experimentally investigated. A first-order planar gradiometer based on the double relaxation oscillation SQUID (DROS) was operated without low-frequency magnetic shielding, and was exposed to radiated electromagnetic fields in the frequency range of 300 MHz-2 GHz. The experiments have been performed in a fully anechoic chamber to minimize reflected waves in the chamber. The flux-to-voltage transfer and the field sensitivity of the gradiometer have been observed under the irradiation of fields. Results show that the performance of the gradiometer was significantly affected at a typical power level of the digital wireless telephones for both horizontally and vertically polarized electromagnetic waves. The noise level of the DROS output voltage also increases as the strength of the external fields becomes higher.

64-channel second-order axial gradiometer system based on DROS for magnetocardiogram in a thin shielded room

We fabricated a magnetocardiogram (MCG) system having 64 second-order wire-wound gradiometers for operation inside a thin magnetically shielded room (MSR). In order to provide good quality MCG signals in noisy environments using a thin economic shielded room, we used second-order gradiometers with a baseline of 70 mm. In order to reduce the liquid level necessary to cool the sensor into superconducting state, the total length of the gradiometer was reduced to 160 mm by using a special superconducting connection method between pickup coil and input coil. The SQUID sensor is the low-temperature double relaxation oscillation SQUID (DROS) having large flux-to-voltage transfers to simplify the readout electronics. The coverage area of the 64-channel sensor array is about 256 mm. The MSR is consisted of 1 layer of 1.4-mm thick Permalloy and 1 layer of 12-mm thick Al. The thin MSR has shielding factors of about 17 at 0.1 Hz and 300 at 10 Hz. When the second-order gradiometer system was operated inside the thin MSR, the average noise level is about 7 fT/√Hz at 200 Hz.

64-Channel Second-Order Planar Gradiometer System for Magnetocardiograms

A 64-channel planar gradiometer system was developed to measure magnetocardiogram (MCG) signals. The planar gradiometers are second-order planar gradiometers with a baseline of 50 mm, fabricated from Nb thin films on Si wafers using thin film processes. The SQUIDs were fabricated from Nb/Al-oxide/Nb junctions, and a double relaxation oscillation SQUID (DROS) scheme was used to have a large flux-to-voltage transfer, typically about 1 mV/Phi0. The planar gradiometers were arranged to measure magnetic field components tangential to the chest surface, which has a field distribution peak just above the current dipole. The diameter of the sensor coverage area is 190 mm, which was proved to be large enough to provide MCG field distribution in a single measurement. Inside a thin-wall shield room, the white noise level of the planar gradiometer system is about 8 fTradicHz. The 64-channel system was operated in a thin-wall shield room, and MCG signals were measured.

Characteristics of magnetocardiograms measured using various pickup coil structures

Magnetocardiograms (MCG) measured using multichannel SQUID sensors of different pickup coil configuration have different magnetic signal waveforms and field distributions. In order to compare MCG data measured using different multichannel systems and to make standard format of data, we fabricated and characterized three different MCG systems each having different structure of pickup coils. Pickup coils are integrated magnetometer, planar first-order gradiometer with baseline 4cm, planar second-order gradiometer with baseline 5cm. The magnetometer system measures normal component of MCG fields with 61 channels, and the two planar gradiometer systems measure tangential components of MCG fields each with 64 channels. The SQUIDs are double relaxation oscillation SQUID type fabricated using Nb/Al-oxide/Nb thin film process on Si wafers. From the measured tangential field maps, normal component waveforms and field maps were reconstructed using minimum norm estimation method. The average correlation values over one cardiac cycle between the measured Bz waveforms and reconstructed Bz waveforms were higher than 90% both for the first-order and second-order gradiometers, meaning that we can transform reliably the tangential measurement results to the conventional normal measurement scheme.

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

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

64-channel magnetocardiogram system based on double relaxation oscillation SQUIDplanar gradiometers

We developed a magnetocardiogram (MCG) system measuring tangential field components usingplanar gradiometers. The SQUID insert has 64 planar gradiometers, of which 32 measure thex-component and theother 32 measure the y-component, where the chest surface is in thex and y plane. The SQUID sensor is a double relaxation oscillation SQUID type with highflux-to-voltage transfers. Thus, a room-temperature dc preamplifier with a modest inputvoltage noise could be used to detect the SQUID output voltage directly. The pickup coilsare first-order planar gradiometers with a baseline of 4 cm, and they were connected to theinput coils using a superconductive bonding of Nb wires. The sensor coverage area of the64 channels is 190 mm in diameter, and the average boil-off rate of liquid He is3.6 L/d in everyday operation of the SQUID system. The SQUID operation of the 64channels was done by digital controls and the average noise of the MCG system was4.6 fT Hz−1/2 at 100 Hz, inside a magnetically shielded room.

Development of a SQUID Readout System for the MiniGRAIL

The MiniGRAIL is one of the three similar spherical gravitational wave detectors that are currently being developed. The detector has a resonant frequency of about 3 kHz and will be operated at 20 mK. The ultimate goal is to use a readout system consisting of six transducers coupled to nearly quantum limited two-stage SQUIDs. The two-stage SQUIDs are based on double relaxation oscillation SQUIDs, which enables a direct voltage readout scheme. We have developed nonintegrated two-stage SQUIDs and experimentally verified the proper operation of the system coupled to a capacitive transducer. Based on the results that were achieved, integrated two-stage SQUIDs were designed. Special attention has been paid to the sensor SQUID, the back action of the SQUID and the feedback scheme that is used for linearizing the SQUID output.

Tangential Cardiomagnetic Field Measurement System Based on Double Relaxation Oscillation SQUID Planar Gradiometers

We present a 62-channel double relaxation oscillation SQUID (DROS) planar gradiometer system to measure magnetocardiogram (MCG) signals tangential to the chest surface. The DROSs were fabricated using Nb/Al-oxide/Nb junctions, and provided flux-to-voltage transfers of larger than 1 mV//spl Phi//sub 0/ typically, which is large enough for direct readout by room-temperature flux-locked loop (FLL) circuits. The analog FLL circuits and digital control circuits are made in a single printed circuit board. The planar gradiometers are integrated, series-connected first-order pickup coils with a baseline of 4 cm. White noise level of the planar gradiometer is about 1.5 (fT/cm)//spl radic/Hz or 6 fT//spl radic/Hz, operated inside a magnetically shielded room. The planar gradiometers were arranged to measure dB/sub x//dz and dB/sub y//dz simultaneously. The covering area of the 62 channel system is 162 mm /spl times/ 162 mm, which is smaller than the sensor coverage area of vertical measurement systems, but large enough to measure the essential MCG field distribution in a single position measurement.

Compact Readout Electronics for 62-Channel DROS Magnetocardiogram System

Compact and low-cost SQUID electronics to operate double relaxation oscillation SQUIDs (DROSs) has been constructed for detecting magnetocardiogram (MCG) fields. SQUID electronics consists of low-noise preamplifier with a white noise of 0.6 nV//spl radic/Hz, flux-locked loop (FLL) circuit with auto-reset, and interface circuit to control FLL using 3 digital lines from a computer. The automatic control software adjusts SQUID parameters to the optimum operating condition within 15 seconds for 62 channels. Outputs of FLL circuits are passed through the hardware filter box to the 64-channel data acquisition board. Each SQUID electronics for FLL and digital control is built together on a printed circuit board of 45 mm /spl times/ 95 mm. When SQUID electronics is connected to DROS gradiometer with typical flux-to-voltage transfers of 1 mV//spl Phi//sub o/, the noise contribution of FLL circuit is about 0.6 /spl mu//spl Phi//sub o///spl radic/Hz or 0.6 fT//spl radic/Hz at 100 Hz, low enough to measure MCG fields.

Double Relaxation Oscillation SQUID with a 4JL On-Chip Digital Flux Locked-Loop Circuit

Double relaxation oscillation SQUID (DROS) combined with an on-chip superconducting digital flux locked-loop (FLL) circuit has been proposed. In this study, we described superconducting digital FLL circuit which consists of an 8-bit up/down counter and an 8-bit R-2R ladder D/A converter. The circuit was designed using 4-Junction Logic (4JL)-gates which are driven by a two phase power supply. In the up/down counter, we employed binary carry lookahead (BCL) circuits for high-speed operation. Logic simulations for the BCL 8-bit up/down counter showed correct operation up to 4 GHz, assuming a 2.5 kA/cm/sup 2/ Nb/Al-AlO/sub x//Nb junction technology. From simulation results high slew rate of 10/sup 7//spl Phi//sub 0//s and dynamic range of 2.5/spl Phi//sub 0/ can be expected.

Double Relaxation Oscillation SQUID Systems for Biomagnetic Multichannel Measurements

Multichannel superconducting quantum interference device (SQUID) systems based on double relaxation oscillation SQUIDs (DROS) were developed for measuring magnetocardiography (MCG) and magnetoencephalography (MEG) signals. Since DROS provides large flux-to-voltage transfer coefficients, about 10 times larger than the DC SQUIDs, direct readout of the SQUID output was possible using compact roomtemperature electronics. Using DROSs, we fabricated two types of multichannel systems; a 37-channel magnetometer system with circular sensor distribution for measuring radial components of MEG signals, and two planar gradiometer systems of 40-channel and 62-channel measuring tangential components of MCG or MEG signals. The magnetometer system has external feedback to eliminate magnetic coupling with adjacent channels, and reference vector magnetometers were installed to form software gradiometers. The field noise of the magnetometers is around 3 fT/ Hz at 100 Hz inside a magnetically shielded room. The planar gradiometer systems have integrated first-order gradiometer in thin-film form with a baseline of 40 mm. The magnetic field gradient noise of the planar gradiometers is about 1 fT/cm/Hz at 100 Hz. The planar gradiometers were arranged to measure field components tangential to the body surface, providing efficient measurement of especially MCG signals with smaller sensor coverage than the conventional normal component measurements.

A multichannel double relaxation oscillation SQUID magnetometer system for magnetoencephalogram

We developed multichannel SQUID magnetometer and gradiometer systems based on double relaxation oscillation SQUIDs (DROS) for use in biomagnetic studies, such as magnetoencephalogram (MEG). DROS consists of two SQUIDs (signal SQUID and reference SQUID) in series, and a relaxation circuit of an inductor and a resistor. Specially, we used single reference junction instead of the reference SQUID. Since DROS magnetometer has about 10 times lager flux-to-voltage transfer coefficient than dc SQUID, we can use simple flux-locked loop electronics for SQUID operation. The measured noise at 100 Hz was 3 fT/ for the magnetometer and 4 fT/ for the planar gradiometer, which are low enough for neuromagnetic measurements. We also constructed two kinds of multichannel SQUID systems, 40-channel and 37-channel system. Using this system, we measured several MEG signals. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)