Amane Hayashi

Active compensation in combination with weak passive shielding for magnetocardiographic measurements

We studied different schemes of noise reduction using active compensation of environmental magnetic field noises in order to develop a high-temperature-superconductor (HTS) magnetometer system for the measurement of magnetocardiographic signals. The active compensation was combined with weak passive shielding of about 20 dB by the use of a magnetically shielded room (MSR) having a single layer of surrounding μ-metal. A novel method using a normal detection coil and compensation coils that were wound around the walls of the MSR to enable magnetic coupling was examined. Effective suppression of environmental field noises of more than 20 dB at 0.5–10 Hz and 10 dB at 10–100 Hz, covering the low-frequency range of biological signals, was obtained. In an alternative method using a reference HTS magnetometer in the MSR for noise detection, a compensation coil was set in the MSR and served as a feedback coil in operating the magnetometer. Residual field noises that were limited by the intrinsic noise of the refer...

High-Tc SQUID magnetometers for use in moderate magnetically-shielded room

We have fabricated and characterized high-T/sub c/ planar SQUID magnetometers and first derivative gradiometers with directly-coupled pickup loops. The devices were made from single layer of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film on LSAT bicrystal substrate with 30/spl deg/ or 24/spl deg/ misorientation angle. Magnetic properties were investigated by applying a magnetic field B/sub 0/ for the SQUID magnetometer patterned with holes to reduce the maximum structural width. We found an increasing low frequency noise with cooling fields B/sub 0/ larger than 1.5 /spl mu/T. This value consists with the threshold field estimated from the maximum structural width. A magnetic field noise level of 500 fT/Hz/sup 1/2/ at 10 Hz was observed by using FLL electronics with a bias current reversal in a moderate magnetically-shielded room consisting of only two 1 mm thick layers of permalloy. Measurements of magnetocardiograms demonstrate the suitability of this magnetometer for biomagnetic applications. On the other hand, the gradiometer with two symmetric pickup loops was operated without any shielding. The performance obtained was a field gradient resolution of about 1 pT/cmHz/sup 1/2/ at 1 kHz and 10 pT/cmHz/sup 1/2/ at 1 Hz, with a baseline of 4 mm. The imbalance of this gradiometer was around 0.7%, limited by the sensitivity to homogeneous field of the SQUID itself.

ON THE AC BIAS READOUT SCHEMES FOR HIGH-TC SUPERCONDUCTING QUANTUM INTERFERENCE DEVICES

The ac bias method is used in the readout system of dc-superconducting quantum interference devices (SQUIDs) to suppress the low-frequency noise that originates in the fluctuation of parameters of two junctions in the SQUID. From the analysis of various schemes of the ac bias method, we propose a simple form which expresses the alternation of the operation variables, i.e., the bias current, modulation flux, and bias flux. We considered the modification of the previous schemes in the context of this form and have found a possible method to directly detect the noise component due to the parameter fluctuation. In the modified ac bias scheme, the low-frequency noise of the parameter fluctuation can either be detected or suppressed by manipulating a single variable. We fabricated the electronics to verify this function, and confirmed the versatility of this method from the measurements of the high-TC SQUID.

Adaptive cancellation of line-frequency noise in magnetocardiogram measurements

In the recording of magnetocardiogram (MCG), the largest environmental magnetic field interference is the line-frequency noise. Even with multi p-metal layer shielding, 50 or 60 Hz interference is not completely rejected. In this paper, we report an extension of active noise cancellation to the magnetic field interference, in which a digital signal processor (DSP) was used. A sinusoidal wave generated in the DSP was filtered adaptively in the least mean square algorithm, and its output was supplied as cancellation field by means of a coil that was wound around a small room magnetically shielded with a single g-metal wall. With the help of the low-frequency noise compensation, which was also applied to the shielded room, wide band MCG recording was possible inside the room using a high temperature superconducting quantum interference device.

Transmission line high-T/sub c/ dc-SQUIDs

Because of high permittivity of SrTiO/sub 3/ (STO) substrate, YBCO high-Tc SQUIDs have distributed impedance. Based on the simulation by the analytical method to calculate the I-V characteristic, we have designed and fabricated YBCO dc SQUIDs that have a transmission-line-like geometry of thin parallel strips with a wide slit between them. The geometrical parameters were determined to give high modulation voltage. The fabricated SQUIDs had a white noise of (0.5-1.0)/spl times/10 /spl Phi//sub 0//Hz/sup 1/2/. We also designed a direct coupling magnetometer, in which the transmission line SQUID was located within the hole of the pickup coil. On a small substrate area of 5 mm/spl times/10 mm, the fabricated magnetometer had an effective area of 0.22 mm/sup 2/. The field noise of about 100 fT/Hz/sup 1/2/ was obtained in the white noise region.

High-Tc squid magnetometer system with active cancellation

Recent developments of high-Tc SQUIDs have enabled high sensitivity magnetometers to be used in wide range of places, such as laboratory and outdoor fields. At the early stage of developing multichannel system for measurement of magnetocardiogram (MCG) in clinical application, we have fabricated a single channel high-Tc SQUID magnetometer system. The system includes a direct-coupled SQUID with slot structure, a simple magnetically shielded room (MSR), and some active compensation electronics for the purpose of reducing various environmental field noises. A novel active noise cancellation was made by using a combination of a normal conducting detection coil that was horizontally wound in the middle height of the MSR, and two compensation coils that were wound at the top and bottom of the MSR. In addition, adaptive noise cancellation was supplemented by means of adaptive digital filter that was implemented in a digital signal processor. A total noise field attenuation of 50–60 dB was attained at 0.5–100 Hz....