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 room-temperature 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/ 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/ 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.

This study is to clear how the impedance and the current of a simple printed line model involve to the near field electromagnetic noise radiation, by computer simulation and experiment. Frequency characteristics of the impedance and the current of the printed line and the near field noise are considered, from low to high frequency components. The model size 225 60 0.51 mm3, length of the line is 185 5 mm2 and 1 kΩ termination resistance is connected as non-matching load. FDTD method is used to calculate the impedance, the current waveform and the near field noise. Measurements of the line impedance and the near field noise spectrum by clock pulse are compared with simulations. It is cleared that using FDTD method, the characteristic of impedance of the printed line model, the current waveform, and the near field noise can be calculated at the same simulator. As results, from calculation and measurement, the near field noise has a relationship with impedance of printed line model. Emission at frequency less than 200 MHz, which is near the wavelength of λ/4, is observed at significant intensity. So, it is suggested that near field noise emission should be discussed from low to high frequency and analysis of the characteristics of the printed line and magnetic near field noise using FDTD method and measurement is useful to basic examination of complex PCB models.

Switching device used on digital and inverter circuits such as a stabilizer of fluorescent lamp is one of main sources of electromagnetic noise. To make such noise characteristics clear, using a simple printed line model with a TTL IC as a switching device, electric far field noise radiating from that model is measured in an anechoic chamber. It is shown typical results and that noise characteristics can be evaluated by comparing the spectrum and spectrum change of the harmonics of 3 MHz switching pulse using the same switching device. And the characteristics of the electric field noise with PCB thickness and strip line width changed are compared with the magnetic near-field noise measured by a small shielded loop antenna. The results indicate that the electric field noise strength, on the case where the width is 7 mm and the thickness is 0.51 mm, is larger than that on other cases in the range from 50 to 150 MHz. And it is confirmed that the magnetic near-field noise increases as the loop antenna approaches the IC and varies depending on the PCB thickness and the line width. However, the spectral profile of the electric field noise is different from the magnetic near-field noise.

A new 19-channel SQUID magnetometer system has been developed for research use in order to measure the neuromagnetic fields originating from cortices of the human brain.The system could function for 6 days with a one-time supply of about 25 L of liquid helium. The system consists of Nb/Al-oxide/Nb SQUID sensors with 2nd-order gradiometers, tank circuits, readout electronics, a liquid helium dewar, a gantry, and a prefabricated shielded room. The gradiometers cover a circular area of 15 cm radius. We used fine stainless steel leads for electric connection between the sensors and room-temperature electronics with low thermal conduction in a low helium consumption dewar. The system could function for 6 days with a one-time supply of about 25L of liquid helium. The system can be thermally cycled for repeated measurements, with an intervening nonusage period at room temperature. The noise characteristics, for both the time and frequency domains, of all channels were measured. From an analysis of the voltage output at the phase-sensitive detector, the flux-origin noise which is generated by external sources was dominant in the white noise frequency. The power spectra of the noise field were below 10 fT/Hz1/2 at 10-100 Hz and below 18 fT/Hz1/2 at 1-10 Hz. Some other peaks of power line frequencies such as 50 Hz and 150 Hz were observed at several channels. Sound-evoked magnetic fields were measured from the temporal area of the head upon application of tone bursts. The evoked fields were recorded with the amplitude of about 250 fTpp. The isofield contours of the peak response showed that the measurement area is large enough to estimate current dipoles. It is confirmed that the system has the ability to measure magnetic fields from the human brain.

This study shows that using the direct offset integration technique (DOIT) and additional positive feedback (APF) in a high-Tc dc superconducting quantum interference device (SQUID) improves the effective flux-to-voltage transfer function and reduces the flux noise of a magnetometer, thus improving the magnetic field noise. The effective flux-to-voltage transfer function and the flux noise with APF were measured at different values of the positive feedback parameter βa, which depends on the resistance of the APF circuit. These quantities were also compared between conditions with and without APF. This investigation showed that a βa condition the most suitable for minimizing the flux noise of a magnetometer with APF exists and that it is βa=0.77. The effective flux-to-voltage transfer function with APF is about three times what it is without APF (93 µV/Φ0 vs. 32 µV/Φ0). The magnetic field noise of a magnetometer with APF is improved by a factor of about 3 (242 fT/Hz vs. 738 fT/Hz).