Jari K. Hällström

Large-area low-noise seven-channel dc SQUID magnetometer for brain research

The design, construction, and performance of a new high‐sensitivity dc SQUID magnetometer, covering a circular area of 93‐mm diameter, is described. The device, now used routinely in our brain research, comprises seven asymmetric first‐order gradiometers, located on a spherical surface of 125‐mm radius and with the symmetry axis tilted 30° with respect to the vertical. The pickup coil diameter is 20 mm, and the channels are separated by 36.5 mm from each other in a hexagonal array. The overall field sensitivity of the system, measured inside our magnetically shielded room, is 5 fT/(Hz)1/2, mainly limited by the thermal noise in the radiation shields of the Dewar. The optimization of the coil configuration and the measurement system is discussed in detail, and a system to determine automatically the position and orientation of the Dewar with respect to certain fixed points on the subject’s head is described. Finally, some examples of measurements carried out with the new device are given.

A 24-Channel Magnetometer for Brain Research

This paper describes the hardware of the 24-channel SQUID magnetometer being completed in the Low Temperature Laboratory. The overall system, including computer hardware and software, is discussed elsewhere (Hamalainen 1989). The instrument will be used in a magnetically shielded room for brain research. We hope that this apparatus will enable us to locate current sources underlying evoked magnetic fields without moving the dewar.

On-site calibration of a current transformer using a Rogowski coil

This paper describes an outdoor calibration of an instrument current transformer connected to 400 kV transmission line. The method of generation of the high current and the accuracy of calibration are discussed. The reference current measuring system consists of a current shunt, a Rogowski coil, two digitizing voltmeters and a computer to control voltmeters. The Rogowski coil is calibrated on-site with the current shunt. The output voltage of the Rogowski coil is not integrated, but it is scaled according to the on-site calibration result. Estimated uncertainties for ratio and phase displacement are less than 0,01% and 100 murad, respectively.

High-Accuracy Comparison of Lightning and Switching Impulse Calibrators

This paper summarizes the results of a comparison on two calculable impulse voltage calibrators and one commercial calibrator between Helsinki University of Technology, Finland (MIKES-TKK), Nippon Institute of Technology, Japan (NIT), and Physikalisch-Technische Bundesanstalt, Germany (PTB). The peak voltage levels of the comparison ranged from 10 to 300 V. Both lightning and switching impulse voltage calibrators were studied using both positive and negative polarity impulses. The comparison was performed by applying impulses from the calibrators to a 12-b digitizer and then comparing the reported digitizer errors. For the switching impulse peak value, the agreement between the two calculable calibrators was better than 0.02% for all the measured voltages and better than 0.4% for the time parameters. For the lightning impulse peak value, the disagreement between the same two calibrators was smaller than 0.05% for all voltages examined and smaller than 1% for the time parameters

Comparison of Asynchronous Sampling Correction Algorithms for Frequency Estimation of Signals of Poor Power Quality

The measurement of many signal parameters using digital sampling relies on synchronization between the sampling clock and the signal under analysis. Deviations in the frequency of the measured signal from that expected can lead to significant measurement errors. Many techniques for overcoming these problems have been developed. Appraisals of the performance of these algorithms usually involve relatively benign steady-state signals, and the treatment is often heavily mathematical. This paper describes the testing of several algorithms for accurately recovering frequency and harmonic amplitude and phase information from mains-borne waveforms. This paper focuses on their practical use while avoiding abstract mathematical concepts.

Influence of busbar geometry on AC current measurement using Rogowski coil

Readout of a Rogowski coil is affected by external magnetic fields e.g. from the energizing transformer and the return current conductor. The influence of these external magnetic fields on the reading of a high precision Rogowski coil was measured. The results are used to estimate the achievable uncertainty in calibration of current transformers up to 10 kA.

A Low-Noise Seven-Channel DC SQUID Magnetometer for Brain Research

The design, construction, and performance of a high-sensitivity DC SQUID magnetometer, covering a circular area of 93 mm diameter, is described. The device, now used routinely in our brain research, comprises seven asymmetric first-order gradiometers, located on a spherical surface of 125 mm radius and with the symmetry axis tilted 30° in respect to the vertical. The overall field sensitivity of the system, measured inside our magnetically shielded room, is 5 – 6 fT/√Hz ,mainly caused by thermal noise in the radiation shields of the dewar.

Worldwide Comparison of Lightning Impulse Voltage Measuring Systems at the 400-kV Level

An international comparison of lightning and switching impulse voltage measuring systems was arranged and coordinated by the Helsinki University of Technology (MIKES-TKK), Espoo, Finland, between 1999 and 2002. The number of participants was 26, including the coordinator. This paper summarizes the results obtained by those eight National Metrology Institutes that participated in the comparison. A 400-kV transfer reference measuring system for measuring lightning impulse voltages was prepared by the coordinator. In addition, a 1-kV impulse voltage calibrator system, including calibrators for both standard lightning and switching impulses, was circulated

A calculable impulse voltage calibrator for calibration of impulse digitizers

Design and construction of a calibrator for lightning and switching impulse voltage measuring instruments is presented. The operating voltage range is from 50 mV to 300 V. The estimated uncertainty for the peak value is 0.03% and for front time and time to half value less than 1%.

Calculable impulse voltage calibrator for calibration of impulse digitizers

Design and construction of a calibrator for lightning and switching impulse voltage measuring instruments is presented. The operating voltage range is from 50 mV to 300 V. The estimated uncertainty for the peak value is 0.03% and for front time and time to half value less than 1%.