A modular low noise 7-channel SQUID-magnetometer

A modular low-noise seven-channel SQUID-magnetometer for biomagnetic measurements was developed. The DC-SQUIDs are prepared by thin film technology based on the materials NbN-MgO-NbN, and show a flux noise below 10 mu Phi /sub 0// square root Hz. The signal is picked up with first-order wire-wound gradiometers of 2-cm diameter and coupled to the SQUIDs by integrated coupling coils. Individual channels can be replaced from the system, and various gradiometers can be used giving a flexible modular system. The electronics are based on the flux-locked-loop method extended by a bias modulation technique to reduce 1/f noise. The overall system noise is below 20 fT/ square root Hz. Biomagnetic measurements on auditory evoked fields have been carried out.