MAGNETOMETERS BASED ON (DOUBLE) RELAXATION OSCILLATION SQUIDS

Relaxation Oscillation SQUIDs (ROS) and Double Relaxation Oscillation SQUIDs (DROS) combine the sensitivity of dc-SQUIDs with a large transf er, thus enabling flux locked loop operation with direct voltage reado ut by a room-temperature dc-amplifier, without flux modulation techniq ues. We fabricated several ROSs and DROSs with relaxation frequencies from 5 to 180 MHz. The experimental flux-to-frequency curves of the RO Ss can well be fitted with theory, and the transfer (up to 170 MHz/PHI 0) is large enough for practical use in a FLL without flux modulation. The experimental flux-to-voltage characteristics of the DROSs show ve ry large transfer values of typically 10 to 30 mV/PHI0. A DROS operate d in a simple FLL with direct voltage readout showed a white. noise le vel of about 2 muPHI/square-root Hz. A three channel DROS magnetometer system for biomagnetic measurements is under construction. In the DRO Ss that are to be used in this system, the signal flux is coupled to t he SQUID by a tightly coupled 50-turn input coil.