The use of (double) relaxation oscillation SQUIDs as a sensor

Relaxation Oscillation SQUIDs (ROSs) and Double Relaxation Oscillation SQUIDs (DROSs) are based on relaxation oscillations that are induced in hysteretic dc SQUIDs by an external L-R shunt. The relaxation frequency of a ROS varies with the applied flux Φ, whereas the output of a DROS is a dc voltage, with a typical flux-to-voltage transfer of ∂V/∂Φ≈1 mV/Φ0. The flux-to-frequency response of several ROSs has been measured and compared with theory for frequencies up to 7 GHz. Various DROS designs-a multi-loop direct coupling DROS, a DROS with a washer type signal SQUID and a DROS with gradiometric signal SQUID-will be discussed in this paper. The integration of a DROS with a digital flux locked loop (“Smart DROS”) will also be analyze