Digital-Analog Magnetometer Utilizing Superconducting Sensor

Abstract

A magnetometer was developed which utilizes a superconducting quantum interference device (SQUID) as a sensor, and permits digital and analog measurements of magnetic fields. The digital mode permits measurement of large field changes with a constant accuracy of approximately 1/10 the natural periodicity of a SQUID. (The natural period is typically 10−6–10−3 G.) Thus, the precision of measurement of large field changes is high. The analog mode is advantageous for high precision measurement, of the order of 10−8 G, but for small field changes. Combined mode measurements permit the very high precision of the analog mode to be combined with the large range of the digital mode. The digital mode, which represents an improvement over the original lock-on magnetometer, utilizes two synchronous detectors to provide signals which uniquely determine the instantaneous magnetic bias of the SQUID. The synchronous detector outputs drive a novel logic circuit which produces appropriate add-subtract pulses to a digital counter, which reads out field changes in units equal to the periodicity of a SQUID.