Abstract
To operate a SQUID magnetometer in a magnetically unshielded environment, it is important to ensure that a flux-locked loop (FLL) circuit has a very wide dynamic range without decreasing its resolution. Digital FLL (D-FLL) satisfies these conditions: its dynamic range is expanded by jumping a working point on the SQUID's periodic Φ-V characteristic and counting the number of such events. We propose a double-counter system, which has two counters: one on a one-chip microcontroller and the other on a host computer. We developed a D-FLL system by using a double-counter with general-purpose operational amplifiers, digital-to-analog converters, and a microcontroller. The effectiveness of the double-counter is shown, and a description is given of how the D-FLL system was used to operate a magnetocardiogram (MCG).
