Wide Dynamic Range Analog FLL System Using High- $T_{c}$ SQUID for Biomagnetic Measurements

In a system that can perform biomagnetic measurements, the most eagerly desired features include without liquid helium, unshielded operation, and low cost. We developed a high-critical-temperature (high-T_c) superconducting quantum interference device (SQUID) system to be used for taking biomagnetic measurements. The proposed device consists of a small cryostat, high-T_c SQUID magnetometer, and analog flux-locked loop (FLL) circuit. A wide dynamic range was achieved through an analog FLL circuit that used the flux quanta counting (FQC) method. We also proved the feasibility of this high-T_c SQUID system with the help of a simple prototype. The circuit performance of the analog FLL circuit was evaluated using a high-T_c SQUID. A wide dynamic range was obtained by using the analog FLL circuit.