Wide Dynamic Range Analog Flux-Locked Loop System Using Low-Tc SQUID for MCG Measurements Without MSR

Recently, biomagnetic measurements using a superconducting quantum interference device (SQUID) magnetometer have been widely performed and utilized for clinical applications. In biomagnetic measurements, such as magnetocardiogram (MCG) and magnetoencephalogram, a magnetically shielded room (MSR) is used for noise reduction. However, because the MSR is expensive and heavy, there is a problem to which used environment is restricted. We developed an analog flux-locked loop system (low-Tc SQUID magnetometer) using the flux-quanta counting (FQC) method for MCG measurements, which does not require the MSR. This FQC-based system is operated using high-speed analog switches and comparators to realize a wide dynamic range. The comparator is given in the hysteresis property for stability behavior. We show the noise spectrum, dynamic range of the developed analog system and MCG waveforms measured inside and outside the MSR. The developed analog system can measure MCG waveforms (~30 pT) under large environmental magnetic noise (~1000 pT) conditions.