Development of Digital FLL System for SQUID Using Double Counter Method

We have developed a digitally controlled flux-locked loop (FLL) system using a double-counter method to achieve both high performance and low cost. This system is composed of generally used electric devices. The system slew rate is about 1.3 k Phi₀/s, where Phi₀ is the flux quantum. The measurable flux amplitude and dynamic range are 218 Phi₀ and 141 dB at 1 Hz and 22 Phi ₀ and 121 dB at 10 Hz, respectively. A white noise level measured in the magnetically shielded room (MSR) is 36.1 fT/Hz^1/2 . Magnetic measurements with a first-order gradiometer were done outside the MSR, and the developed system can detect a magnetocardiogram in an unshielded condition