Development of Digital FLL System for SQUID Using Double Counter Method

Abstract

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 Phi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /s, where Phi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the flux quantum. The measurable flux amplitude and dynamic range are 218 Phi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and 141 dB at 1 Hz and 22 Phi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and 121 dB at 10 Hz, respectively. A white noise level measured in the magnetically shielded room (MSR) is 36.1 fT/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2 </sup> . Magnetic measurements with a first-order gradiometer were done outside the MSR, and the developed system can detect a magnetocardiogram in an unshielded condition