Design and development of a flux-locked loop for low magnetic field measurements using dc squids

A novel instrumentation to process the SQUID response with the objective of measuring low magnetic field is presented. A computer model of the dc SQUID and the instrumentation system is developed and its performance such as its stability and slew rate are evaluated using PSpice circuit simulator. The feedback electronics uses room temperature impedance matching circuit, an ultra-low noise bipolar pre-amplifier and a resonant amplifier. Synchronous detection Is achieved by means of simple CMOS switches and the feedback flux to the dc SQUID is derived from a single pole integrator. The entire Flux-Locked Loop (FLL) electronics is built on three different printed circuit boards; one each for amplifier, detector and oscillator sections and are separated by shielding for good noise suppression. Also, a novel method is illustrated to test the FLL electronics with a computer simulating the dc SQUID's response. © 1998 Taylor & Francis Group, LLC.