We describe a system for the compensation of time-dependent stray magneticfields using a dual channel scalar magnetometer based on non-linear Faradayrotation in synchronously optically pumped Cs vapour. We detail the activecontrol strategy, with an emphasis on the electronic circuitry, based on asimple phase-locked-loop integrated circuit. The performance and limits of thesystem developed are tested and discussed. The system was applied tosignificantly improve the detection of free induction decay signals fromprotons of remotely magnetized water precessing in an ultra-low magnetic field.
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