The Effect of Low Frequency Disturbance to SQUID Based Low Field NMR

The influence of low-frequency magnetic field disturbances on SQUID based low field (LF) nuclear magnetic resonance (NMR) measurements is investigated. Two types of sinusoidal fields, a homogenous field and a linear gradient field, were applied as artificial disturbance sources. The influences on the free induction decay (FID) signals as well as on the spectra are discussed. The homogeneous disturbance field caused a frequency modulation of the FID signal. The measured spectra were found to be in good agreement with calculated traces obtained from a solution to the Bloch equation. The gradient disturbance field yielded an amplitude-modulated FID signal. In both cases, frequency mixing lines were observed. It is shown that for disturbances at the power line frequency and harmonics, the influence on the NMR spectra is negligible.