A method of determining the frequency and amplitude of a perturbing cyclic EM signal in the field of view of an NMR or MRI system during acquisition of a spin-echo spectrum. The frequency ν of the perturbing electromagnetic signal is determined by acquiring a plurality of n 2D NMR spectra with n different values of TR; selecting a peak in each of the n NMR spectra; determining the area of the peak; calculating possible frequencies along the ΩTR axis; and eliminating results that do not match the position along the Ωτ axis, thereby obtaining ν. The amplitude α of the perturbing electromagnetic signal is determined by calculating the square of the area of the peak.