The article deals with studies of the structure of the nuclear magnetic resonance signal, which is recorded using the modulation technique. The influence of the properties of the medium on the possibility of registering an NMR signal in a weak magnetic field using a modulation technique for measuring is established. A new method for describing the registered NMR signal using the modulation technique is proposed, taking into account the contributions of absorption and dispersion signals. The features of the use of spectral analysis in the study of the NMR signal from liquid media are determined. The results of theoretical calculation and experimental studies are compared.