Excitation circuit of a probe for measuring and recovering the signal emitted in return by this latter in an NMR spectrometric apparatus, connecting said probe, under conditions defined by switching means forming a portion of said circuit, to a high frequency excitation signal generator by a portion of the line or emission line and, by a portion of the line or reception line, to a receiver for the acquisition and exploitation of signals emitted by said probe as a function of the specimen to be analyzed which it contains, a device being, moreover, provided to carry out or verify the tuning of the measuring probe with the excitation signal generator, associated with or integrated into said circuit, which circuit is characterized in that said device comprises a directive coupler (7), mounted in series in or in parallel to the portion of the emission line (4).