This paper reports an experimental study to detect and distinguish surface and sub-surface cracks in non-ferromagnetic materials using eddy current testing with a multi-harmonic excitation signal. A minimization of the crest factor of the excitation signal was done using a Shapiro-Rudin phase sequence. The chosen range of frequencies was based on a corresponding range of standard depths of penetration (SDP) in order to ensure that the eddy currents penetrate more or less an aluminum plate with 4 mm of total thickness. Experimental tests were realized on surface and sub-surface cracks with depths of 0.5, 1, 1.5 and 2 mm. The results obtained by the ECT probe are analyzed in the frequency domain.