Electrical impedance plots of EXC_3 placed in L1, panel (a), and C, panel (b). Rigid masses are progressively added on the exciter to alter its mechanical resonance (f_md). The dashed lines represent the first modes of the board; mode (0, 0) (f₀ = 118 Hz) is located near the first small local maxima in panel (a), and in between the peaks f_l and f_h (“low” and “high”) in panel (b). A good excitation point (a) can be identified when a single peak f_md (resonance of DUT + transducer) is visible. When mass is added, f_md shifts down in frequency, while smaller local maxima of the curve are not affected.