  The dynamic response of a parametric system constituted by a spin precessing
in a time dependent magnetic field is studied by means of a perturbative
approach that unveils unexpected features, and is then experimentally
validated. The first-order analysis puts in evidence different regimes: beside
a tailorable low-pass-filter behaviour, a band-pass response with interesting
potential applications emerges. Extending the analysis to the second
perturbation order permits to study the response to generically oriented fields
and to characterize several non-linear features in the behaviour of such kind
of systems.
