Properties of the phase fluctuations of a nonlinear oscillator were investi
gated. The multiscale aspect of synchronization is experimentally confirmed
with the observation of lockings at any rational ratio. This global proper
ty of synchronization is associated with a local one by the study of the ph
ase locked loop (PLL) under nonlinear conditions. We show that the disconti
nuity between the locking and the free-run state is characterized by the pr
esence of slow temporal variations that induce a low frequency spectrum. An
experimental verification of the increase of fluctuations confirms the cla
ssical results, which associate 1/f noise to nonlinearity, autosimilarity a
nd regularity, represented here by the synchronization.