  We investigate the stochastic resonance phenomenon in a physical system based
on a tunnel diode. The experimental control parameters are set to allow the
control of the frequency and amplitude of the deterministic modulating signal
over an interval of values spanning several orders of magnitude. We observe
both a regime described by the linear response theory and the nonlinear
deviation from it. In the nonlinear regime we detect saturation of the power
spectral density of the output signal detected at the frequency of the
modulating signal and a dip in the noise level of the same spectral density.
When these effects are observed we detect a phase and frequency synchronization
between the stochastic output and the deterministic input.
