  We study analytically and numerically the phase-modulation properties of a
biased classical Josephson tunnel junction in the zero-voltage state and
phase-locked to an external ac field. We show that the phase-locked state is
being modulated in the transients, or in response to perturbations, and the
modulation frequency is calculated as a function of relevant system parameters,
such as microwave field amplitude. The numerical analysis is parameterized
similarly to recent experimental results in which a combination of pulsed ac
signals and relative switching from the zero-voltage state are used to probe
the internal excitations of the junctions. Our analysis demonstrates that the
modulation of a phase-locked state in an entirely classical Josephson junction
produces oscillations analogous to quantum mechanical Rabi oscillations,
expected to be observed under the same conditions.
