  We have numerically and analytically studied ac+dc driven Josephson-junction
arrays with a single vortex or with a single vortex-antivortex pair present. We
find single-vortex steps in the voltage versus current characteristics (I-V) of
the array. They correspond microscopically to a single vortex phase-locked to
move a fixed number of plaquettes per period of the ac driving current. In
underdamped arrays we find vortex motion period doubling on the steps. We
observe subharmonic steps in both underdamped and overdamped arrays. We
successfully compare these results with a phenomenological model of vortex
motion with a nonlinear viscosity. The I-V of an array with a vortex-antivortex
pair displays fractional voltage steps. A possible connection of these results
to present day experiments is also discussed.
