  We calculate analytically the Floquet scattering matrix for a periodically
driven double-barrier structure. Our approach takes into account dynamical
effects which become necessarily important when electrons propagate through a
system subject to a fast drive. It is convenient to represent the Floquet
scattering amplitude as a sum of amplitudes corresponding to different times
spent by an electron inside the structure. These amplitudes define dynamical
scattering channels. Then we represent the dc current generated by the
structure as a sum of two generic contributions. The first one is due to
photon-assisted interference processes within the same channel and the second
one is due to inter-channel interference processes. At zero temperature both
contributions are present while at high temperatures and/or high driving
frequencies only the former survives.
