  Examination of an annular system of underdamped Josephson junctions in the
presence of external radiation showed that the ability of the system to lock
with some external radiation was determined not only by the number but also by
the type of rotating excitations (fluxons or antifluxons). Shapiro steps can be
observed in the current-voltage characteristics only in the system with trapped
fluxons or in the system with fluxon-antifluxon pairs. If the trapped fluxons
circulate simultaneously with fluxon-antifluxon pairs, there are no Shapiro
steps regardless of the amplitude or frequency of the applied external
radiation.
