  The effect of external radiation on the phase dynamics of Josephson junctions
shunted by an LC circuit is examined. It is shown that additional resonant
circuit (rc) branches appear in the current voltage characteristics of the
junctions when the Josephson frequency is equal to the frequency of the formed
resonance circuit. We show that the amplitude dependence of the Shapiro step
width crucially changes when the Shapiro step is on the rc-branch in comparison
to the case of Josephson junction without shunt. The experimental
implementation of these effects might give very important advantages for
existing methods and technologies.
