  We designed and developed a master-slave electronic oscillatory system (based
on the 555-timer IC working in the astable mode), and investigated its dynamic
behavior regarding synchronization. For that purpose we measured the rotation
numbers corresponding to the phase-locking rhythms achieved in a large set of
values of the normalized forcing frequency (NFF) and of the coupling strength
between the master and the slave oscillators. In particular we were interested
in the system behavior in the strong-coupling limit, because such problem has
not been extensively studied from an experimental perspective. Our results
indicate that, in such a limit, a degenerate codimension-2 bifurcation point at
NFF=2 exists, in which all the phase-locking regions converge. These findings
were corroborated by means of a mathematical model developed to that end, as
well as by ad hoc further experiments.
