  On base of Hamiltonian formalism, we show that Hopf bifurcation arrives, in
the course of the system evolution, at creation of revolving region of the
phase plane being bounded by limit cycle. A revolving phase plane with a set of
limit cycles is presented in analogy with revolving vessel containing
superfluid He$^4$. Within such a representation, fast varying angle is shown to
be reduced to phase of complex order parameter whose module squared plays a
role of action. Respectively, vector potential of conjugate field is reduced to
relative velocity of movement of the limit cycle interior with respect to its
exterior.
