  Frequency conversion in microresonators has revolutionised modern-day
nonlinear and quantum optics. Here, we present a theory of the multimode second
harmonic generation in microresonators under conditions when the parametric
conversion back to the pump spectrum dominates through the large domain in the
resonator parameter space. We demonstrate that the spectral tunability of the
sideband generation in this regime is governed by a discrete sequence of the
so-called Eckhaus instabilities. We report the transition to modelocking and
generation of solitons, which have a double-pulse structure in the pump field.
These solitons exist outside the bistability interval and on a slightly curved
background.
