  We experimentally study the factors that limit the span in frequency combs
derived from the crystalline whispering gallery mode resonators. We observe
that cavity dispersion is the key property that governs the parameters of the
combs resulting from cascaded four wave mixing process. Two different regimes
of comb generation are observed depending on the precise cavity dispersion
behavior at the pump wavelength. In addition, the comb generation efficiency is
found to be affected by the crossing of modes of different families. The
influence of Raman lasing is discussed.
