  We investigate the threshold of $\chi^{(2)}$ frequency comb generation in
lithium niobate whispering gallery microresonators theoretically and
experimentally. When generating a frequency comb via second-harmonic
generation, the threshold for the onset of cascaded second-order processes
leading to a comb is found to be approximately 85 $\mu$W. The second-harmonic
generation efficiency up to this value is in excellent agreement with a
previously known theoretical framework. This framework is extended here,
showing that the onset of cascaded $\chi^{(2)}$ processes and the maximum of
the second-harmonic generation efficiency coincide. Furthermore, we observe
that the frequency distance between the comb lines is a function of the pump
power. It changes from 4 free spectral ranges at the oscillation threshold to 1
free spectral range at 590 ${\mu}$W.
