  We present an experimental study of optical frequency comb generation based
on cascaded quadratic nonlinearities inside a continuous-wave-pumped optical
parametric oscillator. We demonstrate comb states which produce
narrow-linewidth intermode beat note signals, and we verify the mode spacing
uniformity of the comb at the Hz level. We also show that spectral quality of
the comb can be improved by modulating the parametric gain at a frequency that
corresponds to the comb mode spacing. We have reached a high average output
power of over 4 W in the near-infrared region, at ~2 {\mu}m.
