  Optical frequency combs are revolutionising modern time and frequency
metrology. In the past years, their range of applications has increased
substantially, driven by their miniaturisation through microresonator-based
solutions. The combs in such devices are typically generated using the
third-order $\chi^{(3)}$-nonlinearity of the resonator material. An alternative
approach is making use of second-order $\chi^{(2)}$-nonlinearities. While the
idea of generating combs this way has been around for almost two decades, so
far only few demonstrations are known, based either on bulky bow-tie cavities
or on relatively low-$Q$ waveguide resonators. Here, we present the first such
comb that is based on a millimetre-sized microresonator made of lithium
niobate, that allows for cascaded second-order nonlinearities. This
proof-of-concept device comes already with pump thresholds as small as 2 mW,
generating repetition-rate-locked combs around 1064 nm and 532 nm. From the
nonlinear dynamics point of view, the observed combs correspond to the Turing
roll patterns.
