  The remarkable frequency stability of resonant systems in the optical domain
(optical cavities and atomic transitions) can be harnessed at frequency scales
accessible by electronics using optical frequency division. This capability is
revolutionizing technologies spanning time keeping to high-performance
electrical signal sources. A version of the technique called 2-point optical
frequency division (2P-OFD) is proving advantageous for application to
high-performance signal sources. In 2P-OFD, an optical cavity anchors two
spectral endpoints defined by lines of a frequency comb. The comb need not be
self-referenced, which greatly simplifies the system architecture and reduces
power requirements. Here, a 2P-OFD microwave signal source is demonstrated with
record-low phase noise using a microcomb. Key to this advance is a spectral
endpoint defined by a frequency agile single-mode dispersive wave that is
emitted by the microcomb soliton. Moreover, the system frequency reference is a
compact all-solid-state optical cavity with a record Q-factor. The results
advance integrable microcomb-based signal sources into the performance realm of
much larger microwave sources.
