  Optical frequency division relies on optical frequency combs to coherently
translate ultra-stable optical frequency references to the microwave domain.
This technology has enabled microwave synthesis with ultralow timing noise, but
the required instruments are too bulky for real-world applications. Here, we
develop a compact optical frequency division system using microresonator-based
frequency references and comb generators. The soliton microcomb formed in an
integrated Si$_3$N$_4$ microresonator is stabilized to two lasers referenced to
an ultrahigh-$Q$ MgF$_2$ microresonator. Photodetection of the soliton pulse
train produces 25 GHz microwaves with absolute phase noise of -141 dBc/Hz (547
zs Hz$^{-1/2}$) at 10 kHz offset frequency. The synthesized microwaves are
tested as local oscillators in jammed communication channels, resulting in
improved fidelity compared with those derived from electronic oscillators. Our
work demonstrates unprecedented coherence in miniature microwave oscillators,
providing key building blocks for next-generation timekeeping, navigation, and
satellite communication systems.
