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

Microresonator-referenced soliton microcombs with zeptosecond-level timing noise