This study presents a compact optical frequency division system using microresonator-based frequency references and comb generators to achieve zeptosecond-level timing noise in microwave oscillators. The system utilizes a soliton microcomb generated in an integrated Si3N4 microresonator, which is stabilized to two lasers referenced to an ultrahigh-Q MgF2 microresonator. The soliton pulse train is photodetected to produce 25 GHz microwaves with an absolute phase noise of -141 dBc/Hz (547 zs Hz⁻¹/²) at 10 kHz offset frequency. These synthesized microwaves are tested as local oscillators in jammed communication channels, showing improved fidelity compared to those from electronic oscillators. The system demonstrates unprecedented coherence in miniature microwave oscillators, providing key components for next-generation timekeeping, navigation, and satellite communication systems. The system uses a MgF2 microresonator as a frequency reference and a Si3N4 microresonator for soliton microcomb generation. The MgF2 microresonator has a high Q factor and low thermo-optic coefficient, providing low thermo-refractive noise. The Si3N4 microresonator is suitable for soliton microcomb generation due to its strong Kerr nonlinearity and tight optical confinement. The system achieves a noise reduction factor of 40 dB through optical frequency division, with phase noise of -123 (-141) dBc/Hz at 1 (10) kHz offset frequency. The system is tested in anti-interference experiments, showing a 20 dB advantage over electronic oscillators in signal-to-noise ratio. The system is also compared with other microwave oscillators, showing the lowest timing noise among all miniature photonic microwave oscillators. The system has potential applications in robust communication systems, high-resolution radars, and high-speed wireless communications. The study highlights the importance of frequency references in reducing absolute timing noise and achieving high performance in microwave oscillators.This study presents a compact optical frequency division system using microresonator-based frequency references and comb generators to achieve zeptosecond-level timing noise in microwave oscillators. The system utilizes a soliton microcomb generated in an integrated Si3N4 microresonator, which is stabilized to two lasers referenced to an ultrahigh-Q MgF2 microresonator. The soliton pulse train is photodetected to produce 25 GHz microwaves with an absolute phase noise of -141 dBc/Hz (547 zs Hz⁻¹/²) at 10 kHz offset frequency. These synthesized microwaves are tested as local oscillators in jammed communication channels, showing improved fidelity compared to those from electronic oscillators. The system demonstrates unprecedented coherence in miniature microwave oscillators, providing key components for next-generation timekeeping, navigation, and satellite communication systems. The system uses a MgF2 microresonator as a frequency reference and a Si3N4 microresonator for soliton microcomb generation. The MgF2 microresonator has a high Q factor and low thermo-optic coefficient, providing low thermo-refractive noise. The Si3N4 microresonator is suitable for soliton microcomb generation due to its strong Kerr nonlinearity and tight optical confinement. The system achieves a noise reduction factor of 40 dB through optical frequency division, with phase noise of -123 (-141) dBc/Hz at 1 (10) kHz offset frequency. The system is tested in anti-interference experiments, showing a 20 dB advantage over electronic oscillators in signal-to-noise ratio. The system is also compared with other microwave oscillators, showing the lowest timing noise among all miniature photonic microwave oscillators. The system has potential applications in robust communication systems, high-resolution radars, and high-speed wireless communications. The study highlights the importance of frequency references in reducing absolute timing noise and achieving high performance in microwave oscillators.