An accurate timescale is the backbone of the time-frequency system. In recent years, with the development of optical clocks, a hybrid microwave-optical timescale has been proposed. In this “optical clock steering a microwave flywheel oscillator” architecture, the noise of the microwave clock, typically a hydrogen maser, limits the performance. In this paper, we present a sympathetic-laser-cooled ion microwave frequency standard scheme that enables a longer Ramsey interrogation time than the fountain clock and eliminates the dead time induced by the laser-cooling process, thereby overcoming the limitation imposed by the Dick effect. The optimized configuration of a ${\mathrm{Yb}}^{+}-{\mathrm{Cd}}^{+}$ two-component Coulomb crystal suppresses ion loss, which degrades long-term frequency stability in the past. Using a hydrogen maser as reference for the microwave synthesizer chain, the frequency stability expressed in terms of Allan deviation is measured to be $2.0\times {10}^{-13}/\sqrt{\tau }$, better than that of the laser-cooled mercury ion microwave frequency standard, and reaches $4.0\times {10}^{-15}$ after continuously operating for 85 000 s. The systematic uncertainty is evaluated to be $4.4\times {10}^{-15}$. We identify the residual magnetic-field fluctuation as the dominant limiting factor and discuss that the sympathetic-laser-cooled ion microwave clock promises to achieve a stability of $2.0\times {10}^{-14}/\sqrt{\tau }$ and accuracy at ${10}^{-16}$ level. Its excellent short-and-medium frequency stability is possible to develop a reliable flywheel oscillator or bridge the performance gap between the hydrogen maser and optical clock, therefore breaking through the limitation on local timescale from the flywheel.

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• Atomic, optical & lattice clocks
• Cooling & trapping
• Ions