Research on Clean-Up Technique to Improve Short-Term Stability of Rubidium Clocks

In order to improve the poor short-term stability of rubidium (Rb) clock in time transfer, this paper propose a self-adaptive dual-state control algorithm to clean up the frequency standard. Compared with control algorithms that remain in the laboratory, this algorithm has a strong engineering application background. It has the advantages of long-term stable operation, combining efficiency and simplicity. By measuring the phase difference between the Rb clock and the crystal oscillator, and calculating the predicted value to change the control voltage, a digital phase-locked loop (PLL) is formed. Based on this principle, this paper designs a digital PLL control circuit for the VCXO tracking the Rb clock. After continuous testing, the second-level stability of the Rb clock is 7.68E-13, which is improved by an order of magnitude. The enhancement greatly reduces the usage limitations of Rb clocks, offering the cost-effective of any frequency source. The “crystal oscillator+” control strategy in the paper can also be applied to short-term stability improvement of other frequency sources. When the reference signal is traced to UTC (k), the VCXO can track and replicate the standard time signal.