Fig. 1. (a) Simple schematic diagram of our AFC. The frequency output of the OCXO is transferred to the microwave synthesizer and the synthesizer provides the interrogation frequency for the atomic fountain, yielding a frequency error signal with the electronic control system locking the OCXO to the resonance spectrum of the fountain; simultaneously, the OCXO frequency output is compared with a local H-maser. ADC: analog to digital converter. (b) The short-term frequency stability of the locked LO with respect to the H-maser, with the fitting curve of 2.7×10−13τ−1/2. It reaches 2.7×10−15 at the average time of 10000 s, and arrives at 1.8×10−15 at the average time of 40000 s.

Fig. 2. (a) Equivalent fractional frequency drift of LO in free running during MJD 57080-57103, linear drift rate in fitting: −1.01×10−16/s; (b) RFD scheme diagram of LO in a linear drift. y1¯ is the average of the fractional frequency output of LO at the start of the first Ramsey interaction, TL is the fountain cycle of cold atoms, ΔT is the time before the Ramsey interaction in one fountain cycle, TI is the Ramsey interaction time, while the areas with deep color represent two Ramsey interactions during two successive fountain cycles.

Fig. 3. (a) Recorded fractional frequency error signal ye; (b) evaluation result of the bias and uncertainty of RFD: −3(2)×10−16, with the average time of a day; (c) the Allan deviation of the RFD. Red square: with the sectionalized post-process method; black diamond: original data without the sectionalized post-process method. In the sectionalized method, the data are divided into the same time scale of a day. All data in Fig. 3 are generated in LOL during MJD 57080-57103.