Fountain clocks comparisons in Beijing area

NIM has built three cesium fountain clocks in Beijing area. NIM5 fountain clock has been reported to BIPM since 2014. The microwave synthesizer and computer control system of NIM5 had been modified recently. The type-B uncertainty is 9×10−16, limited by the interferometric switch in the microwave chain. NIM5-M has been sent to 32 km away from NIM and compared with NIM5 via fiber link. The new NIM6 fountain clock has been built and evaluated, the results are presented in the paper. The direct comparison of NIM5 and NIM6 has also been done, a relative frequency difference of 4.4×10−16 was obtained for 20 days averaging time, consistent with the total uncertainty of the two clocks.