Advances in the NIM Cs fountain clocks

Two Cs fountains clocks have been built in NIM. NIM5 fountain clock has been reported to BIPM since 2 014. The microwave synthesi%er 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. The new NIM6 fountain clock has been built and evaluated, the preliminary results are presented in the paper. With an ultrastable microwave signal generated from an optical comb which locked to an ultra-stable laser, the frequency instability of NIM6 fountain clock reached 5×10
^-14
/τ
^1/2
at high density. The direct comparison of two fountain clocks has also been done, a relative frequency difference of 4.4×10
^-16
was obtained for 2 0 days averaging time, consistent with the total uncertainties of the two clocks.