Improvement of the first CS fountain clock, FO1

To date atomic fountain clocks using laser cooled Cs atoms prove to be the most accurate frequency standards. Among the three set-ups at BNM-SYRTE, the historically first Cs fountain clock, FO1, has recently undergone substantial changes. The cold atom preparation and selection have been rebuilt. Selection of the F = 3, m_F = 0 state can be carried out by adiabatic passage allowing the preparation of 100% or 50% of the atom density with a precision of 10^-3. Thereby, we can quantify the collisional frequency shift at a level of 10^-16. Furthermore, a cryogenic sapphire oscillator is used for the synthesis of the microwave interrogation signal. With these improvements FO1 is currently operating at a stability of 2.8 x 10^-14s^-1/2 and an accuracy of a few 10^-16.