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In this paper we report on the implementation and stability analysis of adrift-compensated frequency synthesizer from a cryogenic sapphire oscillator(CSO) designed for a Cs/Rb atomic fountain clock. The synthesizer has twomicrowave outputs of 7 GHz and 9 GHz for Rb and Cs atom interrogation,respectively. The short-term stability of these microwave signals, measuredusing an optical frequency comb locked to an ultra-stable laser, is better than$5\times10^{-15}$ at an averaging time of 1 s. We demonstrate that theshort-term stability of the synthesizer is lower than the quantum projectionnoise limit of the Cs fountain clock, KRISS-F1(Cs) by measuring the short-termstability of the fountain with varying trapped atom number. The stability ofthe fountain at 1-s averaging time reaches $2.5\times10^{-14}$ at the highestatom number in the experiment when the synthesizer is used as an interrogationoscillator of the fountain. In order to compensate the frequency drift of theCSO, the output frequency of a waveform generator in the synthesis chain isramped linearly. By doing this, the stability of the synthesizer at an averagetime of one hour reaches a level of $10^{-16}$ which is measured with thefountain clock.

Drift-compensated Low-noise Frequency Synthesis Based on a cryoCSO for the KRISS-F1