  We describe an optical frequency stabilization scheme of a microwave
oscillator that is used for the interrogation of primary caesium fountain
clocks. Because of its superior phase noise properties, this scheme, which is
based on an ultrastable laser and a femtosecond laser frequency comb, overcomes
the frequency instability limitations of fountain clocks given by the
previously utilized quartz-oscillator-based frequency synthesis. The presented
scheme combines the transfer of the short-term frequency instability of an
optical cavity and the long-term frequency instability of a hydrogen maser to
the microwave oscillator and is designed to provide continuous long-term
operation for extended measurement periods of several weeks. The utilization of
the twofold stabilization scheme on the one hand ensures the referencing of the
fountain frequency to the hydrogen maser frequency and on the other hand
results in a phase noise level of the fountain interrogation signal, which
enables fountain frequency instabilities at the $2.5 \times 10^{-14} (\tau
/\mathrm{s})^{-1/2}$ level which are quantum projection noise limited.
