  We report on the evaluation of microwave frequency synthesis using two
cryogenic sapphire oscillators developed at the University of Western
Australia. A down converter is used to make comparisons between microwave
clocks at different frequencies, where the synthesized signal has a stability
not significantly different from the reference oscillator. By combining the CSO
with a H-maser, a reference source of arbitrary frequency at X-band can be
synthesized with a fractional frequency stability of sub-$4 \times 10^{-15}$
for integration times between 1 s and 10,000 s.
