  High-stability quartz oscillators are needed in a number of space
applications. A short-term stability of parts in 10^{-14} [Allan deviation
\sigma y(\tau) ] is sometimes required, for integration time \tau of
approximately 1-10 s. The Centre National d'Etudes Spatiales (CNES) and the
FEMTO-ST Institute (formerly LPMO and LCEP), have been collaborating for many
years in this domain, aiming at measuring and at understanding the oscillator
noise. The highest stability has been observed on 5 MHz and 10 MHz bulk
acoustic-wave resonators. Yet this stability is still not sufficient, or the
the manufacturing method is not reproducible. Recently, the analysis of a few
premium-stability oscillators has demonstrated that the oscillator frequency
instability is due to the fluctuation of the resonator natural frequency,
rather than to the noise of the sustaining amplifier via the Leeson effect. It
is therefore natural to give attention to the measurement of the resonator
fluctuations.
