  We report observations of the Schawlow-Townes noise limit in a cryogenic
sapphire secondary frequency standard. The effect causes a fundamental limit to
the frequency stability, and was measured through the novel excitation of a
bimodal maser oscillation of a Whispering Gallery doublet at $12.04 GHz$. The
beat frequency of $10 kHz$ between the oscillations enabled a sensitive probe
for this measurement of fractional frequency instability of
$10^{-14}\tau^{-1/2}$ with only 0.5 $pW$ of output power.
