Ultra-Low Vibration Pulse-Tube Cryocooler Stabilized Cryogenic Sapphire Oscillator With ${\hbox{10}}^{-16}$ Fractional Frequency Stability

A low maintenance long-term operational cryogenic sapphire oscillator has been implemented at 11.2 GHz using an ultra-low-vibration cryostat and pulse-tube cryocooler. It is currently the world's most stable microwave oscillator employing a cryocooler. Its performance is explained in terms of temperature and frequency stability. The phase noise and the Allan deviation of frequency fluctuations have been evaluated by comparing it to an ultra-stable liquid-helium cooled cryogenic sapphire oscillator in the same laboratory. Assuming both contribute equally, the Allan deviation evaluated for the cryocooled oscillator is σ
y
≈ 1 × 10
^-15
τ
^-1/2
for integration times 1 <; τ <; 10 s with a minimum σ
y
= 3.9 × 10
^-16
at τ = 20 s. The long term frequency drift is less than 5×10
^-14
/day. From the measured power spectral density of phase fluctuations, the single-sideband phase noise can be represented by
L
_φ
(
f
) = 10
^-14.0
/
f
⁴
+10
^-11.6
/
f
³
+10
^-10.0
/
f
²
+10
^-10.2
/
f
+ 10
^-11.0
rad
²
/Hz for Fourier frequencies 10
^-3
<;
f
<; 10
³
Hz in the single oscillator. As a result,
L
_φ
≈ -97.5 dBc/Hz at 1-Hz offset from the carrier.