[time-nuts] Sapphire oscillators

[Attila Kinali]

On Thu, 10 Nov 2016 11:20:37 +1100
Jim Palfreyman <jim77742 at gmail.com> wrote:

> Anyone got any comments on this?
> 
> http://www.theleadsouthaustralia.com.au/industries/technology/worlds-most-precise-clock-set-for-commercial-countdown/

Cryogenic sapphire or whispering gallery mode oscillators have been around
for quite some time. You basically have a piece of sapphire (aluminium oxide
in crystaline form)[1] in a cavity[2,3], cool everything down to liquid
helium temperatures and use this as an oscillator. There are two popular
configurations, one is to use the sapphire as resonant element like in
an LC or crystal oscillator, or more commonly, to use the sapphire as a
filter element in Pound locking scheme[4].

The short term stability of these oscillators is AFAIK unsurpassed
and flat up to 1000-10'000s, but exhibits drift at longer taus[5].

Their biggest problem is that they need a liquid helium cryo-cooler
which causes vibrations that need to be carefully filtered out.
This also makes them relatively large (fill between one and two 19" racks)


			Attila Kinali

[1] http://www.uliss-st.com/uploads/pics/tech2.jpg
[2] http://inspirehep.net/record/1244235/files/cavity.png
[3] http://www.uliss-st.com/uploads/media/imgmedias.jpg
[4] That's the (original) microwave variant of the Pound-Drever-Hall
locking scheme, see https://en.wikipedia.org/wiki/Pound%E2%80%93Drever%E2%80%93Hall_technique
[5] http://inspirehep.net/record/1409150/plots
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