[time-nuts] non-cryogenic sapphire oscillators (was: Best frequency to start for GHz synth ?)

[Attila Kinali]

On Thu, 01 Apr 2021 11:06:43 -0500
Chris Caudle <6807.chris at pop.powweb.com> wrote:

> What kind of oven temperature range?  I thought sapphire oscillator was 
> pretty much synonymous with "cryogenic sapphire oscillator."  I found a 
> paper which described sapphire as a "low loss material with loss tangent 
> of 5×10^−6 at room temperature, 2×10^−8 at 77 K and 7×10^−10 at 4 K 
> giving Q-values of more than >10^7 at low temperatures."

There are three kinds of sapphire oscillators: cryogenic ones
that work around 4-10K, "high" temperature ones that work
around 80K (liquid nitrogen) and room temperature.

There are two reasons for going to low temperatures:
the loses of sapphire decrease with decreasing temperature
and with that the Q increases. At 4K one can engineer the
sapphire crystal with Cr doping to have a zero temperature
coefficient at the right temperature. For higher temperature
sapphire oscillators, it is possible to engineer the temperature
coefficient by splitting the resonator into two halves and
inserting a spacer with the right temperature coefficient.
Unfortunately, this lowers the Q of the resonator even further,
as now part of the field is in free air/vacuum instead of 
being confined into the crystal.

			Attila Kinali

-- 
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always 
wonder about. And that's why we do research.
		-- Kobayashi Makoto