[time-nuts] Crystal sweet-spot (was: Best frequency to start for GHz synth ?)

[Attila Kinali]

On Wed, 31 Mar 2021 16:44:40 +0000
"Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:

> It used to be that 5MHz was the "hot spot" for crystals on the 
> parameters we care about as time-nuts.

Depends on what kind of time-nut you are ;-)

If you are going for high frequencies beyond 1GHz, then a mesa
type high frequency BAW is the best you can do (given you don't
want to use frequency comb to divide down a cryogenic silicon
cavity).

As Bob wrote, for low-frequency, high stability applications, 
the lower the frequency of the crystal the better. Or rather,
the thicker the crystal the better. I.e. you want to use an
as low frequency crystal with an as high as possible overtone.
Unfortunately, to make full use of the properties of low
frequency crystals, you need to scale the diameter with the
frequency. Otherwise, the energy loss due to the edges of the
crytal will limit the Q.

For historical reasons, 3rd overtone 5MHz turned out to be the
lowest that could be done economically with the avaible tools
and methods and still fit the size constraints.

Today we could probaly go lower, but the market demands for
large crystal units is shrinking steadily and, as Bob wrote
a few times in the past, nobody has the tooling to do so.

			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