[time-nuts] What’s the BEST crystal?

[Attila Kinali]

On Sat, 29 Feb 2020 13:44:59 -0500
Bob kb8tq <kb8tq at n1k.org> wrote:

> One wonders what the result would be of doing a large blank (> 50mm)
>  2.5 MHz 5th OT using modern design, packaging and mounting techniques. 

If we use the venerable Sulzer as guide (see e.g. [1]) and going
by what the FE-405 achieves, I would say sub-1e-13 around 10-100s
should be possible. As far as I can tell, from the paper's I've
read, the major contributors to instability between 1s and 1000s
seem to be temperature and absorption/desorption of material on the blank.
We know how to do temperature control these days (see E1938 [2]).
But, there is not much we can do about absorption/desorption.
The best strategy is to polish the surface as much as possible
and use an as thick blank as possible. And then, backe the whole
thing in high vacuum for as long as possible at as high temperature
as possible (which isn't that high, due to twining).

The diameter of the blank has to be scaled with its thickness, in
order not to compromise the f*Q product. Which in turn makes it
a bit problematic in terms of packaging, but nothing unsolvable.

I am not sure, how much an advantage it is to go BVA. It helps in
terms of Q (seems to be 30-50% higher), but I'm not sure whether
the ab/desorption rate is much lower if there are no metal
electrodes on the crystal. The specs of the HSO-14 are awfully
close to the 8607, even though Rakkon places the electrodes
on the blank itself. But I have not seen any actual measurements
of an HS0-14 yet, so I don't know how it actually performs,
especially at tau >1000.

BTW: this is another advantage of SC cut: an AT cut is ~1.6 MHz*mm
an SC cut is ~1.8 MHz*mm, ie you get about 10% more thicknes at the
same frequency for an SC cut than for an AT cut.

> Given that it would take a rather large pile of nickels to 
> find out (like > $50M worth), I very much doubt we ever will know the answer. 

Why would it cost so much? There are enough companies that make
crystal blanks for various purposes, getting one to make large
ones shouldn't be too difficult. And the FE-405 is very close
to what you are asking for, being a 5th OT 5MHz SC cut.


			Attila Kinali


[1] https://febo.com/for_sale/sulzer/index.html

[2] "The Theory of Zero-Gradient Crystal Ovens", 1997
by Karlquist, Cutler, Ingman, Johnson and Parisek
http://www.karlquist.com/oven.pdf

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