[time-nuts] What’s the BEST crystal?

[Bob kb8tq]

Hi

> On Feb 29, 2020, at 3:07 PM, Attila Kinali <attila at kinali.ch> wrote:
> 
> 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.

The FE-405 crystal is in a package that is already a bit “tight” for a 3rd overtone
at 5 MHz. You can *force* just about anything. The point is to make it at a 
thickness to diameter ratio that is more on the order of what is used at 
10 MHz. That makes it a *very* large diameter blank (at least out of
high performance quartz).

Bob

> 
> 
> 			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
> 
> -- 
> <JaberWorky>	The bad part of Zurich is where the degenerates
>                throw DARK chocolate at you.
> 
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