[time-nuts] time-nuts Digest, Vol 180, Issue 22

[Bob kb8tq]

Hi

Here’s the gotcha with what they are talking about. The SC when done as a 
*fundamental* crystal has a higher Q than the AT. You don’t use fundamental
mode SC’s in a normal OCXO. 

The issue with Q relates very specifically to the sort of HC-40 package AT resonators
you *would* use for maximum Q in a 5 or 10 MHz OCXO. Practical package
size for the resonators is as much a part of it as anything else. ( = that’s about as
big a package as anybody is tooled to do an OCXO crystal in ) 

Bob

> On Jul 12, 2019, at 4:24 AM, Leo Bodnar <leo at leobodnar.com> wrote:
> 
> From: Bob kb8tq <kb8tq at n1k.org>
>>>>> - high-Q crystals require SC-cut 
>>>> ... An SC has a lower Q than an AT of similar size
>>>> and design up to the point acoustic Q losses completely take over. 
>>>> If you are talking about sub 20 MHz OCXO?s with ?doable? crystal 
>>>> package sizes, the AT will have the higher Q by a significant margin. 
>>> 
>>> Could you please back up this claim with verifiable facts?
>> 
>> Order up a few 5 MHz 3rd overtones in HC-40 packages and see what you get.
>> You also could send in an RFQ for a batch of each to any of the people who make them
>> and see what comes back. 
>> Bob
> 
> Here is a random selection of links to back my point of view that, if you have noticed, contradicts Bob's.
> If anybody is interested they will find information themselves without much effort.  I suggest printed books if you don't trust Internet at large.
> 
> http://www.crovencrystals.com/croven_pdf/Old%20Spec%20Sheets/croven_catalogue.pdf (Croven Crystals is Wenzel company)
> The main advantages of these resonators, and in particular the SC-cut type are:
> higher Q-factor (typically 10 - 15% better than equivalent AT-cut resonators)
> 
> https://www.tfc.co.uk/pdfs/SC_cut_crystals_article_TFC.pdf
> SC cut family of quartz crystals:
> Other key characteristics include Higher Q factor
> 
> https://www.rfwireless-world.com/Terminology/AT-cut-vs-SC-cut-quartz-crystal.html
> Specification:Q factor
> AT cut: lower	
> SC cut: higher (it will achieve low phase noise) 
> 
> http://members.femto-st.fr/sites/femto-st.fr.patrice-salzenstein/files/content/Peer-review-journal/smdo160017.pdf
> SC-cut
> It has faster higher Q, warm-up speed and better phase noise close to the carrier.
> 
> http://www.resonal.com/Downloads/John%20R.%20Vig%20-%20tutorial%20on%20Quartz%20Crystals%20and%20Oscillators.pdf
> Advantages of the SC-cut:
> Higher Q for fundamental mode resonators of similar geometry
> 
> http://www.mtronpti.com/sites/default/files/files/crystal-resonator-terminology.pdf
> A typical 10 MHz, 3rdovertone SC may have a Q of 1.0 to 1.3 million;
> a 100 MHz, 5th overtone AT may have a Q of 80 to 100 thousand,
> while a 100 MHz AT fundamental would be much lower, in the range of 20 to 50 thousand.
> 
> https://www.eetimes.com/document.asp?doc_id=1290592# (MptronPTI)
> Since SC-cuts have a much higher Q-factor than AT-cuts, SC-based OCXOs offer better noise performance from 1-Hz offset to 1,000-Hz offset.
> 
> Worryingly, I have started receiving unpleasant personal emails from list members suggesting that I do not question factual correctness of other's opinions.
> This will explain why I am going off the list for the sake of everyone's good.
> 
> Leo Bodnar
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