[time-nuts] Re: pulling some crystals

[Bob kb8tq]

Hi

Having designed / built / shipped a few (million) OCXO’s over the years ….

1) Every one of them got an aging test. There *never* was an assumption made based on this or that crystal process. Some percentage always got rejected. Yes, this bothered the accountants quite a bit :) :). If there had been a way to go with some sort of 3 day “crystal check”, the savings would have been in the millions of dollars. 

2) Crystal aging and its modeling is a “fun” field to dig into. There are a lot of things that can make it difficult. The equations you see in various papers are useful. Often the data they use to demonstrate the fit is from runs that are not quite in the “typical” range …. errr ….  One typical gotcha is fitting an initial “retrace” sort of curve (which damps out in hours / days) and calling that aging. Another gotcha is that the worse the crystal, the better it will fit this or that curve ( = there is one dominant effect driving things). 

3) Ferrite core inductors can (and sometimes do) dominate the aging, even of a VHF design. More than any other part, coils pop up as “hard to manage” items. The first OCXO I ever worked on was a redesign to eliminate a ferrite part.

4) Back in the day (1930’s maybe ….) the theory was that aging doubled every time you bumped the temperature by 10C. Obviously this would make an OCXO at 90C horrible compared to one at 0C. Folks did to the experiment in the 1950’s (and possibly before that). They saw no advantage aging wise and a lot of issues thermal design wise. Yes, this was done with OCXO crystals.

5) The packaging and processing for an OCXO crystal is very different than what is done for something targeted at a room temperature MCU sort of application. There *is* a reason you pay more for an OCXO crystal. 

6) Some level of burn in is a good idea for any OCXO. This includes ones that simply have been on the shelf for a couple years. It is not at all uncommon to just put a part on power for a couple of weeks before you test or use it.

Aging wise, what to do:

One option is to set up something like HP used. Very fancy, very nicely done, lots and lots of data collected. One would guess pretty darn expensive to implement.

The opposite extreme is to just put them on a shelf and wheel up a frequency counter every so often. Is that once a day 5 days a week? Maybe. Take a month (or two) of data and look at it. Folks did aging checks this way “back then”. It’s not ideal, but it is *way* better than nothing at all.

Fun !!!!

Bob



> On Dec 8, 2023, at 11:18 PM, glen english LIST via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> yeah I certainly dont buy the 3 days at 105C based on my own research....  These are AT , by the way, FWIW.
> 
> Rick your point about not observing aging different for a 105C versus an 80C xtal is telling. Today, it is hard to get xtal mfrs interested in the fussy crystals I want and only perhaps 100-200 per annum.  I am beginning to think I need to make a large automated test jig, or go about this a different way. Having a high PLL reference PD frequency is essential - 49MHz  or pref 98 MHz .
> 
> These dont have to be super stable over temperature-  since I disclipine that oscillator to an off-the-shelf cheap arse 10 MHz chip TCXO  that costs $5  (1ppm is plenty ! ) .
> 
> *** Tweaking temperature to tweak frequency might just be useful. Of course, I am going to have to think about this, as the low end will be where the equipment temp can get to...
> 
> Looking at the temperature frequency curves for different angles cuts for 3OT crystals, for example, and my understanding of this stuff is rudimentary compared to a crystal expert :  A 0 second cut yields a 23 ppm from 25 deg C to 87 deg C. at 70 deg C, in this graph this is +8ppm, IE the crystal might need to be cal-ed for 8ppm low.
> 
> DO you feel this might be an avenue to persue ? I certainly need to get some characteristic data first into my spreadsheets before I go off on this.
> 
> -glen
> 
> 
> On 9/12/2023 3:03 pm, Richard (Rick) Karlquist wrote:
>> 
>> 
>> On 12/7/2023 12:07 PM, glen english LIST wrote:
>>> 
>>> All our crystals have undergo the process of preaging, also called "accelerated aging" (i.e. 3 days storage at +105 deg.C). This aging is equivalent to one year aging at normal temperature (according standard IEC 60122-1). The crystals produced for you had frequency shift after this preaging below 1.0 ppm, so it guaranties aging below 10 ppm after 10 years. The aging curve shows deceleration (saturation) in time.
>>> Information to pulling parameter:
>> 
>> This kind of discussion about "guaranteed aging" is completely at odds
>> with everything I observed over many years working for HP.  The top
>> HP experts on crystals never talked about crystals being so predictable.
>> These experts were involved in inventing the SC cut, etc.
>> They taught the rest of the industry how to make crystals.
>> I personally observed many crystals aging vs time and vs temperature
>> trying to "sort" out the good ones.  Crystals would be good for while, then for no reason might drift in the opposite direction. I am
>> especially skeptical of "3 days at 105 deg C is worth 10 years of
>> aging."  The E1938A oscillator had an oven set point of around
>> 105 degrees C.  It did not accelerate the aging compared to an
>> 80 degree set point and certainly didn't accelerate it following
>> a hockey stick curve.
>> 
>> Rick
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