[time-nuts] Designing an embedded precision GPS time

Bob kb8tq kb8tq at n1k.org
Wed Nov 1 09:01:35 EDT 2017


Unfortunately not all TCXO’s are created equal. It depends a bit on the 
original intended use. I’d bet it also depends a bit on the original target
price. Perturbations  (frequency jumps) over temperature are one “feature”
that might be present. Hysteresis at half the temperature spec is another

Even within the same batch or same test run, some will be much better 
than others. You stop the compensation process when they get “good enough”. 
That will mean that a few are right at whatever the production target is and
others exceed the target by quite a bit. 

While crystal curves are indeed cubic, there are higher order terms in the
curve. The “why” is something people get to write papers on.If you are trying
to compensate to tight specs, you will see all sorts of stuff. It is not at all uncommon
to see >9th order curves residual curves. Indeed some of that is from residuals
in the compensation circuit as well as from the crystal. 

Why all this yack? A lot of people come from a background using OCXO’s. An
OCXO generally has a low order temperature characteristic. It also is rare to see
things like frequency perturbations in an OCXO. Moving from one to the other
can be a bit interesting. 


> On Oct 31, 2017, at 10:42 PM, jimlux <jimlux at earthlink.net> wrote:
> On 10/31/17 1:47 PM, Bob kb8tq wrote:
>> HI
>> TCXO is a very loosely defined term. A part that does +/- 5 ppm -40 to +85C
>> is a TCXO. A part that does +/- 5x10^-9 over 0 to 50C may also be a TCXO.
>> Dividing the total deviation of either one by the temperature range to come
>> up with a “delta frequency per degree” number would be a mistake. You
>> would get a number that is much better than the real part exhibits.
>> Working all this back into a holdover spec in an unknown temperature
>> environment is not at all easy.
> Very much so - most of the TCXO curves I've seen tend to be "much" better than the spec over the central part of the frequency range (which makes sense, the underlying crystal is a cubic with temp, most likely)
> Retrace and hysteresis might be your dominant uncertainty.
> I've attached a typical TCXO data plot for your viewing pleasure..
> (that's an expensive oscillator, because it's for space, but I don't think space or not changes the underlying performance)
>> Bob
> <TCXODataVectron 47.pdf>_______________________________________________
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