[time-nuts] Designing an embedded precision GPS time

jimlux jimlux at earthlink.net
Wed Nov 1 09:17:31 EDT 2017

On 11/1/17 6:01 AM, Bob kb8tq wrote:
> Hi
> 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
> “feature”.
> 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.

Indeed - I was looking at algorithmically compensating some cheap TCXOs 
and there's an amazing spread in the "details" of the curves - sure, 
they all met the spec (several ppm, as I recall), but it was clear after 
very little testing that there was no "one algorithm to fit them all"

As you say, good grist for a paper or thesis project.

That's why I wish they'd sell OCXOs, cheap, without the oven. Or maybe 
look for regular XO (no TC).  Those might have a more "pure" (read lower 
order) freq vs temp characteristic.

The problem I see with regular XO is that they tend to be designed to a 
cost point and there might be more of the hysteresis and mechanical 
effects - if you're not claiming better than 100ppm, then 50 ppm of 
hysteresis isn't a problem.

A 1ppb OCXO is going to have to be a better mechanical design - so that 
it can hit that 1ppb every time when you turn the oven on and go from 
cold to hot.

Maybe this is just griping in general - why don't mass production 
manufacturers make exactly the niche part I want to buy (that is of no 
use to anyone else)for $3 each

I suppose if you were going to build little algorithmically compensated 
modules, you'd bite the bullet and design a crystal oscillator and then 
YOU get to choose what crystal in what mount etc.

When all is said and done, the production cost for a design that uses a 
crystal in a can plus half a dozen discrete devices to make an 
oscillator is probably not a lot different than the production cost for 
a design using an oscillator in a can.  it's the "other stuff" in the 
design that will add up.

> Bob
>> 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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