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 <[email protected]> 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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