On Wed, 25 Nov 2015 12:35:44 -0800
Hal Murray <hmur...@megapathdsl.net> wrote:
> If you poke around on the web you can probably find a graph of frequency vs
> temperature, maybe even for that particular vendor/model of crystal if you
> can read the label. Beware: the graph for a 32 KHz crystal is probably much
> different than a high frequency (10 MHz) crystal.
Yes, 32kHz crystals are usually XT cut tuning forks and exhibit a
negative quadratic detuning from the inflection point (usually 25°C)
Because it's quadratic and does not have the nice cubic form of AT cuts,
this can get quite big. E.g. if we use the Abracon ABS07[1] an example
for a typical speciem that you might use in such an application
(i.e. not one of the cheap china crystals with an initial offset of >200ppm)
you see that it has a typical temp coefficient of -0.036ppm/°C^2 relative
to the infection point. Let's say we have a device that is used indoors
in some place that is more or less well ventilated, then we can expect
a maximum temperature of 45°C. This results in an frequency offset of
14.4ppm. Which is already above the limit of 1ns/7us=14.2ppm.
And that's best case. It does not account yet for the inflection point
being off (+/-5°C variation, which give another 8.1ppm) or the coefficient
being higher (can go up to -0.040 -> +1.6ppm).
So already with a relatively good 32kHz crystal and very moderate
temperature requirements, the crystal change its frequency by 25ppm
and thus by 1.75ns @ 70us.
There is reason why I said one needs temperature compensation for
this kind of stability requirement ;-)
Attila Kinali
[1] http://www.abracon.com/Resonators/ABS07.pdf
--
Reading can seriously damage your ignorance.
-- unknown
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