On 10/31/15 7:32 PM, Bob Camp wrote:
Hi
So … how good is the “calibrate and go” (not the tone on second channel)
approach likely to be?
If it’s a bare crystal or normal XO (not a TCXO) that is supplying the clock,
the crystal will follow some
fairly well known curves. Which one of the curves it follows will depend
entirely on the individual sample
you have. What I see on my sound card will not be what you see on a different
sample of exactly the same
sound card installed in a similar machine.
having just done a bunch of measurements over 4 days on some crystals of
this type (16 MHz, nothing special), I can shed some light on it.
over half a day sitting on my desk, they fluctuated about 0.1 ppm (peak
excursion)
AVAR is about 1E-9 at tau=10sec, rising to 2e-7 at 10,000 seconds (after
taking out the temperature dependence)
There is a strong temperature dependence.. with temperature changes of 6
degrees (C) the frequency change is on the order of 6ppm (90 Hz out of
16 MHz)
It’s a pretty good bet that your device will change by 0.1 to 1 ppm per degree
C. Yes it can be a bit better. It
can also be a whole lot worse. Your room environment is likely to move 1 to 3 C
per hour if you have the heat
or air-conditioning turned on. If you are in an unheated garage … who knows. If
you take the most likely 0.5 ppm/C tempco
and the most likely 2C change you get a “wobble” of about 1 ppm. The period of
this wobble probably will be
in the 30 to 90 minute range.
yes, that's about right..
Although my office doesn't have that kind of fluctuation..
And I ran the 4 day test with the oscillators in a cardboard box over
the weekend, so that damps short term variations.
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