Hello Bjoern,
that would work well for static acceleration (tilt) but for vibration
resistance the crystal must be low-g, or complexly compensated with
wide
loop
bandwidths such as the FEI papers describe.
Initial Calibration would also be tricky, and having an algorithm to
measure one result (frequency) against five inputs (aging, tempco,
X, Y, Z
acceleration) and more (crystal jumps, retrace) is also quite
sophisticated :)
Also, Mems, or other accelerometers have inherent noise, and to
compensate a crystal that has say +/-2E-09 per g sensitivity means
one
would have to
add up to +/-2E-09 in offset statically. That's a lot of deviation,
and
any noise from the mems would find its way into the Allan
Variance/phase-noise.
For vibration compensation, the compensation could easily go up to
+/-1.2E-08 and more (for up to +/-6G vibration to be canceled).
Very interesting topic, and I would love to hear what folks think
about
this, or have come up with in terms of solutions.
At the high-end of the spectrum of the technology is the gun-barrel
launched artillery shell with crystal oscillator built-in, that has
to
withstand
and operate with 10,000 to 20,000 g acceleration!
One caveat for the artillery shell: commercial GPS would likely
not work
due to the 1000 Knots verlocity limit.
bye,
Said
In a message dated 7/10/2009 16:53:23 Pacific Daylight Time,
b...@lysator.liu.se writes:
Hi Said & Tom,
The below url links some "low-g"-osc papers.
http://www.freqelec.com/tech_lit.html
Said, did you contemplate adding a cheap 3d-accelerometer and try to
teach
your holdover algorithms use the accelerometer measurements in the
same
way as your temperature measurements?
--
Björn
Hello Tom,
this plot looks very similar to our standard double oven units. We
have our low-g option, which reduces the deviation to about 2-
3E-10
per g, they work great but do cost more than standard units..
Coincidentally they also reduce sensitivity to vibration and
"tapping"
by 5x to 10x... I wish we could offer them at the same price, but
they
are very difficult to manufacture. That's why no one uses them by
default in their product.
Bye, Said
From iPhone
On Jul 10, 2009, at 15:51, "Tom Van Baak" <t...@leapsecond.com>
wrote:
One is do crystal oscillators change frequency when they
are turned. The answer to that is yes. This gravitational
acceleration effect is rather huge, parts in ten to the 9th
or so, and anyone can see this. This is why you never
touch, bump, or move, or rotate a laboratory frequency
standard (this includes GPSDO and cesium standards).
And to give you a *picture* instead of just numbers... Here is
a plot showing frequency changes in an OCXO (this from a
free-running Thunderbolt GPSDO) over the span of one hour.
Every 5 minutes or so I rotated the rectangular box on some
axis by 90 degrees.
<http://www.leapsecond.com/pages/ocxo-2g/TBolt-2g-6axis.gif>
You can see that the sudden frequency jumps due to change
in g-force on the crystal are about -0.5e-9 to +1.5 e-9, which
is 100x the normal frequency noise for this oscillator (about
2e-11 pk-pk or about 2e-12 adev).
Hopefully this result won't come as a big surprise to anyone; the
so-called "2g turn-over" spec is common for quality oscillators.
Again, this is why when you enter the world of precision timing
at 1e-10 and below you tend not to ever touch your standards.
Now if one of you happened to have a fully-programmable 3-axis
turntable and a couple of hours you could slowly create a most
beautiful high-resolution 3D color plot showing the precise shift
in frequency as a function of axis.
/tvb
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