Hi Said, What GPSDO-products do compensate for tilt?
It seem like a major error source -- if the user for some reason want to tilt a unit in holdover. It seems to be a "low hanging fruit" to attenuate this error substantially even with a $2 MEMS accelerometer. Once the ambitions grow -- more complexities can be added. Then again, is there a use-case giving some hope the engineering costs can be regained. -- Björn > 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, > [email protected] 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" <[email protected]> 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 >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
