[time-nuts] 10811 crystal orientation

Bob Camp lists at cq.nu
Sat Jul 11 18:53:45 UTC 2009


Hi

Most of the GPSDO stuff is made for fixed location use. In that case,  
paying for acceleration compensation doesn't make much sense.

About the only people who try to do this stuff mobile (and have the  
ability to pay) are the military.

Bob
KB8TQ


On Jul 11, 2009, at 5:55 AM, bg at lysator.liu.se wrote:

> 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,
>> bg at 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" <tvb at 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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>>
>>
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