[time-nuts] 10811 crystal orientation

Magnus Danielson magnus at rubidium.dyndns.org
Sun Jul 12 10:07:38 UTC 2009


Said and Björn,

SAIDJACK at aol.com wrote:
> 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.

It could work for low-frequency compensation, as it would take a bit 
more analysis to figure out the impulse response needing of 
compensation/equalisation.

> 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  :)

But hey, we are time-nuts, arn't we? :)

> 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.

Which is scaled by the g-sensitivity of the crystal. If you have a 
g-sensitivity of 2E-6 of the crystal and then compensate that with a 
noise of 2E-9 you would end up with a 4E-15 noise contribution. Right?

Now, that was assuming white-noise... the 1/f noise sources needs to be 
estimated for real performance, as they behave differently in ADEV.

> 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!

Hence the military and related industry have worked alot on figuring out 
how to handle it. Using two crystal blanks in opposite direction to 
subtract the vector-field of the g-sensitivity is one way. The patents 
for that is available.

> One caveat for the artillery shell: commercial GPS would likely not  work 
> due to the 1000 Knots verlocity limit.

Not an issue for military receivers, but they need to be built for it 
not to break at the moment of fire rather than the moment of impact.

Cheers,
Magnus



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