[time-nuts] GPS: ADEV or MDEV?

[SAIDJACK at aol.com]

In a message dated 4/23/2007 06:05:12 Pacific Daylight Time,  
tvb at LeapSecond.com writes:

>-6  ppm / 1000 hours (= 2e-12/d)
>-2 ppm / 1000 hours (= 6e-13/d)
>-2  ppm / 3000 hours (= 2e-13/d)
>+6 ppm / 5 months (= 5e-13/d)
>+10  ppm / 12 months (= 3e-13/d)
>-1 ppm / 10 days (= 1e-12/d)
>-1.5  ppm / 30 days (= 6e-13/d)

>These all fall into the range of 2 to 20  e-13 / day drift. It
>confirms my hunch then that Vref drift is a  non-issue for
>a GPSDO.


Hi Tom,
 
I agree. What makes life even better is that these are probably also  
worst-case numbers from the datasheets. Typical numbers can be better :) Also,  drift 
typically does slow over long time frames.
 
What is an issue is the tempcos of the reference, the PCB (thermocouple  
effects on the leads etc), the DAC, and the low pass filter.
 
Active DAC low pass filters add temperature dependent offsets, as well as  
their own noise, drift, and PSRR's etc.
 
BTW: the retrace hysterisis of typical very high-quality OCXO's is speced  at 
1E-08 (24 hours off, 2 hours on) that's probably a lot worse than the voltage 
 reference zener retrace will be.
 
A typical 20ppm Hysterisis on a 5V Zener with the OCXO having a +-22Hz  
(0-5V) adjustment range gives 4.4E-011 retrace. Several orders of  magnitude better 
than these particular OCXO's inherent retrace by  itself. 
 
bye,
Said
 



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