[time-nuts] GPS: ADEV or MDEV?
SAIDJACK at aol.com
SAIDJACK at aol.com
Mon Apr 23 17:51:33 UTC 2007
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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