[time-nuts] GPS: ADEV or MDEV?

Tom Van Baak tvb at LeapSecond.com
Mon Apr 23 09:02:57 EDT 2007


> Tom
> 
> Attached images are drift plots for:
> 
> 1) VRE430 reference utilising a buried zener.
> 2) LM199
> 3) LTZ1000
> 
> Bruce

OK, thanks very much. What I read from the slopes in
those long-term plots are Vref drift rates on the order of:

-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.

> Tom
> 
> Tempco (if the reference isnt in a temperature stabilised environment) 
> and noise can be much more important if the drift is low enough.

Can someone do a similar numerical analysis of these two
factors: tempco and noise? It's not that I don't believe you;
it's just that I'd rather see numbers and plots than words.

> That drift plot in AN713 is the for a bandgap reference.
> However the attached plot from Woodward J Eicke's in April 1964 ISA 
> transactions  paper shows similar drifts for discrete zener reference 
> diodes.
> Buried zeners can be much better.
> 
> Bruce

Wow, a very long-term plot. I see Vref drift rates like:

+75 ppm / 500 days (= 2e-12 / d)
-25 ppm / 400 days (= 7e-13/d)
-60 ppm / 100 days (= 7e-12/d)
-100 ppm / 700 days (= 2e-12/d)

So that one is not too unlike the first three plots. I think
we can put the GPSDO Vref drift issue to rest.

/tvb





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