[time-nuts] Phase measurement of my GPSDO

[Matthias Welwarsky]

On Mittwoch, 12. August 2020 13:23:57 CEST Tobias Pluess wrote:
> On the picture `gpsdo_180_210.png` one can see the last 30 hours of my
> measured data. During this time, the DAC value need to change only by 9
> counts. I think the DAC value has a slight trend to go lower and lower,
> this is perhaps because my OCXO was new and is still a bit aging. Phase
> error stays within +/-15ns during the whole time!

TimeLab can tell you the linear drift from the imported data.

> As I still don't have a proper DMTD system or anything similar which allows
> me to `quickly` estimate the stability of my device, I made some
> experiments and imported my logged data int Timelab. This can be seen in
> the picture `timelab_estimated_from_dac.png`. The blue curve results when I
> analyse the phase error. As expected, since this is essentially the
> performance of the 1PPS signal, the curve goes lower and lower. (I imported
> this data as "phase difference" and scaled it with 1e-9).
> The pink curve results when I let Timelab analyse the DAC values. This idea
> is actually from a colleague on this list. It assumes that the OCXO is
> perfectly stable and the only instability comes from the DAC being
> disturbed by the 1PPS noise. To analyse this, I imported the data as
> "Frequency Difference" and scaled it appropriately (scaling factor is
> 7.6e-12 as I have approx. 5 to 6Hz tuning range of the OCXO per 16 Bits). I
> am not sure how accurate this estimation will be, but if it is only close
> to the reality then it appears that my GPSDO has a stability in the range
> 1e-12. According to John's comparisons here:
> 
> http://www.ke5fx.com/gpscomp.htm
> 
> this would mean it is slightly better than a BG7TBL and quite a bit worse
> than a Tbolt with 10811A oscillator.

The DAC analysis tells you mainly how much "instability" the regulation causes 
for a given tau. The GPSDO output can not be better than this value. 
Typically, it's worse than the DAC ADEV. I've been doing some simulations with 
TvBs GPSDO simulator a while ago (might have posted some graphs here, cannot 
quite remember), the DAC ADEV (or MDEV) starts to influence the OCXO 
performance when they become close. 

> Comments welcome. I hope I get my two Rb standards soon and can then do
> some DMTD tests.

Looking forward to seeing some actual measurements!

> 
> 
> Tobias
> HB9FSX
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