[time-nuts] Phase measurement of my GPSDO

[Matthias Welwarsky]

On Donnerstag, 19. März 2020 20:32:30 CET Tobias Pluess wrote:
> My recorded data is here:
> 
> https://hb9fsx.ch/files/gpsdo/phase.txt
> 
> Now I made two plots (here: https://hb9fsx.ch/files/gpsdo/phase.png). The
> upper is the raw phase data, and I also fitted a linear function to it
> (i.e. if the OCXO has a constant frequency but has some offset from 10 MHz,
> the phase would drift linear into one direction - at least I expect it to
> do so).
> The lower plot is what results when I subtract my linear fitted function
> from the measured phase. I am wondering whether this makes some sense? One
> can see some wiggles and ripple, especially between sampel 0 and 3000. Is
> this the GPS jitter I see there?

I'd say so, yes. What type of receiver did you use? You mentioned a Ublox NEO-
M8T, did you trigger a self-survey when you started the measurement?

It looks like the receiver entered position-hold around 3100 seconds into the 
data file. At least that would explain why the wiggles became much less 
pronounced. Also, you're evidently not correcting the PPS quantization error 
yet.

> Further, I also imported this phase data into Timelab and determined the
> ADEV. Here is my plot:
> 
> https://hb9fsx.ch/files/gpsdo/timelab.png
> 
> This trace is kind of what I expect, but since I use a OSA/UCT 8663 double
> oven OCXO, I would have expected it to be much lower. Something in the
> 1e-12 regime perhaps? is that realistic? As one can see at tau=800s, the
> ADEV is around 5e-11, so is this really all I can expect from this very
> sophisticated OCXO? (However I should maybe add that the OCXO is brand-new
> and has never been used before, so maybe it has some initial aging?).
> I have no plots about the temperature, but this is quite constant.
> For the GPS module, I used an active outdoor antenna at the window ledge.
> Not optimal, but better than an indoor desktop antenna, I guess.
> 
> Is this some plausible data I have here, or do I measure complete crap?

I'd say it's plausible. I got similar results with my Rb-GPSDO when doing the 
same test.

https://www.eevblog.com/forum/projects/diy-gpsdo-project-w-stm32-tdc7200/
msg2860766/#msg2860766

You cannot expect to see the OCXO for small tau if your reference is the GPS. 
The GPS 1PPS jitter and the quantization errors will dominate the ADEV graph 
for small tau. Once you correct the quantization error (you need to parse the 
UBX-TP5 message for that) your 1s ADEV will be in the 1e-9 range.

BR,
Matthias