Gary E. Miller gem at rellim.com
Sun Sep 18 19:53:48 UTC 2016

Yo Achim!

On Fri, 16 Sep 2016 20:17:50 +0200
Achim Gratz <Stromeko at nexgo.de> wrote:

> Gary E. Miller writes:
> > Poll=2s is still the bast for this hardware, but there is a little
> > tradeoff between offset and jitter.  Since NTP is about time, not
> > frequency, we go with the best time.  
> You 've made this argument before, but I think it's circular
> reasoning.

Really?  I think the data is very clear, you can optimize for time
or for frequency.

> You make the local clock follow external jitter faster.

What external jiitter?  Assumption #1 is that the PPS is way
better than the internal XTAL.  An assumption supported by the GPS
data sheets.

> Whether or not that gets you closer to true time isn't something you
> can decide from your data, since the NTP offset value only tells you
> how close the PLL controller thinks it is to the external source.

Yes, it is a bit dodgy, using our dependent cvariable to measure itself.
I'm open to better ways to measure.  But when offset varies by over a
factor of 10 it is certainly indicative.  Large enough that local peers
can see the improvement.

And please notice I am optimizing for self-referencetial stable time,
not some 'true' time.  Until it is stable, no point playing with static
offsets to make it 'true'.

> > Now the good news.  On a quad XEON, with an MR-350P serial GPS:  
> I think as that example amply shows (I think you are using serial line
> discipline for PPS here),

Yes, sort of serial line PPS.  The Xeon is RS-232, the RasPi's are
actually using the GPIO lines, but very similar results.

> having the obviously better (than the rasPi)
> local clock follow a noisy reference (likely more jittery than the one
> on the rasPi) isn't helping the precision.

Uh, lost me.  My RasPi and my XEON are following local PPS.  That local
PPS is far more stable than annything else in the vicinity.  Depending
on who's talking the PPS may be accurate down to 50 or even 10 ns.  No
way to tell.

So the whole point is to track the PPS as well as possible.  When I can
track it down to under 1 us then it will be worth comparing different
PGS to see the best ones.

> BTW, both your plots showed relatively large swings in frequency
> offset in a short period of time.

Both plots?  I'm up to dozens now  Gotta be a bit more specific.

>  Do you have an A/C that is
> producing larger temperature swings?

Nope.  My house has no A/C.  It would likely be better if I did have
A/C.  If you look at the complete graphs (with CPU, Room, HR, etc.
temps) you will see a temperature correlation.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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