Best practices question

[Gary E. Miller]

Yo Paul!

On Wed, 26 Jun 2019 13:14:36 -0700
Paul Theodoropoulos <paul at anastrophe.com> wrote:

> I think maybe I'm  hitting a bug or I'm just not understanding
> (usually the latter), as it doesn't seem possible:

No one totally understands this.

> https://i.imgur.com/LorPR2F.png
> 
> jitter fluctuating between 400 parts per trillion and 800 ppt?? From
> a Raspberry Pi running at 1.4Ghz?

Here is mine:

https://pi3.rellim.com/day/#local_rms_frequency_jitter

My Standard Deviation is 48.504 ppb (10e-9)

Your Standard Deviation is 82.8 10e-12)

So about 500x better then mine.

But my numbers are over 24 hours, not one.  So the diurnal ambient changes
are in mine, not yours.

I get a lot better when I put the RasPi in a temp chamber and control
the CPU temp.  But not 500x better.

> That said, I switched to min/maxpoll 1 for PPS, and saw a
> corresponding change - so Time jitter appears to be inversely
> correlated to Frequency jitter - less frequency jitter, more time
> jitter, and vice-versa. This is all fascinating.

This sort of relates to a current discussion on time-nuts.  They
are discussing the relative merits of a PLL vs an FLL.  Similar effect
here.

The shorter poll keeps the time closer to the PPS, but magnifies the
jitter.

"And if you go chasing rabbits, And you know you're going to fall..."

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

	    Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin
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