[time-nuts] Divide by 3

[Attila Kinali]

On Wed, 8 Jun 2016 18:35:43 +0200
Mike Cook <michael.cook at sfr.fr> wrote:

> It is a no brainer to get a sine from a square wave, BUT , I seriously
> doubt that the excellent ADEV can be maintained with all that flipping
> and flopping going on. I even doubt that it could be kept in a pure sine
> implementation. 

I would not worry too much about ADEV degradation. The two most influencial
parameters for delay changes are temperature and supply voltage. Given that
the power supply is of decent quality (<<1% change per °C), then the supply
variations have little influence. Temperature, in an office or lab, does
not change that much to cause large differences.

I attached a TDEV plot (TDEV_all_pairs.png) of our clock sync system, which
uses Cyclone4 FPGAs (on an undmodified DE0-nano board) as TDCs to measure and
correct the skew between otherwise independent nodes. The measurements were
done using pulse outputs of the nodes feeding an DTS-2075, hence depict the
skew between pairs of nodes[1]. As you can see the curves go down nicely down
(with white phase noise) to <2ps in the 10-100s range. The blue curve looks
worse, due to hysteresis effects in the control system.

The second plot (TDEV_N4_N5_long.png) is an 9d19h run of the blue node
pair from the other plot. As you can see, the TDEV stays below 10ps up to 1e5s.
Most of the instability at taus >10 comes from the above mentioned hysteresis.

All measurements were done in late April, early May this year in Vienna,
with changing weather conditions. I.e. the temperature was anything but stable.
Unfortunately, we didn't record the temperature in the room.

If you take this data as indication, you can guess that any modern CMOS system
in an office environment will add less than 10ps timing uncertainty long term.
More likely it will be less than 1ps, as the plots depict measurements of a
complex system that does a lot of detection and non-trivial processing. 
For a frequency reference that is fed by GPS this level of stability should
be good enough.

Short term effects (aka phase noise) are a totally different story, though.


			Attila Kinali

[1] I am leaving the details of the system out for the moment, as they are
not really important for the discussion at hand. As soon as I have time to
prepare the arxiv version of the paper I will post it on this list.

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson
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