[time-nuts] FW: Question about the PLL of Trimble Thunderbold

[Attila Kinali]

On Mon, 29 Oct 2018 06:38:03 +0000
Ferran Valdés <vantta at hotmail.com> wrote:

> Yes, a GPSDO is already self adjusting, but for my project, I would like to either use a GPS or to synchronize one node’s oscillator on another.
> 
> The synchronization goal is in the order of ps level

Clock synchronization? Possibly fault tolerant? I think that's my
area of expertise :-)

I have something ready, that can synchronize 4 independent clocks
to eachother at the 180ps level, limited by the FPGA based TDC.
The current incarnation does not allow for an external clock source
to syncrhonize to, but that should be easy to add. That is, if you
don't mind using some half-finished we-have-published-a-paper research
tool.

But going to ps level of synchronization, especially if you mean <10ps,
is not going to be easy. There are not many ways to measure pulses
with this accuracy. If you know what you are doing, about 1-3ps RMS is the
practical limit you can achieve, more likely it'll be in the order of 10-30ps,
for a one-off design. Also keep in mind that ~2mm of cable is already 10ps of
phase shift. Ie you will need to calibrate your cables as well. Cables,
which are of course low dispersion and low temperature coefficient cables.
The dispersion is important so that your pulse remains a sharp pulse.
through the cable and doesn't come out grabled as a weird wave packet,
Quite counter-intuitively, limiting the slew rate might help with this.
The low TC is important if there is any distance between the two
oscillators. Otherwise you can get up to several ps per °C temperature
change and meter cable length for run of the mill cables. If you have
PTFE cables, you also want to keep them well above 25°C or well below 15°C,
for the same reason.

			Attila Kinali

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