[time-nuts] Project on precise timing over Ethernet

Javier Serrano javier.serrano.pareja at gmail.com
Fri Sep 11 19:20:53 UTC 2009

Not sure I understand all your questions. Concerning PLL bandwidth, assuming
you want to minimize jitter you'd set the bandwidth so as to minimize the
area below the phase noise vs. frequency plot. At high frequencies the phase
noise is that of the VCO in the PLL, while at low frequencies the phase
noise is that of your input (i.e. that of the source plus that introduced by
the link). When I say high and low frequencies it's wrt the bandwidth of the
PLL. So you look at the phase noise plot of your VCO and your input and
decide where you must place the PLL bandwidth in order to minimize the area
under the curve, i.e. the jitter. In 10 km of fiber which is not buried
underground you can expect some ns of delay change between summer and winter
(in Geneva at least).
If you are referring to the other loop, i.e. that needed to compensate for
fiber delay changes, the bandwidth should be low enough not to start
"fighting" with the other one. Thermal effects are very slow, so I guess 0.1
Hz or so would be adequate. It's fast enough to catch normal thermal effects
and low enough to not collide with the other loop.

On Fri, Sep 11, 2009 at 8:02 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> > Actually, you can see this as a Shannon information channel, analog or
> > digital.
> What sort of bandwidth do I need to run a PLL over a long link?
> I assume it takes enough to cover all the sources of error:
>  drift in the master
>  drift in the local oscillator
>  drift in the communication link
> Does it depend on my target accuracy?  Do I need more or less bandwidth for
> a
> better answer?  Am I even asking a sensible question?
> What sort of changes in delays to I get on 10 km of fiber or coax or
> twisted
> pair?  Is temperature the only interesting variable?
> Can I measure tides with a 10 km link?  Or is that swamped by temperature?
> How about microwaves?  Does the speed of light in air change with
> temperature
> or humidity or whatever?
> What's the final bandwidth out of the control loop on something like a
> --
> These are my opinions, not necessarily my employer's.  I hate spam.
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