[time-nuts] Project on precise timing over Ethernet
magnus at rubidium.dyndns.org
Sat Sep 12 15:08:16 UTC 2009
Hal Murray wrote:
>> Actually, you can see this as a Shannon information channel, analog or
> 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
noise in the communication link, includes systematic/deterministic noise
such as DJ, ISI, head-of-line packet delays etc. etc.
effective sampling rate (some systems, such as SDH/SONET, have
non-linear buffer mechanisms which makes updates rate and quantization
size large), assuming a justification mechanism or asynchronous clock
effective quantization size, assuming a justification mechanism or
asynchronous clock transfer mechanisms.
There are many phase modulations sources, some taking the form of
variations and other takes the form of biases.
> 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?
It is a sensible question, but you need to ask it the right way for the
particular type of system. Also recall, you can trade bandwidth for
resolution. You need to understand the systems information capacity
need, and you need to understand how it deteriorates. For most things,
you rarely discuss it in those terms, but experience show that early
naïve reduction in capacity may hurt alot later to overcome.
> 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?
Temperature is not the only interesting variable, but it is very
important. The group delay over fiber depends on length of fiber,
temperature and frequency of the ligth (aka "wavelength"). Fiber is by
nature dispersive and this causes frequency-dependent group-delay. Fiber
changes physical length with temperature, but also the wave equation
changes and thus shifts the response. A majority of group delay change
in the actual fiber comes from this temperature change of the
dielectrum. The lasers being used also change frequency with
temperature, and the shift accumulates over the distance.
> Can I measure tides with a 10 km link? Or is that swamped by temperature?
Not with a normal system, but an inventive mind could possibly do that.
I think there is more effective systems for that.
> How about microwaves? Does the speed of light in air change with temperature
> or humidity or whatever?
It changes. Wind shaking the towers should not be forgotten.
> What's the final bandwidth out of the control loop on something like a GPSDO?
Notice how that bandwidth may not directly correlate with that of a
transmission channels as I talked about.
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