[time-nuts] Are cable delays frequency dependent?
bruce.griffiths at xtra.co.nz
Sat Sep 12 20:26:31 UTC 2009
Hal Murray wrote:
> If so, what's the mechanism?
> I know that attenuation is frequency dependent due to skin effect but I can't
> turn that into variable delays. Is there a magic term I should google for
> and/or does anybody have a good URL?
> Context is a memory from 20 years ago. I think it was a data sheet or app
> note for clock recovery on a T1 line. Maybe it was just explaining the specs
> for a line amplifier. The idea was that the recovered clock would shift
> depending on the frequency of the signal. The frequency depended on the data
> pattern so you could harass the clock recovery by picking nasty data patterns.
> I think I almost understood it back then when I had the info in front of me.
> I've tried to remember or reconstruct it a couple of times over the years,
> but I've never been successful.
Even in the RF region where cables act like distributed LC transmission
lines with a relatively constant characteristic impedance, all
dielectrics are lossy.
A lossy dielectric has a frequency dependent dielectric constant.
This is particularly evident in the vicinity of an absorption edge.
Even remote from an absorption edge the dielectric constant varies with
The dielectric constant behaviour as a function of frequency can be
approximated by a Cole-Coles relationship.
This in turn can be approximated by a set of Sellemeier equations, one
per absorption edge.
Dielectric loss (and dispersion) increase with the water content of the
At lower frequencies the cable acts like a distributed RC transmission
line with a strongly frequency dependent characteristic impedance and
Google telegraphers equations for details.
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