[time-nuts] Are cable delays frequency dependent?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Sep 12 20:33:44 UTC 2009

Bruce Griffiths wrote:
> 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
> frequency.
> 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
> dielectric.
> At lower frequencies the cable acts like a distributed RC transmission
> line with a strongly frequency dependent characteristic impedance and
> propagation delay.
> Google telegraphers equations for details.
> Bruce
For measurements of the dispersion (variation of propagation delay with
frequency) of coaxial cable in the RF region see:


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