[time-nuts] Are cable delays frequency dependent?

Bob Camp lists at cq.nu
Sat Sep 12 17:19:34 UTC 2009


A lot depends on the structure of the cable it's self. For plastic  
dielectric cables the answer is normally "yes the delay is frequency  

One mechanism is simply that the velocity of propagation it's self is  
frequency dependent in the material used.  Some of the mechanism's are  
pretty crazy. Periodic spacers in air / plastic cables can give some  
very odd frequency dependent reflections. Even without something odd  
like periodic spacers, the cable's characteristic impedance varies  
with frequency.  With constant termination impedances, you have a  
mismatch. Again, you get reflections that are frequency dependent.


On Sep 12, 2009, at 12:33 PM, 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.
> -- 
> These are my opinions, not necessarily my employer's.  I hate spam.
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