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satter
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Re: All well and good, except...
satter   2/4/2014 7:04:21 PM
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Sorry; however I think you are missing the point.  The varied twist of the pairs reduces the metallic coupling between pairs just as you have pointed out.  The coupling I am speaking of here is the longitudinal coupling between the pairs treated as single conductors.  In the article you can see that I have shorted the two conductors of each pair and teat each pair as a single wire.  These conductors do form transmission lines and store energy -- and can get you in trouble if not addressed.

Thanks for reading the article,

 

Jim

dougp01
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All well and good, except...
dougp01   8/17/2013 6:40:55 PM
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This all well and good except for one thing that seems to be ignored in the article. 

An assumption seems to be made that the coupling bewtween the conductors is a constant impedance over the entire legnth of the cable.  In fact, when Bell Labs first developed the concept of paired bundles for telcom wire many years ago, the problem of coupling between pairs was addressed.  The solution they adopted on pair-wises cables was to introduce a different twist rate on each pair of wires.  The effect they were seeking was to ensure that field coupling between pairs was cancelled every few centimeters.  With 8-pair cable this can be easily verified.  With 25-pair or 50-pair is it a little more difficult but this is where twist of pair-bundles comes into play.

 

-doug

 

 



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As data rates begin to move beyond 25 Gbps channels, new problems arise. Getting to 50 Gbps channels might not be possible with the traditional NRZ (2-level) signaling. PAM4 lets data rates double with only a small increase in channel bandwidth by sending two bits per symbol. But, it brings new measurement and analysis problems. Signal integrity sage Ransom Stephens will explain how PAM4 differs from NRZ and what to expect in design, measurement, and signal analysis.

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