[SI-LIST] Re: Serpentining one side of a differential pair
- From: Michael Vrbanac <vrbanacm@xxxxxxxxxx>
- To: zanella_fabrizio@xxxxxxx,"SI List (E-mail)" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 21 Sep 2001 08:55:39 -0500
Fabrizio,
I really don't see a difference except on one thing.
The skew induced by the inserted "serpentine" and/or the skew induced
by the connectors, etc. could just as well be the same amount
resulting in theoretically the same amount of common mode current due
to an identical differential pair imbalance..
The only difference that I see is the loop area involved by the skew
inducing structure or skew management technique. It would seem that
the most appropriate way to deal with this is to manage the problem by
managing the common mode loop area and minimizing skew in the first
place.
The skew problem, as I see it, becomes more problematic on multi-gigabit
interfaces. The skew to UI ratio becomes more critical as the percent of
UI increases. This ultimately compromises the eye pattern and signal
recoverability especially as the data rates continue increase.
My two cents...
Regards,
Michael E. Vrbanac
Consultant
At 10:37 AM 9/21/2001 -0400, zanella, fabrizio wrote:
>On board designs with nets running Gigabit signals, I constantly see that
>one side of the differential pair is serpentined/lengthened to perfectly
>match it to the delay of other side of the differential pair. This causes
>the positive and negative lines to not be coupled for a few hundred mils,
>which can result in common mode noise.
>My question is, which of the two evils is worse, having a little bit of skew
>within the pair or having the pairs uncoupled for a portion of the run?
>
>Thanks very much,
>
>Fabrizio Zanella
>Signal Integrity
>EMC Corporation
>508-435-2075, x14645
>fzanella@xxxxxxx
>
>
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