[SI-LIST] Re: Length matching on Gb lines

Steve,
 
"common mode rejection" is the key.  I don't know why I didn't realise that.   
All is clearer now.  
 
Thanks,
Richard
>>> steve weir <weirsi@xxxxxxxxxx> 18/01/2006 11:50:46 a.m. >>>

Richard, common mode rejection, and EMI compliance are the big 
issues.  Mismatch between the PHY and the CM magnetics will result in mode 
conversion which is very bad.  that will erode all: EMI, EFT, ESD and 
ground noise rejection.  From an EMI standpoint, we want the CM choke as 
close to the box exit as possible.  On the inside, we want the stray 
coupling to be as uniform as possible.

The better you match the members of a GIVEN diff pair, the better off you 
will be vis a vis the mode conversion issues.  Match between multiple pairs 
isn't anything to be concerned about.  So, what you want to do is keep 
these traces away from mounting hardware.

Steve.
At 10:25 AM 1/18/2006 +1300, you wrote:
>Hi all
>
>This is a question about matching Gb signal lengths in the primary-side 
>and secondary-side of magnetics.
>
>Spec sheets for PHYs usually have very tightly-matched lengths within a Gb 
>port between the PHY and the magnetic.  A ethernet cable on the other side 
>of the magnetic (in the outside world) can have huge length mismatching 
>between the wires.
>
>Why is the primary PHY side of a magnetic more sensitive to length 
>matching on the secondary ethernet side?  Is it due to the center-tap on 
>the PHY side?
>
>My next question would be:  how close do lines have to be matched within 
>the diff pair and between diff pairs within the same port?
>
>Any help or links would be great.
>
>Regards,
>Richard
>
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