[SI-LIST] Matching Differntial Pairs

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I am interested in some information regarding the routing of PCIe signals.
I have worked with 2.5Gbps differential signals in the past, but I am new to
PCIe.  I have noticed that PCIe requires add-in cards to have differential
pairs matched to 5mils.  

 

Can anyone explain why the PCIe differential signals on an add-in card need
to be matched with 5mil?  I have never had to match signals this closely in
the past, so I wasn't sure of the rationale.  Is it for EMI reasons?  Is it
really necessary to match this closely?

 

If you are running the differential signal coupled (I have read that this is
not necessary to route them as a coupled pair, but I have never implemented
differential signals as uncoupled etch, and am reluctant to experiment on my
current design - I can't afford to have the board not work because I miss
some detail), then what is the best way to achieve this 5mil matching?  I
have seen several different techniques.  I have always been taught that it
would be better to be mismatched by 30 mils and routed together the whole
way, than to add etch to one signal to match the length.  If I do need to
meet the 5 mils, and I need to add etch, then what is the preferred method?
Certainly, adding the etch at the transceiver side would be preferable.
However, I don't have a good feel for HOW to add the etch.  I have always
been told that serpentining (second pair below) is bad, but I see that this
app note that I found is recommending the serpentine.  

 

Could someone please explain the preferred method for matching coupled
differential pairs, as well as why this method is better?

 

Thank you for your help,

kathy

 

 

 

 




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