[SI-LIST] FWD: Routing Differential PECL Signals

From: Ray Anderson <Raymond.Anderson@xxxxxxx>
To: si-list@xxxxxxxxxxxxx
Date: Wed, 22 Jan 2003 11:57:05 -0800 (PST)

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Date: Sat, 18 Jan 2003 02:13:20 -0000
From: "mikecar9999 <mikecar99@xxxxxxxxxxx>" <mikecar99@xxxxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Subject: Routing Differential PECL Signals
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I have a couple questions about how best to route 1.25GHz PECL 
signals over a distance of about 9 inches on a 12 layer card.  

Item #1:
I have always tried to route Differential signals inside the board 
using the edge-to-edge routing method with ground planes above and 
below.  For my current design I need to route from a GBIC to the 
Gigabit Mac (PM3386) over a 9 inch area but I am unable to get the 
signals between ground planes due to their 150 Ohm Diff Pair 
Impedance (for the TX pair).  I am comfortable with the signal 
integrity of using the top layer of the board (microstrip routing) 
with all other signals being away from this area.  My question is 
what about EMI concerns?  Isn't running 1.25GHz signals over a 9 inch 
area going to cause EMI issues?  Since they are being routed as 
differential signals should I not be concern about the EMI as long as 
they are match impedance, match length and maintaining a tight 
coupling the full distance (so the fields cancel each other)?  I have 
seen an article from National Semiconductor "LVDS Owner's Manual" 
which calls for the use of ground shield traces being along both 
sides of the diff-pair (at a distance of 2s, where s = space between 
the pair) with vias to the ground plane at <1/4 wavelength 
intervals.  Would something like this help reduce the chance of 
having EMI problems?

Item #2:
Does anyone know how far you can safely route PECL signals on FR4 
material?  Would 9 inches be considered a great distance?  Should I 
consider adding more PECL Drivers/Receivers (such as the Maxim 
MAX9321) between the GBIC and the Gigabit Mac?  This would increase 
the jitter.  The GBIC vendor and PMC reference designs both say to 
keep them as short as possible.  How short is short?

Thanks,
Mike C.



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[SI-LIST] FWD: Routing Differential PECL Signals

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