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Date: Tue, 8 Nov 2011 08:23:49 +1100
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Subject: Re: [geda-user] PCIe card?
From: Stephen Ecob <stephen DOT ecob AT sioi DOT com DOT au>
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On Tue, Nov 8, 2011 at 8:04 AM, Russell Dill <Russ DOT Dill AT asu DOT edu> wrote:
>> One idea I had for a quick start was to lay out and edit a PCB with
>> the differential pairs replaced by fat single traces of thickness (2 *
>> differential trace copper thickness + differential trace internal
>> spacing).  A fairly simple bit of code could later convert these from
>> fat single traces to differential pairs.
>
> BTW, it isn't really necessary to route differential traces this way.
> Rather than route them with a very tightly regulated spacing, you can
> route them instead with a minimum spacing as single ended traces,
> which is much easier. Just be sure they have equal lengths impedances.

That's how I presently use PCB to implement differential pairs.  It
works well enough for the ~ 1Gbps signals that I'm working with, but I
wouldn't try it for something like 5Gbps PCIE.
The main limitation is that the spacing between the traces changes
slightly for diagonal lines, changing the impedance.
For high speeds I'd also want to eliminate sharp corners and use arcs
for every change in direction.  Creating pairs of arcs that maintain
even spacing would be tedious using PCB's present UI.  Creating single
thick arcs in the GUI and later having them transformed into correctly
spaced pairs of arcs would be bearable.

S

-- 
Stephen Ecob
Silicon On Inspiration
Sydney Australia
www.sioi.com.au