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Date: Mon, 7 Nov 2011 14:32:04 -0700
Message-ID: <CAN0Jx-_9xc9-Zx6u_JYqagGEx1z2n=+QLWsRxPngqKCyjZsCuQ@mail.gmail.com>
Subject: Re: [geda-user] PCIe card?
From: Russell Dill <Russ DOT Dill AT asu DOT edu>
To: geda-user AT delorie DOT com
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On Mon, Nov 7, 2011 at 2:23 PM, Stephen Ecob <stephen DOT ecob AT sioi DOT com DOT au> wrote:
> 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.

As long as you have them far enough apart, it doesn't matter if the
spacing changes.

> 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.

I don't think that'd have an effect even at 5Gpbs. You'd probably even
be ok with right angle corners, unless your margins are really really
tight already.