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Date: Tue, 8 Nov 2011 09:08:13 +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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Hi Russell,

On Tue, Nov 8, 2011 at 8:32 AM, Russell Dill <Russ DOT Dill AT asu DOT edu> wrote:
> 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.

That sounds rather optimistic to me :-)

See p12 of:

http://www.pcisig.com/developers/main/training_materials/get_document?doc_id=6d37ec2f8543fc1f9d8ace6264d08b469f57e5f1

(tiny URL in case the above gets broken in transit:)

http://tiny.cc/r4q64

Best regards,
Stephen

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