[SI-LIST] Re: Diff stripline, split in one of the gnd planes acceptable?
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: Morten Leikvoll <nerd@xxxxxxxxx>
- Date: Thu, 08 Oct 2009 07:36:16 -0400
Morten,
Assuming your stripline traces are symmetrically placed between the two
planes, half of the plane return
current is still upset by the gap. You can help this in two ways: you
can reduce the return current in
the planes by tighter coupling in the diff pair, or you can make the
stripline asymmetrical, putting it
vertically further away from the plane you want to split. You may find
our paper from DesignCon 2009
useful: Examining the Impact of Split Planes on Signal and Power
Integrity. You can find it at
http://www.electrical-integrity.com/ >> Paper download.
Regards,
Istvan Novak
SUN Microsystems
Morten Leikvoll wrote:
> I got a sandwich config of diff stripline (lvds):
> 1:lvds (microstrip)
> 2:GND
> 3:lvds (diff stripline)
> 4:GND
> 5:lvds (diff stripline)
> 6:GND
> ...
>
> For microstrip I see the importance of keeping the 2:GND plane unsplit for
> common mode return currents, but for normal stripline there are two
> (2:GND,4:GND) return paths for the 3:lvds pair.
>
> I wonder if its ok to make split in 4:GND without disturbing the impedance
> noticeably. I basically want to share the 4:GND plane with areas of power,
> and it will cause gnd/power splits. There will be no signal nets there, just
> low impedance like mixed power and GND.
>
> Should I worried about the return paths if I do add splits?
> Should I worry abut reflections if the gap caused by impedance disturbance?
> Maybe gaps could be "mended" with a MLCC just over/under each pair?
>
> I got rather long distances, maybe up to 50cm but breakouts/vias only close
> to both ends, and the bitrate is 1GHz.. (Altera StratixIII in both ends)
>
>
> If any doubts, the layout geometry terms are taken from here:
> http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=util_pcb_imp_calculator
>
>
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