I just wanted to restate my observation that it is difficult to obtain
a balance coefficent more than 20 dB. That will leave about 10% of the
differential current flowing in the ground plane as a common mode
current. From an EMC point of view, this could be a problem.
Doug
"D. C. Sessions" wrote:
>
> Eric Goodill wrote:
> >
> > Mike Jenkins wrote:
> > >
> > > Eric,
> > >
> > > One line of your question, "My system is running pretty fast
> > > (> 1 Gbps)", caught my eye. At that speed, which I assume might
> > > be Fibre Channel or Gigabit Ethernet, you may well be running
> > > differential. (If not, good luck to you.) But if your lines
> > > are dif'l, they carry their own return current. Depending on
> > > geometry, there is some discontinuity, but MUCH less than
> > > single-ended. If your lines are, in fact, differential, and
> > > if you wish me to elaborate, I will.
> >
> > Mike,
> >
> > Yes, differential. However, we're using edge-coupled pairs, and it's my
> > understanding, though I've done no analysis, that about 10% - 15% is about
> > as much coupling as you can get between edge-coupled lines. Thus, there is
> > still a strong coupling between the trace and it's reference place.
> > Therefore, I suspect that there's non-ignorable amount of return current in
> > the reference planes. I'd be interested to see a
> > return-current-distribution plot for a diff pair both in the reference
> > planes and the coupled traces.
>
> I don't think so. Sure, there's a fair bit of capacitive current between
> each trace and the adjacent plane, but since they're equal and opposite
> the loop is very small and entirely lateral. Cross a plane boundary and
> there's no need for any current across the break.
>
> --
> D. C. Sessions
> [email protected]
>
> **** To unsubscribe from si-list: send e-mail to [email protected]. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
--
-----------------------------------------------------------
___ _ Doug Smith
\ / ) P.O. Box 1457
========= Los Gatos, CA 95031-1457
_ / \ / \ _ TEL/FAX: 408-356-4186/358-3799
/ /\ \ ] / /\ \ Mobile: 408-858-4528
| q-----( ) | o | Email: [email protected]
\ _ / ] \ _ / Website: http://www.dsmith.org
-----------------------------------------------------------
**** To unsubscribe from si-list: send e-mail to [email protected]. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****