[SI-LIST] Re: XAUI
- From: steve weir <weirsi@xxxxxxxxxx>
- To: alexh1@xxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 22 Dec 2005 19:06:58 -0800
Alex,
When we loosely couple we don't have significant "unwanted crosstalk"
between the two lines in this "bus". The reason to consider this view is
that in a practical PCB there is only one very rare, and not very sensible
geometry where coupling to each of the two lines in a diff pair and a
aggressor / victim is even close to being equal. The idea that diff. pairs
have better immunity / susceptibility to aggressors is almost always
guaranteed to be wrong. So, that reduces the diff pair to a two signal bus
with very tight timing match requirements. When we loosely couple the
members of this bus, each line only has one significant propagation
mode. And, in that case, the effective impedance between the lines is
just twice the even mode, 100ohms. Our efforts are then directed at
meeting that tight timing match requirement.
When we couple tightly we have two impedances to manage, and losses can be
higher as well due to proximity. So instead of primarily concerning
ourselves with matching the flight times, we also have to worry about more
impedance discontinuities and sometimes higher loss. That makes tightly
coupled more complicated to get right than loosely coupled. Lee strongly
advocates against intentional tight coupling for the mistaken idea that it
will result in improved immunity / susceptibility that it does not
significantly improve.
I, along with many others agree that loose coupling is easier to get
right. So if you have the space to loosely couple, that is a good approach.
Steve.
At 03:49 PM 12/22/2005 -0800, Alex Horvath wrote:
> I don't completely understand your answer Lee. Questions inline below
> preceeded by --.
>
> Thanks
>
> -----Original Message-----
>From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
>Sent: Thursday, December 22, 2005 2:18 PM
>To: Ravindra Johari; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: XAUI
>
> Actually, neither. Coupling doesn't have any benefit. If you can spread
> the traces out so there is no coupling, you've got the best geometry.
>
> --You mean no coupling between traces so effectively simple stripline
> or microstrip, right?
>
> Real
> world, you can't do this and route a PCB, so route side by side with as
> much separation as your topology will allow. Then adjust trace widths to
> give you 100 ohm diff impedance.
>
> --Meaning route with wide seperation initially to force large spacing
> from possible victim traces but then go back to a 100 Ohm spacing? Is
> thinking here that you might as well route with 100 diff impedance even
> though it doesn't improve performance? Seems to conflict with statement above.
>
> Lee W. Ritchey
> Speeding Edge
> P. O. Box 2194
> Glen Ellen, CA 95442
> Phone- 707-568-3983
> FAX- 707-568-3504
>
> I just used the energy it took to be angry to write some blues.
> Count Basie
>
>
> > [Original Message]
> > From: Ravindra Johari <Ravindra.Johari@xxxxxxxxxxxxxxxxxx>
> > To: <si-list@xxxxxxxxxxxxx>
> > Date: 12/22/2005 11:19:44 AM
> > Subject: [SI-LIST] XAUI
> >
> >
> > Hello Gurus!!
> >
> >
> >
> > Which is a better coupling for running a differential pair at XAUI Speeds
> > i.e at 3 Gbps..
> >
> >
> >
> > Is it Edge coupled stripline or Broadside coupled stripline..
> >
> > What kind of constraints can I face with these?
> >
> >
> >
> > Thanks!
> >
> > Ravi
> >
> >
> >
> >
> >
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