[SI-LIST] Re: XAUI

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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