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Re: XAUI AC coupling




Hi Rich,

NOW we're both talking about the same thing.  Thanks for the
clarification.

-Ed Grivna

> Ali,
> 
> The common mode voltage is too high. Several vendors are doing XAUI in
> 0.25 um. Many are going to 0.18 or better. Some have announced SerDes at
> 0.13 um already. The direction of several key XAUI vendors will be to go
> 0.13 um or better for XAUI by the time the standard is published.
> Therefore, a 0.75 V +/-0.25 V common mode spec is short sighted and will
> be obsolete prior to publication. This is why I'm recommending no spec
> for DC-coupling and no mandate for AC-coupling for XAUI.
> 
> Best Regards,
> Rich
>    
> --
> 
> Ali Ghiasi wrote:
> > 
> > Hi
> > 
> > About 8 month ago we debated the same issue rigorously in another
> > standard.  The XCVR mfg
> > wanted AC coupling and the system company wanted DC coupling.  Can you
> > imagine if you
> > have a 64 ports 10 Gig switch how many capacitor you need.
> > 
> > In that group I proposed the following satisfying the need of both AC
> > and DC:
> >         - every transmitter must provide 0.75+/- 0.25 volts of common mode
> >           else it must be AC coupled.
> >         - Every receiver must be capable of operating with 0.75 V of common
> > mode
> >           else it must be AC coupled.
> >         - All external link the receiver will be AC coupled.
> > 
> > I believe there is genuine need for DC coupled link in the backplane and
> > switch applications.  The
> > above implementation may add two capacitors one for each blade, assuming
> > both serialize and
> > de-serializer do not operate with the 0.75 V common mode.  But the
> > capacitors can be eliminated
> > from the blades if you operate with common mode of 0.75 V.
> > 
> > Why 0.75 V Common mode; it is half 1.5 Volts, support >500 mV on each
> > leg, supports
> > advance CMOS (Sub 0.18 um).
> > 
> > AC coupling will be used often, but lets not force high density
> > applications to use
> > 1000's of caps.  We need to define the common mode voltage, but many can
> > just ignore it.
> > 
> > Thanks,
> > 
> > Ali Ghiasi
> > Newport Communications
>                                    
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