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Re: XAUI AC coupling

my $.02, for what it is worth:

At 08:35 AM 21-09-00 -0500, Ed Grivna wrote:
>Lets start at the beginning.  

not a bad idea, since 'starting at the end' is rather heavily ironic.

>The fastest base frequency that you 
>can get from an 8B/10B coded signal is when sending a D21.5 character
>(which generates a continous alernating 01 bit sequence).  With
>a serial signaling rate of 3.125 GBd, this is equivalent to a 1.56 GHz
>square wave.

so far, so good.

>Now the first thing I usually get thrown back at me when I say this is
>that "but you need to take into account the odd harmonics of this at
>4.7GHz and 7.8GHz.  Bunk.  This is only true if you have a true square
>wave with extreemly crisp edges.  In the case of these links, most
>can barely get a signal to switch at the 3.125GBd signaling rate--
>which is the BEST thing you can have.  Those higher frequency spectral
>components CAUSE much of the dispersion in the interconnect.  Any
>components present that are faster than 1.56 GHz only serve to degrade 
>the link performance.

i disagree, though it can be a moot point for the 'low-pass filter' effect 
that you cite.  for my part, i don't care to preserve those first  three odd 
harmonics for the sake of vertical pulse edge shape, i do so to avoid
reducing pulse transmission to a bunch of single-frequency sinewaves.

i see an optimum tradeoff in this -- a gradual transition will certainly help 
minimize optic dispersion (also helps electrically), but a low transition 
slope introduces more jitter.  and while it is true that one goal of line
code development is to minimize accumulated jitter, it is also true that
no line code, on an AC-coupled link, is entirely free of DCD (please
correct me if you know of one such code).

so i see this as less than "trying to get a square wave" and more a case 
of striking a balance between extremes.

as for the remainder, i agree that description is a good start but i suggest
the committee should generate numeric examples, perhaps with Widemar's
(sp?) presentation as basis.

i hope this has helped.