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Re: 8b/10b and EMI


A Fibre Channel Idle is the following four character ordered set:
/K28.5/D21.4/D21.5/D21.5/ It's fairly ugly from an EMI perspective since:

1) The beginning and ending RD is always negative meaning the exact same
disparity versions of the Idle word are used;
2) D21.5 is repeated within the Idle word and is the high-frequency
b'0101010101' pattern. D21.4 is almost as bad being only a hamming distance of 1
from D21.5.

>From an EMI perspective, making a worse choice would be difficult.

For XAUI/XGXS, the /A/K/R/ Idle pattern bit stream does not repeat for 32
code-groups, 8X that of the Fibre Channel Idle. A very simple method of randomly
shifting /R/ columns at the initial transmitter would result in a non-repeating
Idle pattern. There is essentially no similarity between FC Idles and XAUI/XGXS
Idles and XAUI/XGXS Idles are already defined to be EMI "friendly".

Best Regards,

"Benjamin J. Brown" wrote:
> Ed,
> I picked up on your very last paragraph:
> Ed Grivna wrote:
> >
> > The 8B/10B code, when sending random data, has a fairly wide emissions
> > spectrum (which is what you want), but if you sit on the same
> > character or small group of characters, you can see the discrete
> > spectral peaks quite clearly.
> >
> When we look at the idle sequence for HARI, it looks like a
> stream of /K/R/K/R/ with an occasional /A/. Is this the type of
> thing you are referring to when you say small group of
> characters?
> Ben
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> Benjamin Brown
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> Nortel Networks
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