Re: 8b/10b and EMI
Scrambling is more spectrally "smooth" than a short block code if repeated
characters are used. However, this is not the case with the 10 GbE 8B/10B
patterns so the issue is mute.
Longer run lengths make CDR designs much more complex than for a short run
length code such as 8B/10B. It is straightforward these days to implements
multiple 8B/10B CDR units on a single substrate. I don't believe that I know of
any multiple scrambled code CDR units on a single substrate.
The significantly shorter run lengths of 8B/10B systems directly translate into
a much narrower signal bandwidth allowing the use of a relatively high frequency
low pass filter to keep most lost frequency noise from affecting the CDR. The
inability of scrambled systems to do the same results in significantly more
coupled noise form extraneous sources and adds significantly to the
implementation of multiple CDRs on the same substrate.
"Booth, Bradley" wrote:
> Does scrambling give better spectral density than 8b/10b? I was under the
> impression that scrambling has longer run lengths of 1's or 0's compared to
> 8b/10b. Does this not make it harder to perform clock and data recovery
> with just a single scrambler, hence the reason SONET uses two scramblers?
> Just curious,
> Brad Booth
> bradley.booth@xxxxxxxxx <mailto:bbooth@xxxxxxxxxx>
> Intel Network Interface Division, Austin Design Center
> (512) 407-2135 office
> (512) 589-4438 cellular
> -----Original Message-----
> From: Tom Truman [mailto:truman@xxxxxxxxxx]
> Sent: Wednesday, March 15, 2000 10:13 AM
> To: Ed Grivna
> Cc: stds-802-3-hssg@xxxxxxxx
> Subject: Re: 8b/10b and EMI
> << File: Card for Tom Truman >> Ed,
> Thanks for the response.
> If 8b/10b were to be scrambled, then it would appear
> to me that all it is providing at the XAUI interface is
> packet delineation
> and some error monitoring capability. I imagine that each
> lane would need
> a separate scrambler/descrambler, initialized to different
> states so that
> the transitions across the lanes are uncorrelated.
> Synchronizing these
> and deskewing the lanes would require some thought -- it
> isn't difficult,
> but it isn't as straightforward as the "alignment column"
> proposed for HARI.
> At that point, the 25% overhead of the 8b/10b scheme
> seems to be a staggering price to pay for delineation and
> error monitoring -- why not start with scrambling, at a
> lower baud rate, and
> make the overall design problems simpler?
> Best regards,
> Tom Truman
> Lucent Technologies
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