RE: 8b/10b and EMI
Tom and ALL:
Usually the board is inside a cabinet. The EMI test is based on a system,
but not a component or a board. The enclosure is usually well designed to
shield all radiations both in and out. The only possible leak is the copper
cables. In an optical system, a cable is not a concern at all.
Although, if the transceivers are not well designed for EMI concerns, and an
excessive radiation parts may cause some headache; however, in general, the
8B/10B code has been used for many years, and I have not heard anyone had
EMI problem. I never had experienced any EMI problem at all in my products.
The key is to design a enclosure being well shielded from radiation.
Regards,
Edward S. Chang
NetWorth Technologies, Inc.
EChang@NetWorthtech.com
Tel: (610)292-2870
Fax: (610)292-2872
-----Original Message-----
From: owner-stds-802-3-hssg@ieee.org
[mailto:owner-stds-802-3-hssg@ieee.org]On Behalf Of Tom Truman
Sent: Wednesday, March 15, 2000 11:13 AM
To: Ed Grivna
Cc: stds-802-3-hssg@ieee.org
Subject: Re: 8b/10b and EMI
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
scramblers,
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
Ed Grivna wrote:
>
> Hi Tom,
>
> a good source of data onthis can be found in the IEEE 1394b development
> archives. Since 1394b maskes use of 8B/10B encoding, and they spend
> a lot of effort on EMI reduction, there should be a significant number
> of papers/presentations available on the subject.
>
> As a background on what I remember, it is definately possible to
> create "hot spots" in the radiated emission spectrum if you keep
> repeating a short sequence of characters. This occurs in Fibre Channel
> systems with their 4-character Idle sequence. To get around this,
> 1394b added a level of scrambling to both the source data cahracters
> AND to the command characters used, prior to sending them through the
> 8B/10B encoder. By doing this they were able to achieve some dramatic
> (sorry, I can't remember the number of dB) reduction in radiated
> emissions.
>
> 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.
>
> Regards,
>
> Ed Grivna
> Cypress Semiconductor
>
> >
> > I would like to raise the issue of EM emissions with 8b/10b
> > vs. scrambling (spectrum comparisions can be found in the SLP
presentation
> > or in Joel Goergen's presentation). My impression is that component
vendors
> > are leaving this problem for the system integrators to solve.
> >
> > Has anyone looked into the issue of EMI/EMC within the context of system
> > cost and time-to-market?
> >
> > While system integration and implementation are
> > beyond the scope of the standard, it would be grossly negligent to
ignore
> > these issues, as ultimately the goal is to get product out the door that
> > satisfies FCC/ETSI regulations.
> >
> >
> > Tom