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Re: 8b/10b and EMI

Dear Ed,

> "Edward Chang" <edward.chang@xxxxxxxxxxxxxxxx> writes:
> If the only reason to scramble the 8B/10B code is to minimize the
> probability of EMI emission caused by the occasional, repetitive IDLE
> signal, we may have to ask ourselves a question: have we done enough home
> work to prove it is required?  Even a simple circuit, it is not free.

The problem is also with repetitive input data patterns, such as all
zeroes, or all ones.  Such patterns are very common in computer data

> So far, in the real industry-wide installations, no one has the 8B/10B IDLE
> EMI problem.  Furthermore, no one has proved that 8B/10B IDLE signals will
> cause EMI problem for 10 GbE in an enclosed environment.

Were have you been hiding?  :-).

This is a big problem in the industry at 1G and 2.5G.  The problem gets
worse in proportion to the square of the frequency.  

If you are out in the field listening to the gossip, you'll find that
many supply contracts have collapsed due to EMI problems at numerous
companies.  Of course, no sane company advertises when they have an EMI
problem.  This may be why you haven't heard about these issues.

You may be correct for mil-spec double-shielded enclosures, but such a
requirement is not consistent with a low cost datacom standard. 

Best regards,
Rick Walker