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RE: 8b/10b and EMI


I don't think anyone has suggested that problem is emmissions disrupting the

computer system. The difficulty with respect to EMI from idle is passing 
EMC testing. If the computer system was sensitive to the radiated noise, it
would probably take a pretty short sequence of bits to cause the disruption
(something on the order of 10s or 100s of bits). The worst case sequences
internal disruption will occur even with scrambled codes. Therefore, these 
sequences must not cause interference with system operation.

On the other hand, EMI testing looks at power averaged over very long
- millions of bits. Packets having data fields with a repeated pattern 
such as a string of zeros or ones don't have a significant effect in this
time frame. Those I've consulted have said that the data sent for this
needs to represent normal traffic. One is not required to pack packets with
worst case data pattern and send them repeatedly because one would not
this in normal operation. Sending idle for extended periods is part of
operation so one should pass the test sending idle. This is why the
from a repetative idle pattern are an issue.


-----Original Message-----
From: NetWorthTK@xxxxxxx [mailto:NetWorthTK@xxxxxxx]
Sent: Friday, March 17, 2000 7:40 PM
To: walker@xxxxxxxxxxxxxxxxx; stds-802-3-hssg@xxxxxxxx
Subject: Re: 8b/10b and EMI 

Dear Walker:
>  The problem is also with repetitive input data patterns, such as all
>  zeroes, or all ones.  Such patterns are very common in computer data
>  transmisssion. 
The all "0" or "1" input data is different problem from the IDLE issue being

discussed here.  

Regardless of line code, all PCB must be designed to sustain all "0" or all 
"1" input signals simultaneously for all parallel data to shake up the whole

circuits to generate the maximum noise, crosstalk, radiation..etc, and still

meet all the requirements and BER.  This is the first test every PCB
will do to find out if all the design rules are correctly implemented.  If a

PCB does not pass this most fundamental test, the board should not be 
introduced for production.           

>  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.

I never heard that the EMI is so strong to collapse a computer system.  Only

an intentionally designed EMI generator will have such power.  Normally a 
correctly designed equipment may have the EMI problem to cause increased
or nothing at all.  However, it will cause interference to TV receivers, 
wireless receivers or other radio receivers.   

Some company has problem does not mean everyone has to have the same

EMI is a very complex issue which involves the source reduction and leaking 
prevention of the whole circuit design inside a cabinet.  The worst thing is

that you cannot measure it correctly, until it becomes the final product,
it requires a certified EMI lab. to provide an accurate data. If you got the

EMI problem, you may have to go through another development cycle to find
if the fix works.  While in the development stage, your past success in the 
EMI design counts a lot.  The IDLE issue is only the very small part of the 
whole EMI issue.  I will never use IDLE as an excuse for any EMI design 

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

In today's fiercely competitive, cost-effective market, no one can afford to

use the old technique, "solve the problem at any cost."   Only the one who 
knows how to solve the problem with the minimum cost will survive in the 
market.  It takes more than just engineering knowledge to do it.

>  Best regards,
>  --
>  Rick Walker 

Ed Chang
NetWorth Technologies, Inc.