[SI-LIST] Re: Relativity in EMC design

[olaney@xxxxxxxx]

The measurement repeatibility at at test lab is nominally 3 dB.  To
account for manufacturing variation and the uncertainties that apply to
retesting, a margin of at least 6 dB below spec limit is wanted to
guarantee that sample testing by a regulatory authority will not result
in a failure.  Thus, at 7 dB below spec you are only 1 dB from the limits
set by prudence.  Of course, you won't be shipping a bare board to the
end user (I hope). The box integrity, filtering of power leads, treatment
of connectors to outside cables, etc. are major determinants of
compliance or the lack thereof.  For instance, conducted emissions
originating in the board might or might not be converted to radiated
emissions by the interconnect cabling, depending on how it is designed. 
Cleaning up emissions at the board level is a fundamental approach (it's
good that you are paying attention to it!), but the possibility of a
successful test is dependent upon too much else to extrapolate from the
board itself.  Involvement of an EMC engineer from preliminary planning
onward is best if you want a truly clean product, and you should look
over his/her shoulder as much as possible to understand the principles
that are applied.  There are many courses available for EMC compliant
board design (not all created equal), and some good books worthy of self
study.  A personal favorite is Introduction to Electromagnetic
Compatibility by Clayton Paul.  It offers system level understanding that
covers much more than just PCB design.  It shows and explains the results
of months of lab time, which is lots more efficient than trying to
empirically derive the knowledge on your own.  That said, you are on the
right track by working to understand EMC.

Orin Laney PE, NCE

On Tue, 13 May 2008 18:03:46 +0800 Zhangkun <zhang_kun@xxxxxxxxxx>
writes:
> Hi all
>  
> Currently, we have measured the radiation of signal line on external 
> layer
> of PCB. Based on the measurement result, we could gotten some
> design-guideline for layout.
>  
> For example, there is a  external line of 6000mil. When the stimulus 
> is 5mV
> at 400MHz, the RE radiation at 3m is 40dbuV/m. The FCC RE limitation 
> is
> 47dbuV/m. There is remaining margin of 7db.
>  
> My question is 7db is enough or not. On the PCB, there will be more 
> than one
> line, which carry signal of 400MHz. The RE radiation maybe 
> reinforce. How to
> evaluate the relativity effect of diffrent line carrying 400MHz 
> signal?
>  
> Any advice will be helpful.
>  
> Best Regards
>  
> Zhangkun
> 2008.5.13
> 
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