[SI-LIST] 答复: Relativity in EMC design
- From: Zhangkun <zhang_kun@xxxxxxxxxx>
- To: olaney@xxxxxxxx
- Date: Wed, 14 May 2008 09:38:39 +0800
Hi Orin
Thank you for the reply.
Based on the measurement result, the radiation of a signal line of =
6000mil
on external layer is about 40dbuV/m. How to evaluate the radiation of =
two
signal line of 6000mil on external layer? I do not know the phase =
relation
between the two signal line.
Best Regards
Zhangkun
2008.5.14
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=D6=F7=CC=E2: Re: [SI-LIST] Relativity in EMC design
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.=20
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
> =20
> Currently, we have measured the radiation of signal line on external=20
> layer of PCB. Based on the measurement result, we could gotten some=20
> design-guideline for layout.
> =20
> For example, there is a external line of 6000mil. When the stimulus=20
> is 5mV at 400MHz, the RE radiation at 3m is 40dbuV/m. The FCC RE=20
> limitation is 47dbuV/m. There is remaining margin of 7db.
> =20
> My question is 7db is enough or not. On the PCB, there will be more=20
> than one line, which carry signal of 400MHz. The RE radiation maybe=20
> reinforce. How to evaluate the relativity effect of diffrent line=20
> carrying 400MHz signal?
> =20
> Any advice will be helpful.
> =20
> Best Regards
> =20
> Zhangkun
> 2008.5.13
>=20
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