[SI-LIST] Re: 答复: Relativity in EMC design
- From: olaney@xxxxxxxx
- To: zhang_kun@xxxxxxxxxx
- Date: Tue, 13 May 2008 22:28:30 -0700
Worst case is a doubling of emissions to +43 dBuV (in-phase case), best
case is far field cancellation (equal magnitude, opposite phase, parallel
traces with close spacing i.e. differential pair). If the data streams on
the two traces are independent, you will get some of each! The rest of
the layout (discontinuities causing ringing et al) can create dips and
peaks that violate this general amplitude rule at specific frequencies.
Good EMC design does not proceed along the lines you are inquiring about,
because it is too simplistic. Compliance happens reliably when a system
approach is used. The board considered in isolation has no context to
work with. But, it's a start.
BTW, 40 dBuV at what measurement distance? Using what probe or what
test? The number alone has no interpretive value.
Orin
On Wed, 14 May 2008 09:38:39 +0800 Zhangkun <zhang_kun@xxxxxxxxxx>
writes:
> 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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> =B7=A2=BC=FE=C8=CB: olaney@xxxxxxxx [mailto:olaney@xxxxxxxx]=20
> =B7=A2=CB=CD=CA=B1=BC=E4: 2008=C4=EA5=D4=C213=C8=D5 23:06
> =CA=D5=BC=FE=C8=CB: zhang_kun@xxxxxxxxxx
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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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