Re: Suppression of radiated emissions and spread spectrum clocking

[emc-p...@ieee.org]

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Isn't it fun, that's why I am not sure if being called an EMC expert is a good
thing.

 

Now to the point:

 

1) Pulling back the Vcc plane away from the edge of the "ground" plane changes
the distribution of the EM field at the edge. While the strength at the edge
decreases, it can indeed increase radiation in another direction. There have
been papers published on that, e.g. search for Gisin and Tanner in the IEEE
EMC Symposia or Transactions records. However, what is the practical
application and usefulness? While the radiation pattern is changed, one should
notice that on the typical PCBs none of the mechanisms of radiations
("edge-fringing" or Patch-antenna) are not efficient radiators, and in my
experience the direct radiation off the PCB edge or off the (undercut) Vcc
plane is typically not a problem, because Vcc planes are typically at an
electrically very small distance to the "ground" planes (even in a not-so-good
PCB stackup), and the field falls-off quickly with the distance from the edge
of the Vcc plane (wherever it is! ). Also, in many cases, the Vcc planes are
in between other plane ("ground") layers, which again changes the field
distribution and coupling and radiation mechanisms (there is no patch-antenna
in the PCB if it is sandwitched in the inner layers).

 

The typical practical problem with the edge-radiation that I have seen is not
the direct radiation, rather it is when the PCB-edge is close to a piece of
metal that is electrically larger, e.g. card-cage in a chassis. The field at
the edge in that case feeds the larger antenna (piece of metal), which then
radiates as a secondary-radiator much more efficiently. Another similar
situation is in the areas of the PCB-moats. The Vcc that is extended all the
way to the "ground" plane edge (of the moat) can very efficiently couple the
EM field into the moat, which then easily guides it and couples to the I/O
across the moat, e.g. a transformer. In such a case, the transformer and/or
common-mode choke across the moat do not suppress the common-mode energy
coupled to them.

 

The papers I have seen only concentrate on the redirection of energy, but fail
to recognize that the real practical issue is typycally not the direct
radiation but the secondary radiation.

 

I'd say, the expert who is stating that it is uselles is definitelly wrong -
sometimes it is very benefitial as described above (and witnessed in extensive
practice). It is also true that sometimes it does not matter. The expert who
said it CAN definitely help is right - it depends on the circumstances.

 

It is typically easy to pull the Vcc planes back and stitch the “ground”
planes together at the edges, which eliminates the issue with emission coming
>from secondary radiators - so why not do it? However, the energy is still
going somewhere inside the PCB stackup, so the signal routing, PCB-stackup,
bypassing etc. can/should/must be used to minimize the levels and the coupling
with the PCB structures that can radiate or carry it to the I/O sections.

 

2) Spread-spectrum is frequency modulation of the clock signal. If one looks
in any textbook on the topic (or remembers from the school), in FM the
side-bands receive power from the carrier, so that the deeper the modulation
is the power (hence the amplitude) of the carrier is lower and the power (the
amplitude) of the side-bands is higher – the total power in the signal is
constant. The ideal SS technique, from the point of view of minimizing the
radiated emission maximum amplitude) is such that the level at the carrier
frequency and the level at the sidebands are equal. Any deeper modulation and
the sidebands are higher than in the equilibrium, and any lower modulation and
the carrier is higher (thus they are not optimal). Also, the waveform of the
modulating signal can make large difference, check Lexmark web page on that.

 

I'd say the expert was wrong on the second one. Again - see the math of how FM
works.

 

Cheers, Neven


 



Conformity and SI List

 

I attended an electronic conference where a well-known EMI/EMC speaker/author
stated that the 20 H rule is useless for suppression of radiated emissions. 
On the other hand, another well known EMI/EMC speaker/author states that it
can definitely help.  What is the opinion of conformity (SI) members and what
empirical results have been obtained?

 

Second question.  A well-known EMI speaker/author also states that spread
spectrum clocking (SSC) does not lower peak radiated emissions.  Rather the
harmonics of SSC are spread out where the Quasi Peak Detector Spectrum
Analyzer (QPDSA) is missing (incorrect frequency band) their peak value
readings, i.e. fpeak is outside the frequency band that SPDSA is set for
monitoring.

 

Thanks.

 

Robert Hanson

 
 
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Conformity and SI List   I attended an electronic conference where a well-known EMI/EMC speaker/author stated that the 20 H rule is useless for suppression of radiated emissions.  On the other hand, another well known EMI/EMC speaker/author states that it can definitely help.  What is the opinion of conformity (SI) members and what empirical results have been obtained?   Second question.  A well-known EMI speaker/author also states that spread spectrum clocking (SSC) does not lower peak radiated emissions.  Rather the harmonics of SSC are spread out where the Quasi Peak Detector Spectrum Analyzer (QPDSA) is missing (incorrect frequency band) their peak value readings, i.e. fpeak is outside the frequency band that SPDSA is set for monitoring.   Thanks.   Robert Hanson - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc

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