pravidlo 20H

balu@home daniel.valuch@wanadoo.fr
Středa Červenec 11 22:42:02 CEST 2007


mno... ja nemam ten spravny akademicky nadhlad, takze keby som problem 
riesil ja tak by som sa odputal od vceliciek a kvetiniek, duchov a 
chrapunov, anteniek a dratikov a pouzil napriklad google. Urobil si 
kratky resers z dostupnych zdrojov a urobil si obraz o co vlastne ide :-)
Najprv si vezmeme tie doveryhodnejsie zdroje ako napriklad IEEE alebo 
papiere od vyrobcov rychlych obvodov, pripadne EM simulatorov. Ani 
papiere z MIT nie su na zahodenie. Dostaneme napriklad nizsie uvedene 
citaty - efekt celej tejto veci je otazny, vsade doporucuju urobit zemnu 
klietku okolo celej dosky a nechat napajacie vrstvy uzavrete vo vnutri.

http://ieeexplore.ieee.org/iel5/7576/20666/00955851.pdf
Conclusions: As harmonics of clock frequencies from high speed
digital logic circuits extend well beyond the 1 GHz frequency range,
radiation along PCB edges is becoming a significant problem. One
way to reduce radiation is to add two outside ground planes that are
shorted out to each other along the edge of the PCB. If the grounding
is along the entire periphery of the PCB, then internal reflections can
occur, increasing the likelihood of creating signal integrity problems.
A common signal integrity method of reducing these internal
reflections is to pull back one of the planes. This has the opposite
effect of reducing radiation. If complete grounding along the
periphery is not maintained, then the PCB will resonate at different
frequencies, depending on which sides are grounded.

http://ieeexplore.ieee.org/iel5/7576/20666/00955851.pdf
One of the primary printed circuit board mechanisms that
produce radiated from PCB edges are via currents that cxcite
radially propagating electromagnetic waves between power
and/or ground plane structures. When these waves reach the
edge of the printed circuit boards, a portion of the energy
radiates into space (usually the cavity of the electronic
enclosure housing the PCBs), and a portion is reflected back
into the PCB, where it induces currents into the same vias that
were the original source of the initial radially propagating
wave. These induced currents then conductively flow into the
components mounted on the surface of the PCB, producing
secondary radiation. Thc waves inside the PCB excite
resonant modes whose lrequencies are dependent on the
length and with of the PCB, as well as the type of edge
treatment used to reduce radiation. The most common
technique of reducing edge radiation is fencing, where a series
of shorting vias are used to connect top/bottom ground planes
into a Faraday shield. This increases internal reflections. An
alternate approach is to pull back one of the ground/power
planes to form a smootlicr~iinpeclati7cetr ansition region. While
this increases edge emissions, it reduces PCB resonance
effects. It is not clear whether this latter method consistently
produces an overall reduction in emission levels.

http://www.national.com/appinfo/adc/files/questweb_dec_2001.pdf
Q. What about the so-called 20H rule for power planes? Useful?
A. The 20H rule says that extending the ground planes beyond the power 
planes by 20 times the distance
between the ground and power planes will reduce board emissions. A 
theoretical study says the
opposite is true and experimental results support this. (See
http://www.sigrity.com/papers/epep2000/epep_20h.pdf). I do not know the 
origins of the 20H rule, but
it does not seem to be valid. I have never tested the rule.

http://piers.mit.edu/piersproceedings/download.php?file=5A6_2172.pdf
As the above analyses are all based on bare PCB, unlike real PCB which 
has a lot dynamic digital
components on, the real analyses can be very complex. Though we have not 
got the 20H rule's
working mechanism, one thing is certain that he use of 20H rules 
resolved many EMC issues of
PCB design.

celkom nazorne od Ansoftu 
http://www.ansoft.com/thewave/Shielding_and_Decoupling.pdf, strana 9

aj s obrazkami http://dspace.mit.edu/bitstream/1721.1/3996/2/HPCES004.pdf

The effects of the 20H rule will highly depend on three
factors, namely, operating frequency, length of PCB and
separation distance. The operating frequencies play an
important role on the 20H rule because it also affects function
of the other two factors. So the normalized length and
separation distance with regarding to wavelength should be
used to compare their relationships with each other. For our
case studies in which the frequency and the length are fixed,
smaller separation distance strengthens the function of the 20H
rule. Also in our case studies, higher frequencies are found to
cause resonance problem. In this sense, effects of the 20H rule
are determined by some specific conditions. More explorations
are expected so as to gain more physical insight into this
design rule.


http://www.mwr.medianis.net/pdf/Vol7No3-12-ZPantic-Tanner.pdf
...It is not clear whether this latter method consistently
produces an overall reduction in emission levels.


http://www.freelists.org/archives/si-list/12-2002/msg00032.html




Honza aaa wrote:
> Zdravim konferenci,
> 
> podarite mi nekdo nejakou peknou literaturu (nebo osobni zkusenosti), jak vyrazne ovlivnuje nedodrzeni pravidla 20H EMC vyzarovani? Konkretne me zajima pripad procesorove karty (frekvence procesoru 48MHz) uzavrene v hlinikove bedne s tlostkou steny cca 2mm (bez chladicich otvoru), bedna a zem karty nejsou galvanicky spojeny...
> 
> Honza
> 
> P.S. Pravidlo 20H rika, ze by zemni plocha mela o dvacet "tloustek plosneho spoje" presahovat vsechny ostatni drahy, pripadne ze ma byt ve vsech vrstvach okolo desky draha, ktera je spojena se zemi...
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