pravidlo 20H

Pátek Červenec 13 23:33:17 CEST 2007

Pozri, musis najprv vyriesit principialne zdroj rusenia. A na to je len
jedna cesta. Pouzit procesor, ktory ide na 1,8V, nema ziadnu spotrebu a
taktuje sa najmensim moznym taktom, ktory este vyhovuje danej aplikacii.
Zial, ak si tam das napr. ARM na 40MHz a spotrebou 40-50mA, tak si akurat
koledujes o problemy. Tam ti pravidlo 20H samo o sebe nic nepomoze. Ak by
som mal byt uprimny, tak si pre zaciatok kup nejaku knihu o EMC a PCB a
precitaj si ju. 

A.

> Panove dekuji vam... proctu si uvedenou literaturu a pokud se ukaze, ze
> je to k necemu (a ja si myslim, ze je), tak mi snad u nas v praci daji
> pytel zlata, abych to mohl vyzkouset v praxi, na nejake karte co hezky
> vyzaruje to zkusime a porovname vysledky ze zkusebny... Pokud to vyjde,
> tak dam vedet...
> 
> H.
> 
> 
> > ------------ Původní zpráva ------------
> > Od: balu@home <daniel.valuch@wanadoo.fr>
> > Předmět: Re: pravidlo 20H
> > Datum: 11.7.2007 22:43:13
> > ----------------------------------------
> > 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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