Re: pravidlo 20H

Pátek Červenec 13 21:51:36 CEST 2007

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...
> > _______________________________________________
> > HW-list mailing list  -  sponsored by www.HW.cz
> > Hw-list@list.hw.cz
> > http://list.hw.cz/mailman/listinfo/hw-list
> >
> ---------------------------------------------------------------------------------------
> > Orange vous informe que cet  e-mail a ete controle par l'anti-virus mail. 
> > Aucun virus connu a ce jour par nos services n'a ete detecte.
> > 
> > 
> > 
> 
> _______________________________________________
> HW-list mailing list  -  sponsored by www.HW.cz
> Hw-list@list.hw.cz
> http://list.hw.cz/mailman/listinfo/hw-list
> 
> 
> 