Slavek the issues with ground fences: 1. The reflected energy hits vias which provide a path to the top of the board and features that radiate. 2. The decoupling response exhibits a series of peaks and valleys on quarter wave multiples. An ideal plane termination would absorb all the energy and eliminate both effects. Steve. At 09:07 AM 12/4/2004 +0100, Slawek Guzek wrote: >Dear Zhangkun, > >[....] > > > According to the transmission line theory, when the transmission line > is matched, there is no resonance. At the edge of > > PCB, the EM energy is reflected because the transmission line is OPEN. > >I think you've made some small error in your concept. Indeed, EM energy is >reflected IF line IS open, >but at the end of the PCB line in not perfectly open - ground plane and >power plane form a radiating >element - a kind of a dipole. Some of the EM energy is radiated outside, >because of variable E field >lines at the PCB edge closed through air from power to ground plane > > > > Traditional decoupling capacitors are of low ESR, > > which could not damping the resonance. Therefore I need capacitor with > high ESR, whose ESR should be matched with the > > transmission line. On the other hand, the ESL is critical at high > frequency. The ESL should be as small as possible. The > > package of capacitor mainly determines ESL so that the package should > be as small as possible, such as 0402 or 0603. The > > capacitance should be as large as possible so that some benifit could > be gotten at lower freqeuncy domain. > > >It is much easier get close to perfect short circuit than close to perfect >open, so in my opinion, >from EMI point of view, it could be much more efficient to finish power >plane with some margin to >PCB edge, and fill up this space with vias connected to ground plane, >spaced not more than 1/16 >(one says even 1/100) of shortest wavelength you expect to be excited >between power and ground planes. >That case RF energy really reflects back into PCB, noises on traces >perhaps would be greater, >but significantly smaller portion would be radiated. > >Regards, >Slawek > > > > >------------------------------------------------------------------ >To unsubscribe from si-list: >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > >or to administer your membership from a web page, go to: >//www.freelists.org/webpage/si-list > >For help: >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > >List technical documents are available at: > http://www.si-list.org > >List archives are viewable at: > //www.freelists.org/archives/si-list >or at our remote archives: > http://groups.yahoo.com/group/si-list/messages >Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu