Ihsan, I do not think this is just a rule of thumb: this is an experience that comes from measurements and simulations done in the last years. Please give a look to the following link, I used HFSS and Hspice to simulate. I do not like rules of thumb, this was the reason of a short presentation for ANSOFT Partners in design - "Analisi critica delle rergole di buon progetto" (critical analysis of "rules of thumb") http://www.ansoft.com/partnersindesign/IT_guasti_italtel.pdf Another presentation during the EMC of Rome 2005 "The strategic role of EM field solver in Power and Signal Integrity" spent some slides on the optimization of vias. I would be glad to send you the file if you are interested in. I do not want with this to convince anyone. But at this point, in order to change my opinion, I need a research that states otherwise in a peer-refereed publication. :-) Design costs: in my mail I spoke about an analysis needed first, to understand if a gnd via can be useful or not. In the most complex PCB I followed there were only four differential pairs that could benefit from this technique. So this is not a high cost, I think. Board thickness and signal frequency spectrum must always be considered. I would never suggest to double the number of via! Best regards, Giovanni ________________________________ From: Ihsan Erdin [mailto:erdinih@xxxxxxxxx] Sent: 23 July 2006 16:33 To: Giovanni Guasti Cc: si-list@xxxxxxxxxxxxx Subject: Re: [SI-LIST] Re: Current Return Vias Giovanni, The EMC justification of a ground via in the close proximity of a switching via is to minimize radial waves to the edges of the card by providing a "return path" (I hate this phrase...) A two-wire kind of transmission line -as you put it- would be another rationalization against the impedance discontinuity for high-speed signals. In practice, however, the placement of a ground via close enough to a switching via in order to provide a matching impedance to -say 50 ohm- or to mitigate radial wave propagation is in most cases -if not all- physically impossible. I think this kind of SI/EMI rule-of-thumbs are based on a qualitative understanding of electromagnetic theory rather than rigorous research results. In this context, I share Lee's stance to debunk these recommendations because they have significant effect on the design cost by closing routing channels and eating up on the valuable board real-estate. If anybody has come across any research that states otherwise in a peer-refereed publication I'd like to hear that. Regards Ihsan On 7/22/06, Giovanni Guasti <giovanni.guasti@xxxxxxxxxx> wrote: Kenny, GND vias near the signal are not dedicated to return currents, but they are often used to optimize the impedance of the via. As the via is a short transmission line, only the higher speed signals can benefit of the difference between an optimized via and a "usual" via. You have to compare the higher frequency component of your signal, its wavelength and the via length. This will give you an idea of the effective needing to optimize this short transmission line. Of course you could have a 133MHz signal with very sharp edges and high frequency components, even if it seems very unusual... In this case it would be wise to choose a slower transmitter! The rule is to understand if the via behaves like a transmission line for your signal or not, and in the first case to do the best to reduce impedance discontinuities. Best regards, Giovanni -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Lee Ritchey Sent: 22 July 2006 18:36 To: Kenny Frohlich; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Current Return Vias Kenny, It is not true that you need a "return current" via next to each layer changing signal via. I continue to be amazed that engineers who are looked upon as SI experts say such things. Imagine you have a 4 layer PCB, such as the mother board in a PC, where there are only two planes, one Vdd and one ground, where would such vias connect? There have been billions of these made to date that work just fine and have very fast signals on them. The return currents you are concerned about find their way from plane to plane through the collection of decoupling capacitors and interplane capacitance that you had to engineer into the power delivery system in order to make it stable. Focus on this and the return currents take care of themselves. EMI is minimized he same way.. > [Original Message] > From: Kenny Frohlich <kenny_frohlich@xxxxxxxxx> > To: <si-list@xxxxxxxxxxxxx> > Date: 7/22/2006 6:45:56 AM > Subject: [SI-LIST] Current Return Vias > > Dear Experts, > I understand that I need to provide ground vias next to via explictly for the purpose of letting return currents jump between layers. I know it's a requirement for high speed signals, especially differrential signals. Is this also required for low speed single-ended signals (133Mhz or slower)? =20 > If this is a requirement, what would be a good signal via to ground via ratio? For example, there are five signal vias within a 1 inch area, how many ground vias do I need? > =20 > Thank you > Kenny > =20 > __________________________________________________ > Do You Yahoo!? > Tired of spam? Yahoo! Mail has the best spam protection around=20 > http://mail.yahoo.com=20 > > ------------------------------------------------------------------ > 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: =20 > //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 > =20 ------------------------------------------------------------------ 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 <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: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages <http://groups.yahoo.com/group/si-list/messages> Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ 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 <//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