Giovanni Several clarifications: 1. If you are optimizing the impedance of a via transition with ground vias, then you are controlling the return path. They are one in the same. 2. A via is quite a complex transition line. The closest parallel in planar transmission line theory is that of a trace crossing a split plane. best regards, Scott Scott McMorrow Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 (401) 284-1827 Business (401) 284-1840 Fax http://www.teraspeed.com Teraspeed® is the registered service mark of Teraspeed Consulting Group LLC Giovanni Guasti 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! 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