Lee, when contemplating layer switches, one can readily use a field solver to establish cross talk coefficients for any stack-up and edge rate. At this time, we do not share ours ( Teraspeed's ) with the general public. What we have published is an examination of via distance effects for a single signal via and plane to plane stitch via. That publication should provide some insights into when to start looking at these things. Regards, Steve. At 08:39 AM 7/25/2006, Lee Ritchey wrote: >Steve, > >Complex answer, but good. Problem with this answer is it confuses me. At >what frequencies does an engineer have to do the special via treatment? >I'm sure it is above where most logic operates and is likely around 10GE >launches, etc. > > > > [Original Message] > > From: steve weir <weirsi@xxxxxxxxxx> > > To: <leeritchey@xxxxxxxxxxxxx>; Ihsan Erdin <erdinih@xxxxxxxxx>; ><giovanni.guasti@xxxxxxxxxx> > > Cc: <si-list@xxxxxxxxxxxxx> > > Date: 7/24/2006 10:05:43 PM > > Subject: [SI-LIST] Re: Current Return Vias > > > > Lee, et-al I agree that there is a good deal of misinformation that > > circulates on this subject. It is another topic that ultimately > > comes down to the numbers. Single stitch vias are incapable of > > "maintaining return path continuity". What they can do is act in > > concert with other vias and planes, so that the net scattering has > > acceptably low loss. > > > > From my experience path impedance continuity to high frequencies as > > determined by low, linear S21 loss, and good S11 loss requires a > > pattern of vias associated with each launch. Teraspeed designs such > > launches everyday. But a return via pattern on a signal by signal > > basis eats up expensive real estate far too quickly to be practical > > for managing routing layer transitions. > > > > Usually we see these circumstances / effects from stitch vias: > > > > For boards without either multiple Vss layers, or puddles, there is > > nothing to stitch. It is down to bypass caps. > > > > For boards with multiple Vss layers and modest edge rates, the Vss > > vias of the bypass capacitors and components are usually well within > > the quasi static extents. Additional stitch vias directed at > > individual signals have very little impact. If for some reason a PCB > > has a lot of open real-estate, ( more often test vehicles than real > > PCBs ) then the situation changes and distributing stitch vias > > provides quantifiable benefits. For most practical designs the > > density and distribution is usually OK without adding extra stitch > > vias. When in doubt, do the homework. > > > > For boards with very fast edge rates, the quasi static extents are > > small, and stitch vias associated with individual signals display > > more pronounced effects. But, the dimensions become such that we > > really have to throw away the quasi static model and look at the > > various wave modes. This requires a 3D full wave tool like HFSS, or > > CST. For really fast signals, I reiterate that I cannot think of a > > case where one stitch via associated with one signal could make the > > difference between acceptable and unacceptable S parameter > > results. Either a pattern of vias is needed to make the difference, > > ( sic launch ) or none at all. > > > > Regards, > > > > > > Steve. > > > > At 03:34 PM 7/24/2006, Lee Ritchey wrote: > > >Ihsan, > > > > > >Well put. > > > > > > > > > > [Original Message] > > > > From: Ihsan Erdin <erdinih@xxxxxxxxx> > > > > To: <giovanni.guasti@xxxxxxxxxx> > > > > Cc: <si-list@xxxxxxxxxxxxx> > > > > Date: 7/23/2006 7:36:40 AM > > > > Subject: [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! 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