I'd like to see an example of a failure. I've got clients who manufacture tens of thousands of boards with several hundred differential pairs each making transitions between all combinations of signal layers with no "shorting" vias and see none of the problems described in this thread. Signal paths range to 28 Gb/S in some of these designs. -----Original Message----- From: Eric Bogatin Sent: Monday, September 08, 2014 8:39 AM To: si-list@xxxxxxxxxxxxx Cc: 'Eric Bogatin' Subject: [SI-LIST] One stitching via or more vias is better for 25Gbps Guys- When a diff signal transitions from one Vss return plan to another, the return currents of the diff signal mostly overlap in the cavity and a pure diff signal does a poor job of exciting cavity resonances between the two Vss planes. It is not necessary to have "stitching" vias in a cavity if you are only transitioning differential signals. However, it is impossible to engineer a real interconnect that is so symmetrical as to not have any mode conversion. This means there will always be some common signal components along with the diff signals. It's the common signals which will excite the cavity modes, as Steve has been emphasizing. When you do a simulation, unless you are clever enough to anticipate a possible asymmetry in the interconnect and include it in the simulation, you will not generate common signals in your simulation and the diff response through the cavity may look just fine. How big an issue is the common signal in the cavity? .it depends. If you are really, really lucky, you may have only -30 dB of common signal (3% of your diff signal) and the common noise injected in the cavity modes may not be enough to couple anywhere and screw up the system. But, if you are not so lucky, you may have -15 dB common signal. This is still within many specs, not a problem by itself, but will contribute 15% of the diff signal as common current into the cavity and screw up channel to channel cross talk and ground bounce to some other channel or couple out as EMI. If you want a robust system, use shorting vias in the cavity to suppress modal resonances- how many? It's not just about 1 or 2 per diff via. The shorting vias are to suppress model resonances. Their spacing is important. You will see the first model resonance with a wavelength equivalent to 6 x the via to via spacing. For a 1 GHz lowest excited mode, this is 1 inch apart. If you want to rely on luck in your system, maybe the first few, maybe even the first 20 boards will work, but as soon as you have too much mode conversion in a channel, your product may fail. If you are interested in this topic, I cover it in the Advanced Gigabit Channel Design (AGCD) class in the SI Academy, (www.beTheSignal.com) and in my graduate course at CU. I plan to post my lectures from this course on the SI Academy by the end of the year. --eric ******************************************************* Dr. Eric Bogatin, Dean <http://www.bethesignal.com/> Teledyne LeCroy Signal Integrity Academy <http://www.bethesignal.com/> www.beTheSignal.com Adjunct Prof, ECEE Univ of Colorado, Boulder 105 S Sunset St, Suite J Longmont, CO 80501 USA Twitter @beTheSignal e: <mailto:eric@xxxxxxxxxxxxxxx> eric@xxxxxxxxxxxxxxx cell: 913-424-4333 ****************************************************** Msg: #6 in digest From: David Banas <DBanas@xxxxxxxxxx> Subject: [SI-LIST] Re: One stitching via or more vias is better for 25Gbps Date: Mon, 8 Sep 2014 14:38:15 +0000 On 9/7/2014 8:49 PM, leeritchey@xxxxxxxxxxxxx <mailto:leeritchey@xxxxxxxxxxxxx%3cmailto:leeritchey@xxxxxxxxxxxxx> <mailto:leeritchey@xxxxxxxxxxxxx> wrote: > All of the things proposed don't seem to have any basis. I, on the > other hand, have built test boards to find out. My findings are > available in my classes and do not show such vias have any value. > Hi Lee, Our simulation results suggest that, while these stitching vias have no discernable effect on signal integrity at the receiver at these speeds, they do noticeably affect electromagnetic emissions from the PCB edges, potentially threatening electromagnetic compliance testing failure, when they are omitted. And I was wondering if you have any experimental findings, in this regard, that you can share. Thanks! David Banas Sr. Member of the Technical Staff Altera Corp.<http://www.altera.com/> ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ----- No virus found in this message. 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