Lee I've done this in one design con paper with Jason Miller and the Oracle and LSI crew a few years s back. It is backed by theory, modeling, simulation, and measurement. All matched. I've included the link at the end of this email Lee, you have personal knowledge of how good my rf connector launches are. There are theoretical reasons for this, having to do with what I have learned about ground planes and vias. The same theory applies to differential vias. There is more latitude with differential signaling as I have explained, but you need to know what you are doing if you push the limits. My rule is conservative intentionally. If we take your advice to the limit, there is no reason to stitch ground planes except for dc current carrying capacity. In that case it would be perfectly reasonable to provide those stitch via s in one corner of a plane. Try it some time. Take square planes, stitch the planes in one corner, and pass two differential pairs and vias through it. Now measure the insertion loss return loss and crosstalk for both pairs. By the way, I've done a more sophisticated version of this experiment with many more vias. What you will find is that there are several modes of resonance that will appear in the measured data. Those resonances will cause crosstalk peaking that will wreck havoc. Everyone recognizes that plane pairs have distributed capacitance. Few seem to remember that they also have distributed inductance. When we combine the two we have a transmission line. In the case of planes we form a 2d radial parallel plate transmission line. Ignore this at your peril. http://www.electrical-integrity.com/Paper_download_files/DC11_8-TA3_Miller-paper.pdf&ved B0QFjAA&usg¯QjCNFIfLVcgR23TFqH63GKtO-wuow_0g On Oct 16, 2014 7:49 PM, "Lee" <leeritchey@xxxxxxxxxxxxx> wrote: > This discussion seems to be terribly theoretical. We all know that there > are thousands, perhaps tens of thousands of assemblies shipped every day > with differential signals as high as 28 Gb/S on them that work just fine > without the need to put in "ground vias" nearby. > > What comes to my mind when I read discussions such as this is that old > Burger King commercial where Clara Peller asks "where's the beef?" > > For all of the postulated problems mentioned in these discussions, > "where's the proof?" > > We do too much speculating on this forum and not enough proving! > > My position on this whole thing as well as many others that appear on this > discussion group is, if you are going to put forth a rule, be prepared to > offer the proof that the rule is valid as well as where it is valid. If > you are not prepared to do this, it is a disservice to those who are asking > for advice to make such a posting. > > -----Original Message----- From: Scott McMorrow > Sent: Thursday, October 16, 2014 3:29 AM > To: Yishan.Li@xxxxxxxxxxxxxxxxxxxx > Cc: Gert.Havermann@xxxxxxxxxxx ; aaditya.kandibanda@xxxxxxxxx ; si-list > Subject: [SI-LIST] Re: AW: Ground vias around signal via > > Gert has good advice about the distance of the ground via to the signal > via, but I would like to modify it a bit. A well-referenced set of ground > planes will have ground stitch vias that are separated by no more than 1/10 > the wavelength of the Nyquist frequency of the highest differential bit > rate, or the bandwidth of the signal edge rate of the fastest single ended > driver. > So for some numbers in the English system. > > Let Dk = 4 > Tdelay = 170 ps/inch > > For DDR3/4 with 100 ps driver edge rates > BW = .35/100ps = 3.5 GHz > lambda = 285 ps > 1/10 lambda = 28.5 ps > In Dk = 4 ground vias should be separated by a maximum of 167 mil to > maintain a good return path for these singled-ended DDR signals. > > For 10 Gbps > Nyquist is 5 GHz > lambda = 200 ps > 1/10 lambda = 20 ps117 > In Dk = 4 ground vias should be separated by a maximum of 117 mil for 10 > Gbps signalling to maintain a good return path. > > For 28 Gbps > Nyquist is 14 GHz > lambda is 71 ps > 1/10 lambda is 7.1 ps > In Dk = 4 ground vias should be separated by a maximum of 42 mil for 28 > Gbps signalling. > > These recommendations apply to the region in proximity of the signal > transition vias. They serve to tie the ground cavities together, provide > shielding for the power/ground cavities, eliminate resonances in the signal > passband and first harmonic, and reduce via-to-via crosstalk. If the > signal transition is at the balls of a semiconductor device, or in the pin > field of a connector, there are "usually" enough ground vias in these > regions to meet these requirements. If that is the case, then no > additional grounds are required. But, there are often cases that we > encounter at Teraspeed Consulting where these rules are violated. Here are > some common areas to look at. > > Via transitions around dc blocking capacitors. > > Boards with outer layer buildup microvias, where drilled vias do not carry > the package or connector grounds down through the board. > > Areas with asymmetric stripline crossing power splits on the distant plane > side of the stripline (the stripline is close to ground.) > > In all these regions it is necessary to close the return path loop with > ground vias as described above. Use the above as a guide to current and > future designs. > > best regards, > > Scott > > > > > > Scott McMorrow > Teraspeedî Consulting - A Division of Samtec > 16 Stormy Brook Rd > Falmouth, ME 04105 > (401) 284-1827 Business > http://www.teraspeed.com > > On Thu, Oct 16, 2014 at 3:39 AM, LI Yishan <Yishan.Li@xxxxxxxxxxxxxxxxxxxx > > > wrote: > > Hi, Aaditya: >> According to 3D simulation, ground vias around signal via deeply effect >> high frequency impedance. If your signal is low speed signal, it seems the >> ground vias are not necessary. >> >> Best regards >> Li Yishan >> -----Original Message----- >> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] >> On Behalf Of Havermann, Gert >> Sent: 2014å¹´10æÅË16æâÂ¥ 14:59 >> To: aaditya.kandibanda@xxxxxxxxx; si-list >> Subject: [SI-LIST] AW: Ground vias around signal via >> >> Aaditya: My opinion is, if we have a proper return path, they are not >> necessary. >> >> -> As Wolfgang said, GND vias are needed for proper return path because >> without any GND via there will be no proper return path. You don't have to >> put multiple of those around the signal via. with proper placement one is >> enough even for very high speeds. >> >> Aaditya: Any situations they are useful other than shielding? >> >> -> Placing many stitching vias doesn't necessarily provide any shielding. >> Thatââ¬â¢s an old Myth. Take a look at waveguide filters. The placement of >> screws into the dielectric (air) looks and functions very similar to a PCB >> where the energy also travels in the dielectric. Stitching vias can act as >> a filter, meaning that some frequencies are not shielded but guided to the >> outside. >> >> Aaditya: How will they help? When do we need them? >> >> -> As already said, use GND vias to provide a proper GND return. as a rule >> of thumb there should be a return via within the range of 1/8 wavelength >> (Nyquist) to the signal via. >> >> BR >> Gert >> >> >> ---------------------------------------- >> Absender ist HARTING Electronics GmbH, Marienwerderstraße 3, D-32339 >> Espelkamp; Registergericht: Amtsgericht Bad Oeynhausen; Register-Nr.: HRB >> 8808; Vertretungsberechtigte Geschäftsführer: Dipl.-Kfm. Edgar-Peter >> Düning, Dipl.-Ing. Torsten Ratzmann, Dipl.-Wirtschaftsing. Ralf Martin >> Klein >> >> -----Ursprüngliche Nachricht----- >> Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] >> Im Auftrag von Aaditya K >> Gesendet: Mittwoch, 15. Oktober 2014 19:42 >> An: si-list >> Betreff: [SI-LIST] Ground vias around signal via >> >> Hello Experts, >> I have a question on ground vias placement around signal via. >> >> My opinion is, if we have a proper return path, they are not necessary. >> Am I correct? >> >> Any situations they are useful other than shielding? >> >> How will they help? When do we need them? >> >> Please help. >> >> Thanks >> Aaditya >> >> >> ------------------------------------------------------------------ >> 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 >> >> >> ------------------------------------------------------------------ >> 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 >> >> >> ------------------------------------------------------------------ >> 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 >> >> >> >> > ------------------------------------------------------------------ > 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. > Checked by AVG - www.avg.com > Version: 2014.0.4765 / Virus Database: 4040/8400 - Release Date: 10/16/14 > ------------------------------------------------------------------ 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