It seems to me that the via (thru and stub) can be modeled with segments of loaded transmission lines. The anti-pads usually provide periodic loading causing the transmission line impedance and prop. delay to be altered as below: This change in impedance and delay will appear as an Er change even though the actual Er of the material is unaltered. A fully loaded multi-drop backplane bus transmission line will have a lower impedance and higher prop. delay than when it is lightly loaded. We don't usually attribute that to an Er change. Same for the via. Thanks, Vinu On 01/12/2012 01:41 PM, Scott McMorrow wrote: > Bert and Ralph > There are layered-anisotropic variations in Er for many materials, > especially those that include fiberglass weave. However, Er does not > change with non-TEM modes or different TEM modes (stripline, via-coaxial, > circular cavity ... etc). Different propagation modes merely concentrate > the field in different directions and select a different set of localized > material characteristics. > > I've read the papers and seen the claim that the dielectric constant of > layered fiberglass material is higher for propagation through a via, due to > the direction of the field, however, I've not seen a systematic study of > this. (Adjustment of material Er(effective) to obtain a match to modeling > does not constitute proof.) My experience for launch vias with coaxial > ground rings has been that the resonance computed by full wave solvers > matches measurements quite well in a multitude of materials, if the > dielectric has been characterized correctly. I find that most of the > mis-correlations that I've seen are due to improper material > characterization. I do not discount the possibility of a higher localized > Er region around a via in some measurements, its just that I find little > evidence for fiberglass being the sole culprit. In many cases I've found > that mismatch in stub resonance could be easily accounted for by adding the > correct amount of soldermask to the bottom pad in modeling. In other cases > I've found that material variations between layers were not correctly > modeled. > > As a thought experiment, take a section of a PCB with a via along the > z-axis that is fully surrounded by a coaxial metal wall. Calculate the > average Er from top to bottom, and then calculate it radially out. The > volume of material is the same. The composition of the material is the > same. Thus the average Er is the same. The only way to come up with a > higher Er for the radial direction is to conclude that somehow the > drilling process selectively removes more epoxy than fiberglass from the > mixture. There are layered variations as we travel down the via passing > through fiberglass rich, and then epoxy rich layers. But I see no reason > why they would not average out. I can make a case that individual pairs of > signal and ground vias can have Er variation, just as I can for traces, but > I cannot come up with any reason why the Er would not average out in the > limit. > > There is one other potential reason why a via could have a higher localized > average Er. But it has nothing to do with the fiberglass itself. I will > probably use it as a topic for next year's DesignCon paper, as a follow up > to the paper I'm involved with this year. > > > regards, > > Scott > > > On Thu, Jan 12, 2012 at 3:51 PM, Lambert Simonovich< > bertsimonovich@xxxxxxxxxx> wrote: > >> Ralph, >> When building your 3D model, you should also take into account the >> anisotopic factor of the material. I.E. Dkx-y can be 15-20% higher than >> Dkz. Since conventional FR4 type laminates are fabricated with a weave of >> glass fiber yarns and resin, they are >> anisotropic in nature. Because of this, the dielectric constant value >> depends on the direction of the electric fields. In a multi-layer PCB with >> vias, >> there are effectively two directions of electric fields. The one we are >> most familiar with has the electric fields running perpendicular to the >> surface of the PCB -as is the case of stripline traces. The dielectric >> constant, >> designated asDkz in this case, is normally the bulk value of the >> dielectric specified by the laminate manufacturer's data sheet. The other >> case has the electric fields >> running parallel to the surface of the PCB, as is the case when a signal >> propagates through a differential via structure. >> >> I know for a fact that HFSS allows you to have a different value for Dkx-y >> vs Dkz, and when you take this into account, from my experience, it agrees >> quite well with measured results. I don't know about the 3D tools you >> mentioned though. >> >> >> -Bert Simonovich >> >> >> ________________________________ >> From: Ralph Wilson<ralph.wilson@xxxxxxxxxxxxxxxxxx> >> To: Antonis Orphanou<aorphanou@xxxxxxxxxxxxxxxxx> >> Cc: "si-list@xxxxxxxxxxxxx"<si-list@xxxxxxxxxxxxx> >> Sent: Thursday, January 12, 2012 3:13:54 PM >> Subject: [SI-LIST] Re: Via stub math help needed.... >> >> Antonis, >> >> With this, I disagree. Based on other feedback I've gotten, and looking >> at some of the cross references and reference papers, since the via >> does not meet the TEM conditions of a typical transmission line, the >> Dk associated with the PWB stackup is not appropriate to use. Calculating >> an "effective" Dk using one of several methods gets me close (3D >> simulations >> are the best). Due to the non-TEM boundary conditions (pads, anti-pads, >> orthogonal >> reference planes, etc.), the "effective" Dk can be 4x the FR4 Dk. This >> causes >> the delay to be twice that of a stripline in the same material, and leads >> to the x2 factor in the null frequency. >> >> Ralph >> >> On 1/12/2012 1:52 PM, Antonis Orphanou wrote: >>> The first resonant frequency occurs at half and not a full wavelength. >>> In your calculation/formulation you assume full wavelength and this is >> why you are a factor of 2 off. >>> >>> >>> >>> -----Original Message----- >>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] >> On Behalf Of Beal, Weston >>> Sent: Thursday, January 12, 2012 11:41 AM >>> To: Ralph Wilson; si-list@xxxxxxxxxxxxx >>> Subject: [SI-LIST] Re: Via stub math help needed.... >>> >>> Ralph, >>> >>> Your calculation assumes TEM propagation. This happens on uniform >> transmission lines, line long traces or coaxial cables. The via is not >> uniform long enough to support at TEM field propagation. It really is a 3-D >> geometry that needs to be analyzed as such. A 3-D field solver should give >> the most accurate results. Some good calculations based on the analysis of >> the 3-D geometry as HyperLynx does can give a reasonable answer very >> quickly. >>> Weston >>> >>> >>> -----Original Message----- >>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] >> On Behalf Of Ralph Wilson >>> Sent: Thursday, January 12, 2012 4:24 AM >>> To: si-list@xxxxxxxxxxxxx >>> Subject: [SI-LIST] Via stub math help needed.... >>> >>> All, >>> >>> While working on some SERDES nets, specifically trying to quantify the >> effects of some via stubs, I ran across something that has me stymied. I >> expect a null in S21 where the stub length (delay, actually) is 1/4 >> wavelength. However, the simulations are showing a null at half the >> frequency I predict. I've subsequently run the via model through several >> different tools, and although the null frequency varies a little bit due to >> modeling / parasitic issue, they all come up with a null frequency roughly >> half of what my "math" predicts. So my fundamental question is where is my >> theory or my math wrong? >>> Null frequency = 1/wavelength = 1 / (4 x via-stub-delay) >>> >>> via-stub-delay = d / s, where d = distance (length of stub) and >>> s = wave propagation speed >>> >>> s = c / sqrt(Dk), where c = 299,792,458 m/s, or >>> c = 299,792,458 m/s x 1/0.0254 in/m x 1E-9 s/ns = 11.8 in/ns >>> >>> So, if I pick a via stub length of 80 mils in FR4 with a Dk of 4... >>> >>> s = 11.8 in/ns x 1/sqrt(4) = 5.9 in/ns >>> via-stub-delay = 80 mils x 1/5.9 ns/in x 1E-3 in/mil = 0.0136 ns >>> >>> Hence, the predicted null frequency = 1 / (4 x 0.0136 ns) = 18.44 GHz >>> >>> However, all of my simulation tools (Hyperlynx, IE3D, CST MWS) show a >> null in the range of 9-10 GHz. Digging deeper, they show the via delay to >> be in the range of 27 ps rather than the 13.6 that my math shows. >>> What gives? Why is my delay calculation off by (roughly) a factor of 2? >>> Is the lumped capacitance of the via stub somehow affecting the >> propagation delay in the via stub? That's somehow mixing t-line theory with >> lumped model approximations... I'm at a loss. >>> Thanks for any insight. >>> Ralph Wilson >>> Alcatel-Lucent >>> >>> >>> >>> ------------------------------------------------------------------ >>> 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 technical documents are available at: >>> http://www.si-list.net >>> >>> 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 technical documents are available at: >>> http://www.si-list.net >>> >>> 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 technical documents are available at: >> http://www.si-list.net >> >> 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 technical documents are available at: >> http://www.si-list.net >> >> 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 technical documents are available at: http://www.si-list.net 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