Hello Charles, In order to simulate and understand such an effect you could use a 3D electromagnetic solver. Ansoft has already an HFSS parametric model of a differential via on their Online Technical Support site. There is also a paper descirbing the design assumptions behind the 3D model. SI engineers ussually continue the stripline/microstrip away from the via so the higher modes will fade, and then use deembeding to mathematically get closer to the via. In your case,however, I recommend to get closer to the via, and change the port settings to calculate higher modes. For each mode you get a schematic field diagram or draw an accurate field distribution for each mode, the mode impedance (Z0), the attenuation constant real(Gamma), and the propagation constant im(Gamma). For the first two modes (diff/common) the attenuation constant will be small and the propagation constant high (as in sin(x) function). For the other modes you will have a bigger attenuation constant and a small propagation constant (as in a decaying exponent function). The resulting S-matrix contains the transmission coefficient. For example, if the impedance of the via matches the impedance of the line, you will see a clean transmission (with attenuation ofcourse). If however, the via stub is one quarter of a lambda you will see that the via will return all of the energy in this frequency. In order to understand this effect you could animate the surface currents as a function of the phase and "see" the standing wave on the via stub. The S-matrix has also information on mode conversion. For example, how much energy started as differetial mode and ended up in common mode. A few technical notes: 1) Change the default port solver accuracy from 2% to 0.1%. 2) Notice that re(Gamma) units are in Nepper 3) If you "fear" a 3D EM tool, you should consult with your RF colleague. I am sure they are fammiliar with these type of tools. I hope this helps, Itai --- In si-list@xxxxxxxxxxxxxxx, Charles Harrington <ch_harrington@xxxx> wrote: > Hello, > > When a TEM (or quasi-TEM) signal propagating on a > microstrip trace meets a discontinuity (e.g., a via) > on a substrate at microwave frequencies, higher-order > modes are excited at the discontinuity, leading to > reflections. > > Can anyone tell me exactly what kind of modes are > excited and why they are excited. How far away from > the discontinuity can they propagate and how does this > depend on the operating frequency. > > I'm a postgraduate student currently working on > discontinuities on PCB. My search for relevant > literature led me to this list. So, I'll appreciate > any > help. > > Charles. > > > > Yahoo! Mail > Stay connected, organized, and protected. Take the tour: > http://tour.mail.yahoo.com/mailtour.html > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxx 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@xxxx with 'help' in the Subject field > > List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu