Istvan, given a PCB in which the differential traces are routed near reference planes the 'differential traces' are not tightly coupled. Most of the coupling is to the reference plane(s). For example, a 100 ohm differential trace is close to actually being two non-coupled 50 ohm stripline traces driven out-of-phase. So, the 'return' path thru the reference plane does need to be unbroken. Dave Cuthbert On 11/29/08, Istvan Nagy <buenos@xxxxxxxxxxx> wrote: > > hi > for a differential pair, like PCIe or SATA, some people (some of my > colleagues) say, its not needed to have a continous good current return path > since the differential voltage is referenced only to the other signal-trace > in the pair. > > I think the current return path has to be continous even for diffpairs, > because: > 1.) Common mode noise: > if the return path is not continous, then it creates loops for the common > mode noise, so the common mode noise will be bigger. If the CM noise is > bigger than it introduces jitter into the differential signal. which is bad. > 2.) Impedance: > if the return path is poor, the the return current has to flow around to > get a path between the source and destination, so on some areas the return > current doesnt flow unterneath the signal traces so basically their > single-ended (odd mode) impedances (so the diff impedance too) will be > different than its calculated in the imp.calculator (polar, or MMTL), which > assumes a perfect ground plane. > > am i right, or wrong? why? > > Istvan Nagy > Concurrent Technologies plc, UK. > > ------------------------------------------------------------------ > 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 > 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 technical documents are available at: http://www.si-list.net 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