Pras, in that example the Vdd plane is the signal image plane. Once a signal launches into a nice continuous impedance transmission structure, it merrily propagates along without concern for much of anything other than the material properties and physical composition of the structure. Steve Pras venki wrote: > Hello Guys, > I have this confusion regarding "*Return currents*"- > > 1) In the paper "SSN & power plane bounce in CMOS technology" by Larry Smith > ( http://www.csee.umbc.edu/vlsi/reports/ssn_pwr_planes.pdf > <http://www.csee.umbc.edu/vlsi/reports/ssn_pwr_planes.pdf+>). This is > available online for free, so i m pasting it here. (I hope i can) > > In the following excerpt- > > "Suppose the transition is from low to high and the cross-section of the > package has the transmission line located above a Vdd plane as shown in > figure 1.The driver connects the Vdd plane to the transmission line through > a low impedance.Current flows from the Vdd plane onto the transmission line > which is low, say ground potential.As the wave front propagates down the > transmission line, charge flows into the capacitance between the trace and > the Vdd plane, raising the potential on the trace up to Vdd. The current > path is complete because charge from the Vdd plane flows in a complete loop > from the Vdd plane, through the driver and onto the transmission line that > is referenced to the Vdd plane. If there is a ground plane underneath the > Vdd plane, it is not disturbed because it is not part of the current loop." > > Where does the return current flow? Does the return current flow through the > inter-plane capacitance? Doesn't it need to flow thru a reference plane? > > If it can't flow thru the Gnd plane, is it possible for it to flow thru the > same power plane which is supplying the current (i hope not coz it will > totally screw my fundamentals on current flow). > > 2)What if the package does not have an explicit power or ground plane > i.e. Power > & Gnd are normal, thin traces (like other signal traces), distributed > sporadically along with other I/Os, signal, clock traces etc. (Although the > power & ground traces are de-coupled inside the chip & if the I/O traces r > driven using a CMOS buffer) > > How is the return current going to flow now, when the I/O traces r driven > using the buffer? > > Will the return current still look to flow through the nearest Power or Gnd > reference trace it finds, given they are randomly routed in the package like > ordinary traces? > > I'd really appreciate if somebody can clear this nagging doubt. > > Thanx in advance. > > Regards, > pras > > > ------------------------------------------------------------------ > 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 > > > > > -- Steve Weir Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 California office (408) 884-3985 Business (707) 780-1951 Fax Main office (401) 284-1827 Business (401) 284-1840 Fax Oregon office (503) 430-1065 Business (503) 430-1285 Fax http://www.teraspeed.com This e-mail contains proprietary and confidential intellectual property of Teraspeed Consulting Group LLC ------------------------------------------------------------------------------------------------------ Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC ------------------------------------------------------------------ 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