Bi, Whether you model it or not in a real chip and board there will be a CM=20 return path. You would be well advised to control it within the package so= =20 that it can match what is achievable on the board. Then you can terminate= it. In unshielded twisted pair applications the transformer buys a huge CMR. Steve. At 11:32 AM 7/30/2005 +0800, Bi Han wrote: >Hi, SI-listers: > >The design task is to design on-chip differential transmission line to=20 >send signal. According to a lot of papers, edge coupled stripline is used. > >The cross-section is as below graph(might be lost). One key thing is that= =20 >there is no underneath common-mode return path. > >___________________________________________________ > > ______ ______ >dielectric |__+__| |__-___| >___________________________________________________ > > >Hi-Z substrate > >___________________________________________________ > > >Then what is going to happen to common-mode signal? Common mode should not= =20 >be a propagating mode, however, its S21 parameter after several=20 >reflections should be as below. > >S21=3DA*Hx*T/(1-R2*R1*Hx*Hx) > >R1 is source reflection coefficeint; > >R2 is load reflection coefficeint; > >Hx is propagation function, Exp(-gamma*L); > >Common mode impedance will be quite high if there is no good return path.= =20 >Therefore, both source and load are not well terminated. Source could be=20 >very like short, and load could be very like open. > >Then R1=3D-1, R2=3D1; the S parameter could go infinity if Hx(w)=3D-1; It= could=20 >resonate; > >It looks that twisted pair could survive this condition and goes well,=20 >why? Also several IEEE papers also used above configuration on-chip and=20 >goes fine, why? > > > > >--------------------------------- >DO YOU YAHOO!? > =D1=C5=BB=A2=C3=E2=B7=D1G=D3=CA=CF=E4=A3=AD=D6=D0=B9=FA=B5=DA=D2=BB=BE= =F8=CE=DE=C0=AC=BB=F8=D3=CA=BC=FE=C9=A7=C8=C5=B3=AC=B4=F3=D3=CA=CF=E4 > >------------------------------------------------------------------ >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 > ------------------------------------------------------------------ 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