Jeff, My talented friend, Gustavo Blando, with whom we analyzed this situation a few years back, reminded me that the voltage-transfer function formulas can be found for instance in "MicroWave Transistor Amplifiers", Guillermo Gonzales, Page 185. You can also look at L. S. Dutta, T. Hillmann-Ruge, “Application of Ring Oscillators to Characterise Transmission Lines in VLSI Circuits,” IEEE Transactions on Components, Packaging, and Manufacturing Technology - Part B., Vol. 18, No. 4, November 1995, pp. 651-657. Note that you will find various expressions; you may want to pick the Vout/Vsource formulas, as opposed to Vout/Vin. You will see that the voltage transfer ratio is, in general, different from S21. Regards, Istvan Novak SUN Microsystems Loyer, Jeff wrote: >Hello Istvan, >You're keying in to what was surprising to me. For the asymmetric lossy >network I simulated, I noted that the pulse response remained constant, >regardless of direction of simulation. Of course, S11 and S22 changed >dramatically. > >The pulse response was not a derivation of the s-params; it was merely >Vout (the circuit driven with an arbitrarily chosen 2V source) of a >simulation of the actual network (3 lossy lines). > >So, it appears that the pulse response and insertion loss remained >constant, even though the return loss varied dramatically. I wasn't >surprised to have the insertion loss stay constant, but couldn't have >predicted whether the pulse reponse would also remain constant, though I >suspected it would. This implies the eye diagram would also be the >same, regardless of direction of excitation. > >Having the pulse response remain constant appears contrary to your >experience. Any explanation? > >Thanks for your thoughts, >Jeff Loyer > > >-----Original Message----- >From: Istvan.Novak@xxxxxxx [mailto:Istvan.Novak@xxxxxxx]=20 >Sent: Monday, October 01, 2007 4:46 AM >To: Loyer, Jeff >Cc: istvan.novak@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx >Subject: Re: [SI-LIST] Re: AC Coupled Signals > >Hi Jeff, > >Let me see if I can summarize this in simple physical terms. The >scattering matrix deals with power values. In a reciprocal network, >the 'through' power is the same regardless of the direction we go. The >node voltages, on the other hand, depend on the local impedance levels, >which are related to S11, S22 and source and load reflection >coefficients.=20 > >So probably a more generalized condition is that we can shuffle around >reciprocal building blocks inside a cascaded network, and the resulting >eye diagram will stay the same as long as we do not change S11 and S22. > >Going back to your simulations: if you calculate the pulse response of >your example circuits strictly from S21 of the network, I agree, it will >not change, since S21 will not change as you move the reciprocal >building blocks around.=20 >Your eye diagram, however, should be usually calculated as Vout/Vsource >(and for sake of simplicity, we usually assume linear driver and >receiver impedances, where their linearity does not change this >argument). This voltage transfer ratio can be analytically calculated if >you wish, from the S-parameter flow graph. The resulting formula is >symmetrical in S12 and S21, but asymmetrical in S11, S22 as well as >source and load reflection coefficients.=20 >If you simulate the transfer response or the pulse response of >Vout/Vsource in HSPICE, you should see the change as soon as the >electrical symmetry is changed. > >Regards, > >Istvan Novak >SUN Microsystems > > > > > > >Loyer, Jeff wrote: > > > >>Hi Istvan, >>Could you point me towards more information on the "voltage transfer=20 >>ratio", and the difference between it and s21/s12? When I did the=20 >>pulse response of a system where S21 =3D3D S12 (though S11 and S22 were = >> >> > > > >>very different, and the reflections were significant), it came out=20 >>equal (p21 =3D3D p12, see my posting of 9/15). Thus, I think the eye=20 >>diagram will =3D come out equal if S21 =3D3D S12. =3D20 >> >>Thanks, >>Jeff Loyer >> >> >>-----Original Message----- >>From: si-list-bounce@xxxxxxxxxxxxx=20 >>[mailto:si-list-bounce@xxxxxxxxxxxxx] >>On Behalf Of Istvan Novak >>Sent: Saturday, September 29, 2007 12:38 AM >>To: Chris.Cheng@xxxxxxxx >>Cc: lifeatthesharpend; signalintegrity@xxxxxxxxxxx; ron@xxxxxxxxxxx;=20 >>si-list@xxxxxxxxxxxxx >>Subject: [SI-LIST] Re: AC Coupled Signals >> >>Chris and All, >> >>I think the source of misunderstandigs might stem from the fact that in >> >> > > > >>linear, time-invariant, reciprocal networks S21=3D3DS12 regardless of = >> >> >the > > > >>sequence of the smaller building blocks within the network, BUT, the=20 >>eye diagram shows us voltage transfer ratio between source and load,=20 >>and it is NOT S21; the voltage transfer ratio DOES depend on the=20 >>sequence of contributing blocks. This happens also with linear source=20 >>and load and even if the source and load are perfectly matched.=3D20 >> >>The only case when the voltage transfer ratio remains the same in spite >> >> > > > >>of moving building blocks around, if/when reflections at the boundaries >> >> > > > >>of the particular building block are negligible. >> >>Thanks >> >>Istvan Novak >>SUN Microsystems >> >> >> >>Chris Cheng wrote: >>=20 >> >> >> >>>I think the point is tuning S22 for the non-ideal load. S12 or S21 = >>> >>> >=3D3D > > > >>>=3D >>> =20 >>> >>> >>> >>=20 >> >> >> >>>remains being equal (symmetric). >>> >>>-----Original Message----- >>>From: lifeatthesharpend [mailto:lifeatthesharpend@xxxxxxxxx] >>>Sent: Friday, September 28, 2007 2:44 PM >>>To: signalintegrity@xxxxxxxxxxx; ron@xxxxxxxxxxx; Chris Cheng >>>Cc: si-list@xxxxxxxxxxxxx >>>Subject: Re: [SI-LIST] Re: AC Coupled Signals >>> >>> >>>What is the total loss at each point? (reciever, driver side of cap, = >>> >>> >=3D > > >>> =20 >>> >>> >>> >>=3D3D >> >>=20 >> >> >> >>>receiver side of cap) in dB=3D3D20 >>> >>>It seems to stand to reason that if you lose xxx mV when the signal is >>> =20 >>> >>> >>> >>=20 >> >> >> >>>=3D3D stronger at the driver and some smaller portion of that when=20 >>>the=3D20 caps =3D3D are placed closer to the recever is normal since = >>> >>> >loss=20 > > >>>has=3D20 ocurred in the =3D3D media. The total signal loss of the = >>> >>> >system=20 > > >>>could=3D20 still be the same.=3D3D20 >>> >>>Leonard.=3D3D20 >>> >>> >>> >>>----- Original Message ----=3D3D20 >>>From: Stephen Zinck <signalintegrity@xxxxxxxxxxx>=3D3D20 >>>To: ron@xxxxxxxxxxx; Chris.Cheng@xxxxxxxx=3D3D20 >>>Cc: si-list@xxxxxxxxxxxxx=3D3D20 >>>Sent: Friday, September 28, 2007 12:19:22 PM=3D3D20 >>>Subject: [SI-LIST] Re: AC Coupled Signals=3D3D20 >>> >>> >>>Hello SI-LISTers,=3D3D20 >>> >>>I thought for my part in this discussion, I should do some due=3D20=20 >>>diligence =3D3D on=3D3D20 this AC coupling capacitor placement = >>> >>> >location=3D20 > > > >>>question.=3D3D20 >>> >>>Scott McMorrow, Steve Weir and I had some off-line discussions that = >>> >>> >=3D > > >>> =20 >>> >>> >>> >>=3D3D=3D20 >>=20 >> >> >> >>>tended=3D3D20 to suggest my position dependency results may have = >>> >>> >been=3D20 > > > >>>caused by local=3D3D20 resonances from other impedance discontinuities = >>> >>> >=3D > > >>> =20 >>> >>> >>> >>in=3D20 >>=20 >> >> >> >>>the system I was=3D3D20 simulating. Based on this, I set out to = >>> >>> >develop=20 > > >>>=3D >>> =20 >>> >>> >>> >>a=3D20 >>=20 >> >> >> >>>simulation model that =3D3D had=3D3D20 a minimum of discontinuities = >>> >>> >(no=3D20 > > > >>>backplane vias/connectors/trace, etc.). =3D3D >>> >>> >>>I used:=3D3D20 >>> >>>- Spice models of non-linear 3.125Gbit/s silicon (driver and=3D20=20 >>>receiver)=3D3D20 >>>- S-parameter based package models for both driver and = >>> >>> >receiver.=3D3D20 > > >>>- A 0.01uF capacitor and its associated parasitics (via, trace, pad, = >>> >>> >=3D > > >>> =20 >>> >>> >>> >>=3D3D >> >>=20 >> >> >> >>>mount,=3D3D20 component).=3D3D20 >>>- 2D lossy W-Element transmission line (with di-electric and skin=3D20 = >>> >>> > > > >>>effect =3D3D >>> >>>losses included).=3D3D20 >>> >>>I made the capacitor model such that I could "slide" it up and down=20 >>>a=3D20 >>>15 =3D3D >>> >>>inch trace between the driver and receiver. I iteratively = >>> >>> >simulated=3D20 > > > >>>for =3D3D the=3D3D20 following length combinations:=3D3D20 >>> >>>- 500 mil trace from driver to AC coupling capacitor with 14500 mil = >>> >>> >=3D > > >>> =20 >>> >>> >>> >>=3D3D=3D20 >>=20 >> >> >> >>>trace to=3D3D20 receiver.=3D3D20 >>>- 5000 mil trace from driver to AC coupling capacitor with 10000 mil = >>> >>> >=3D > > >>> =20 >>> >>> >>> >>=3D3D >> >>=20 >> >> >> >>>trace=3D3D20 to receiver.=3D3D20 >>>- 10000 mil trace from driver to AC coupling capacitor with 5000 mil = >>> >>> >=3D > > >>> =20 >>> >>> >>> >>=3D3D >> >>=20 >> >> >> >>>trace=3D3D20 to receiver.=3D3D20 >>>- 14500 mil trace from driver to AC coupling capacitor with 500 mil = >>> >>> >=3D > > >>> =20 >>> >>> >>> >>=3D3D=3D20 >>=20 >> >> >> >>>trace to=3D3D20 receiver.=3D3D20 >>> >>>The results show around 125 mV (differential) difference between=3D20=20 >>>the=3D3D20 capacitor at the source versus the capacitor at the=3D20=20 >>>destination, with =3D3D the=3D3D20 benefit going to the capacitor=20 >>>placed=3D20 closest to the receiver. 125 mV is =3D3D a=3D3D20 lot to = >>> >>> >give=20 > > >>>away...=3D3D20 >>> >>>I am not going to pretend to understand the physics behind these=3D20=20 >>>results =3D3D but=3D3D20 I thought it worth while to at least show the = >>> >>> >=3D > > >>> =20 >>> >>> >>> >>basis=3D20 >>=20 >> >> >> >>>for my statements.=3D3D20 >>> >>>I would be happy to evolve the simulation environment if someone=20 >>>has=3D20 a=3D3D20 suggestion...=3D3D20 >>> >>>I have put together a document that I can post to an ftp site or email >>> =20 >>> >>> >>> >>=20 >> >> >> >>>=3D3D if=3D3D20 anyone would like a copy...=3D3D20 >>> >>>Kind regards,=3D3D20 >>>Steve=3D3D20 >>> >>>Stephen P. Zinck=3D3D20 >>>Interconnect Engineering Inc.=3D3D20 >>>P.O. Box 577=3D3D20 >>>South Berwick, ME 03908=3D3D20 >>>Phone - (207) 384-8280=3D3D20 >>>Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx=3D3D20 >>>Web - www.interconnectengineering.com=3D3D20 >>> >>> >>> >>> >>> =20 >>> >>> >>> >>=20 >> >> >> >------------------------------------------------------------------ >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