Stephen, OK so when you say "lossy" or "nonlinear" you mean=20 discontinuous. Discontinuities aggravate resonances based on specific=20 structure material and geometries, in other words the distance on a=20 centimeter or millimeter scale between discontinuities. We have=20 essentially the same opportunities for channel discontinuities at and in = the vicinity of the transmitter as the receiver. So I still do not see=20 a defensible basis for the offered position: that placing a capacitor at = one end of the line versus the other changes the end to end loss. What=20 matters is if wherever I place one discontinuity that it sets up a sharp = resonance with another discontinuity. That can happen equally well at=20 either end of the line. If one looks at a channel with only a TDR I might understand the=20 erroneous perception that placing a discontinuity down the line is=20 better than up front. But that is an illusion. TDR resolution falls=20 with interconnect distance. This ia a result of the inherent loss of=20 the interconnect that shelves bandwidth and hence resolution versus=20 distance for the instrument. This is one of the big limitations of a=20 TDR for channel evaluation. A through measurement with a TDT or VNA=20 does not suffer that limitation, give true measure of S21 and so report=20 the real channel performance. Eric Bogatin spends some time on the=20 issue of bandwidth versus interconnect length in his book. Regards, Steve. Stephen Zinck wrote: > Scott, > We may have some nomenclature issues here...=20 > > When I say "lossy interface to the capacitor" I mean with impedance dis= continuities. So I think we are on a similar page given your statement: > > "The only time position matters is in the face of discontinuities." > > Again, most often, my role is to simulate the customers system at the 1= 1th hour. I don't recommend this, I just work within the customer's needs= /requirements. I make real world recommendations from simulation results = for designs where these discontinuities you mention are a fact of life. G= ranted my customers are not doing 5+ Gbit/s designs (right now ;-). > > Above these data-rates, all you mention, capacitor transition (pad, via= s, etc) are of the utmost importance. And I would absolutely agree that t= he more perfect you make these transitions, the less it matters where the= y are placed... > > So I do believe AC coupling capacitor position does matter, as you stat= e, for the bulk of the designs occurring these days where component footp= rint and via optimization, etc. is NOT occurring... > > Steve > > Stephen P. Zinck > Interconnect Engineering Inc. > P.O. Box 577 > South Berwick, ME 03908 > Phone - (207) 384-8280 > Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx > Web - www.interconnectengineering.com > > ----- Original Message -----=20 > From: Scott McMorrow=20 > To: Stephen Zinck=20 > Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; npatel@micro= n.com ; si-list@xxxxxxxxxxxxx=20 > Sent: Tuesday, September 25, 2007 11:08 AM > Subject: Re: [SI-LIST] Re: AC Coupled Signals > > > Steven, > > I would not agree with your following statements. > > "I agree in theory with all you state. Assuming a lossless interface = to the capacitor, it shouldn't matter where you place it, given a purely = linear system. But the real world is lossy, even when one makes great 3D = solved structures. Manufacturing and other tolerances tend to take the tr= ek towards perfection to task." > > "Would either of you agree that AC coupling capacitor location may ma= tter with a lossy interface to the capacitor?" > > > Insertion loss in a flat impedance linear lossy system will be indepe= ndent of capacitor location. Run the math and see. The only time positi= on matters is in the face of discontinuities. In fact, given a low loss= interconnect with discontinuities and a high loss interconnect with disc= ontinuities, the low loss system, with it's higher Q, will often have wor= se behavior. > > An improperly designed 0402 capacitor transition for a 50 ohm line ca= n easily exhibit a discontinuity of 35 ohms for 50 ps. If attached to po= orly designed via transitions, the discontinuity will be even worse. Whe= n this is coupled closely to a high capacitance receiver input, a high ca= pacitance transmitter output, a low impedance via stub discontinuity, or = a low impedance connector discontinuity, it can form a 1/2 wave resonant = circuit. This is most likely the problem you are seeing. =20 > > If the interconnect has essentially flat impedance, position does not= matter. If the capacitor transition is properly designed, position does= not matter. All of the data we have on this is proprietary at this time= =2E Our understanding of the physics has been verified by full wave mode= ling, simulation and measurement. > > > > regards, > > Scott > > > > Scott McMorrow > Teraspeed Consulting Group LLC > 121 North River Drive > Narragansett, RI 02882 > (401) 284-1827 Business > (401) 284-1840 Fax > > http://www.teraspeed.com > > Teraspeed=AE is the registered service mark of > Teraspeed Consulting Group LLC > > > Stephen Zinck wrote:=20 > Hi Scott and Steve, > > To answer both of your questions, it is the resulting Hspice (with = S-parameters) differential eye patterns, as viewed at the receiver die, t= hat were used to make a comparison of source versus destination AC coupli= ng capacitor locations. The system was excited with a string of ones, fol= lowed by a single zero, followed by a string of ones.=20 > > I have not specifically designed a test board that varies the AC co= upling capacitor location along a trace. > > I understand the "shades of gray" here and agree that one can't mak= e a "rule of thumb" generalization in our line of work these days.=20 > > I agree in theory with all you state. Assuming a lossless interface= to the capacitor, it shouldn't matter where you place it, given a purely= linear system. But the real world is lossy, even when one makes great 3D= solved structures. Manufacturing and other tolerances tend to take the t= rek towards perfection to task.=20 > > Do either of you have real world measured results, that you could s= hare, that show no marked difference in received signal characteristics w= hen the AC coupling capacitor position is varied through a 30 inch backpl= ane system (or similar)? > > I believe my experience with capacitor location may prove true if t= he capacitor interface is lossy (which is the case). A lot of my customer= s are just looking for quick ways to maximize performance using standard = component packages and standard layout practices (in the end, I don't lik= e to give anything away that is low lying fruit). Most of the time I am d= oing my analysis after the board is in layout, where I have limited abili= ty to change the design (unless it is really broken). In a perfect world,= where I am involved early, the package optimization and layout structure= s can be optimized as you state, but only if the margins warrant it (syst= em performance issues are expected after initial "what-if" simulations ha= ve occurred). The right tool for the right job rules the day... > > Would either of you agree that AC coupling capacitor location may m= atter with a lossy interface to the capacitor? > > All the best, > Steve > > Stephen P. Zinck > Interconnect Engineering Inc. > P.O. Box 577 > South Berwick, ME 03908 > Phone - (207) 384-8280 > Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx > Web - www.interconnectengineering.com > > ----- Original Message -----=20 > From: Scott McMorrow=20 > To: Stephen Zinck=20 > Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; npatel@m= icron.com ; si-list@xxxxxxxxxxxxx=20 > Sent: Tuesday, September 25, 2007 9:44 AM > Subject: Re: [SI-LIST] Re: AC Coupled Signals > > > Stephen > > Define "better" and then relate your simulations and conclusions = to linear system theory and measurements. =20 > > I contend that the only difference an AC coupling capacitor can p= ossibly have due to position in a linear interconnect is a result of impe= dance mismatch. I contend that the capacitor will form a 1/2 wave resona= nt circuit with other interconnect discontinuities (connectors, vias stub= s, packages, Tx die, Rx die ... etc) and that this interaction is system,= chip, connector and package design dependent. I contend that it is this= 1/2 resonance that can cause differences that can be measured, but that = there is no "rule of thumb", since the position and magnitude of disconti= nuities are different in every system. In some systems the receiver cons= titutes a larger discontinuity than the transmitter. In other systems th= is is reversed. In yet other systems, connectors and vias represent larg= er discontinuites than do either the transmitters or receivers. It all "j= ust depends". To state a specific rule is just plain incorrect. > > I contend that once you remove the magic and myths surrounding AC= coupling capacitors, analysis of the 3D structure shows that by reducing= the signal path discontinuity through the capacitor, you will necessaril= y improve performance. An AC coupling capacitor, with it's associated vi= a and pad transition design, can be viewed as a black box which has inser= tion loss and return loss, and can be modeled quite well using either lum= ped element approximations or (my favorite) S-parameters. As such it wil= l cascade in a simulation model just like any other linear element. If w= e start with a system with flat 50 ohm impedance from end to end, it can = be easily shown that no matter what the position of the capacitor along t= he interconnect is, the insertion loss of the system is identical. It is= only the return loss, as seen from each end that changes. > > I've been designing AC coupling capacitor mounting transitions pr= operly for quite a few years now and have some 0402 designs that keep S12= above -0.2 dB up to 7.5 GHz, S12 below -20 dB @ 5 GHz, and below -15 dB = @ 10 GHz. For all practical purposes, these designs are transparent and = may be placed anywhere in an interconnect design where there is space, si= nce there is little resonance interaction with other devices and structur= es. > > > Scott > > > Scott McMorrow > Teraspeed Consulting Group LLC > 121 North River Drive > Narragansett, RI 02882 > (401) 284-1827 Business > (401) 284-1840 Fax > > http://www.teraspeed.com > > Teraspeed=AE is the registered service mark of > Teraspeed Consulting Group LLC > =20 > > Stephen Zinck wrote:=20 > Hi Scott, > > My simulations show that the capacitor is best placed at the re= ceiver end of the transmission-line. Do you disagree? If so, why? > > Steve > > Stephen P. Zinck > Interconnect Engineering Inc. > P.O. Box 577 > South Berwick, ME 03908 > Phone - (207) 384-8280 > Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx > Web - www.interconnectengineering.com > > ----- Original Message -----=20 > From: Scott McMorrow=20 > To: signalintegrity@xxxxxxxxxxx=20 > Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; npat= el@xxxxxxxxxx ; si-list@xxxxxxxxxxxxx=20 > Sent: Tuesday, September 25, 2007 8:30 AM > Subject: Re: [SI-LIST] Re: AC Coupled Signals > > > Stephen, > > I'm sorry, this is a linear system. Except for possible reso= nances that are created by discontinuities and modal conversion (which ha= ve absolutely zero to do with signal rise time), there is no difference i= n the attenuation of a capacitor placed at the Tx as opposed at the Rx. = W.R.T. the receiver, if it is "lost in the rise-time degradation of the = system", it will be lost wherever it is placed. > > > > Scott McMorrow > Teraspeed Consulting Group LLC > 121 North River Drive > Narragansett, RI 02882 > (401) 284-1827 Business > (401) 284-1840 Fax > > http://www.teraspeed.com > > Teraspeed=AE is the registered service mark of > Teraspeed Consulting Group LLC > =20 > > Stephen Zinck wrote:=20 > Hi Jory, > > I have simulated this at length and concur with your experience that th= e=20 > capacitor is best placed at the receiver... > > In effect, the attenuation associated with the capacitor placement at t= he=20 > receiver (parasitics/pads/vias) is lost in the rise-time degradation of= the=20 > system. > The classic "don't break it until you have to" rule is applicable... OK= this=20 > is my rule... :-) > > All the best, > Steve > > Stephen P. Zinck > Interconnect Engineering Inc. > P.O. Box 577 > South Berwick, ME 03908 > Phone - (207) 384-8280 > Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx > Web - www.interconnectengineering.com > > ----- Original Message -----=20 > From: "Jory McKinley" <jory_mckinley@xxxxxxxxx> > To: <leeritchey@xxxxxxxxxxxxx>; <npatel@xxxxxxxxxx>; <si-list@freelists= =2Eorg> > Sent: Monday, September 24, 2007 5:31 PM > Subject: [SI-LIST] Re: AC Coupled Signals > > > I will elaborate a bit on what I have seen. I have measured (time dom= ain)=20 > in the lab some effects that appears to be location specific in the=20 > placement of the AC coupling caps at the rcvr. Now this may be due in = part=20 > to the fact that I am using 50-ohm resistor termination in each lead as= =20 > well and the combination (cap plus rcvr reflection) is giving some=20 > imbalance depending on distance. The best rcvr eye that I am seeing is= =20 > when I can move the AC/term as close to the rcvr as I can. By the way = > these are 5Gb/s signals. > If I have time I will try and isolate what I am seeing and even simulat= e=20 > it, has anyone else seen or simulated this? > -Jory > > ----- Original Message ---- > From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx> > To: "npatel@xxxxxxxxxx" <npatel@xxxxxxxxxx>; si-list@xxxxxxxxxxxxx > Sent: Monday, September 24, 2007 1:06:06 PM > Subject: [SI-LIST] Re: AC Coupled Signals > > Nikil, > > I have made measurements on test PCBs and the location is not all that > important. In identical pairs, one with AC coupling capacitors and the= > other without, the loss vs. frequency is virtually identical at leas ou= t=20 > to > 6 GHz. That would be 12 Mb/S. > > Lee Ritchey > > > [Original Message] > From: <npatel@xxxxxxxxxx> > To: <si-list@xxxxxxxxxxxxx> > Date: 9/24/2007 10:21:37 AM > Subject: [SI-LIST] AC Coupled Signals > > Hi all, > In case of AC coupled signals does anyone know of an optimum placement > for the caps? I mean should they be placed near the source, receiver, > middle of the transmission line? > How much difference does it make in the opening of the eye? > The signals are differential CML running at 3.0Gbps > > Thanks, > Nikhil > > > ------------------------------------------------------------------ > 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 > > > > > > > > > > > _______________________________________________________________________= _____________ > Moody friends. Drama queens. Your life? Nope! - their life, your story.= =20 > Play Sims Stories at Yahoo! Games. > http://sims.yahoo.com/ > ------------------------------------------------------------------ > 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 > > > > =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: =20 > //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 > =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: =20 > //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 > =20 > > > > =20 --=20 Steve Weir Teraspeed Consulting Group LLC=20 121 North River Drive=20 Narragansett, RI 02882=20 California office (408) 884-3985 Business (707) 780-1951 Fax Main office (401) 284-1827 Business=20 (401) 284-1840 Fax=20 Oregon office (503) 430-1065 Business (503) 430-1285 Fax http://www.teraspeed.com This e-mail contains proprietary and confidential intellectual property o= f 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