Steve, 125mV is enormous. This indicates that the capacitor represents a huge discontinuity. I would be very interested in seeing that deck so that we can understand: * How transparent or opaque the capacitor and its associated transitions are. * What the form and magnitudes of the other discontinuities are that the capacitor is interacting with. Regards, Steve. Stephen Zinck wrote: > Hello SI-LISTers, > > I thought for my part in this discussion, I should do some due diligence on > this AC coupling capacitor placement location question. > > Scott McMorrow, Steve Weir and I had some off-line discussions that tended > to suggest my position dependency results may have been caused by local > resonances from other impedance discontinuities in the system I was > simulating. Based on this, I set out to develop a simulation model that had > a minimum of discontinuities (no backplane vias/connectors/trace, etc.). > > I used: > > - Spice models of non-linear 3.125Gbit/s silicon (driver and receiver) > - S-parameter based package models for both driver and receiver. > - A 0.01uF capacitor and its associated parasitics (via, trace, pad, mount, > component). > - 2D lossy W-Element transmission line (with di-electric and skin effect > losses included). > > I made the capacitor model such that I could "slide" it up and down a 15 > inch trace between the driver and receiver. I iteratively simulated for the > following length combinations: > > - 500 mil trace from driver to AC coupling capacitor with 14500 mil trace to > receiver. > - 5000 mil trace from driver to AC coupling capacitor with 10000 mil trace > to receiver. > - 10000 mil trace from driver to AC coupling capacitor with 5000 mil trace > to receiver. > - 14500 mil trace from driver to AC coupling capacitor with 500 mil trace to > receiver. > > The results show around 125 mV (differential) difference between the > capacitor at the source versus the capacitor at the destination, with the > benefit going to the capacitor placed closest to the receiver. 125 mV is a > lot to give away... > > I am not going to pretend to understand the physics behind these results but > I thought it worth while to at least show the basis for my statements. > > I would be happy to evolve the simulation environment if someone has a > suggestion... > > I have put together a document that I can post to an ftp site or email if > anyone would like a copy... > > Kind regards, > 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 ----- > From: "ronald miller" <ron@xxxxxxxxxxx> > To: <Chris.Cheng@xxxxxxxx> > Cc: <si-list@xxxxxxxxxxxxx> > Sent: Tuesday, September 25, 2007 6:34 PM > Subject: [SI-LIST] Re: AC Coupled Signals > > > >> Chris >> If your coupling cap is a problem, get a better cap and design the pads >> to have no reflection. >> >> If there are no reflections at the lowest data rate and at the highest >> data rate, the position does >> not matter. >> >> Now, about the S11 and S22, it is much more intuitive and much easier to >> deal with TDR and >> reflection coefficients, or impedance than it is to deal with the >> network analyzer data. >> >> Although I am a microwave engineer, I have learned the hard way, and now >> I try to dtay away from >> the S-Parameters as models and for analysis because they are clumsy and >> non-intuitive. >> >> Ron >> >> Chris Cheng wrote: >> >> >>> Let me try my hand on why position matter. >>> >>> A normal passive channel is reciprocal. e.g. S12=3DS21 It only says the = >>> off diagonal elements are symmetic. It doesn't say the diagonal elements = >>> have to be equal. I believe this was the basis of Jeff Loyer's = >>> discussion a while ago. >>> >>> The presence of the discontinuity affects the S11 and S22 dramatically = >>> different based on whether it is close to the Tx or Rx. >>> >>> In the presences of imperfect loading on the Rx side, it is the = >>> interaction between the S22 and loading that matters.=20 >>> >>> Thus position makes a difference. i.e. we are tuning the S22 with the = >>> non-ideal loading. >>> >>> QED >>> >>> -----Original Message----- >>> From: si-list-bounce@xxxxxxxxxxxxx >>> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir >>> Sent: Tuesday, September 25, 2007 1:35 PM >>> To: Jory McKinley >>> Cc: Stephen Zinck; Scott McMorrow; leeritchey@xxxxxxxxxxxxx; >>> npatel@xxxxxxxxxx; si-list@xxxxxxxxxxxxx >>> Subject: [SI-LIST] Re: AC Coupled Signals >>> >>> >>> Jory, I think this is good example of where intuitively appealing=20 >>> misconceptions can seduce one into translating correlation into=20 >>> causation. If you have more ringing in one case than another, it means=20 >>> that you have set up a resonance that is more severe in the one case. =20 >>> This can easily happen as a result of any number of things going on: =20 >>> suboptimal silicon to package launch, suboptimal IC to PCB, via stubs,=20 >>> connector transitions, etc, etc. >>> >>> The very simple test is to take a VNA, a couple of sections of coax and=20 >>> a DC block. Move the DC block between the transmit end, the junction of = >>> >>> the two cables, and the receiver and look at the behavior of that net=20 >>> channel. With good coax and connectors the channel performance will=20 >>> change almost immeasureably. Now go and add a coax T on one side of the = >>> >>> DC block. Move that whole thing around as a unit and again the channel=20 >>> performance remains the same. Add a second coax T on the other side of=20 >>> the DC block from the first, and again move the whole thing around. The = >>> >>> results will still remain uniform. Now if you go and move one of those = >>> >>> T's someplace else, then the pesky mole you're trying to whack moves and = >>> >>> the resonance will pop up somewhere else. The bottom line is that it's=20 >>> resonance that we need to fight and resonance doesn't know left from = >>> right. >>> >>> Regards, >>> >>> >>> Steve. >>> Jory McKinley wrote: >>> >>> >>> >>>> To add to this, I will ask for release of lab data that I took that=20 >>>> shows RX_EYE clearly improves as the AC cap/term location is moved=20 >>>> closer to the RX. The data indicates that even though overall channel = >>>> >>>> >>>> >>> >>> >>>> loss may not be affected, the 50ps edge rates we are sending through=20 >>>> the channel are affected (in terms of time domain ringing) by the AC=20 >>>> cap/term placement. This kind of feels right. >>>> -Jory >>>> =20 >>>> >>>> ----- Original Message ---- >>>> From: steve weir <weirsi@xxxxxxxxxx> >>>> To: Stephen Zinck <signalintegrity@xxxxxxxxxxx> >>>> Cc: Scott McMorrow <scott@xxxxxxxxxxxxx>; jory_mckinley@xxxxxxxxx;=20 >>>> leeritchey@xxxxxxxxxxxxx; npatel@xxxxxxxxxx; si-list@xxxxxxxxxxxxx >>>> Sent: Tuesday, September 25, 2007 12:52:41 PM >>>> Subject: Re: [SI-LIST] Re: AC Coupled Signals >>>> >>>> Steve, as far as I know where we have agreement that capacitor = >>>> >>>> >>>> >>> location >>> >>> >>> >>>> can only affect performance where the combined capacitor and mount >>>> presents a discontinuity and that discontinuity is located such that = >>>> >>>> >>>> >>> it >>> >>> >>> >>>> forms a resonant structure with another discontinuity in the channel. = >>>> >>>> >>>> >>> I >>> >>> >>> >>>> fail to see where we have moved any closer to supporting your premise >>>> that locating a greater proportion of fixed loss before the capacitor >>>> changes end to end loss than placing that same fixed loss behind it. >>>> >>>> As for lab measurements, we have these as we have characterized many >>>> links. We also have extensive simulations. >>>> >>>> Regards, >>>> >>>> >>>> Steve. >>>> >>>> Stephen Zinck wrote: >>>> >>>> >>>> >>>>> Hi Steve, >>>>> >>>>> I understand your point but I actually thought Scott and I were >>>>> getting close. I guess I still need him to explain his statement: >>>>> "The only time position matters is in the face of discontinuities." >>>>> because this runs counter to your assertion. >>>>> >>>>> It would be good to have some concrete lab measurement results to = >>>>> >>>>> >>>>> >>> back >>> >>> >>> >>>>> either of our points up. I am sorry I don't have any. >>>>> >>>>> We agree on TDR/VNA characteristics... >>>>> >>>>> 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=20 >>>>> >>>>> >>>>> >>>> <http://www.interconnectengineering.com> >>>> >>>> >>>> >>>>> ----- Original Message ----- From: "steve weir" <weirsi@xxxxxxxxxx> >>>>> To: <signalintegrity@xxxxxxxxxxx> >>>>> Cc: "Scott McMorrow" <scott@xxxxxxxxxxxxx>; = >>>>> >>>>> >>>>> >>> <jory_mckinley@xxxxxxxxx>; >>> >>> >>> >>>>> <leeritchey@xxxxxxxxxxxxx>; <npatel@xxxxxxxxxx>; = >>>>> >>>>> >>>>> >>> <si-list@xxxxxxxxxxxxx> >>> >>> >>> >>>>> Sent: Tuesday, September 25, 2007 12:24 PM >>>>> Subject: [SI-LIST] Re: AC Coupled Signals >>>>> >>>>> >>>>> >>>>> >>>>> >>>>>> Stephen, OK so when you say "lossy" or "nonlinear" you mean=3D20 >>>>>> discontinuous. Discontinuities aggravate resonances based on >>>>>> specific=3D20 >>>>>> structure material and geometries, in other words the distance on = >>>>>> >>>>>> >>>>>> >>> a=3D20 >>> >>> >>> >>>>>> centimeter or millimeter scale between discontinuities. We = >>>>>> >>>>>> >>>>>> >>> have=3D20 >>> >>> >>> >>>>>> essentially the same opportunities for channel discontinuities at = >>>>>> >>>>>> >>>>>> >>> and >>> >>> >>> >>>>>> in =3D >>>>>> >>>>>> the vicinity of the transmitter as the receiver. So I still do not >>>>>> see=3D20 >>>>>> a defensible basis for the offered position: that placing a = >>>>>> >>>>>> >>>>>> >>> capacitor >>> >>> >>> >>>>>> at =3D >>>>>> >>>>>> one end of the line versus the other changes the end to end loss. =20 >>>>>> What=3D20 >>>>>> matters is if wherever I place one discontinuity that it sets up a >>>>>> sharp =3D >>>>>> >>>>>> resonance with another discontinuity. That can happen equally well >>>>>> at=3D20 >>>>>> either end of the line. >>>>>> >>>>>> If one looks at a channel with only a TDR I might understand = >>>>>> >>>>>> >>>>>> >>> the=3D20 >>> >>> >>> >>>>>> erroneous perception that placing a discontinuity down the line = >>>>>> >>>>>> >>>>>> >>> is=3D20 >>> >>> >>> >>>>>> better than up front. But that is an illusion. TDR resolution=20 >>>>>> >>>>>> >>>>>> >>>> falls=3D20 >>>> >>>> >>>> >>>>>> with interconnect distance. This ia a result of the inherent loss=20 >>>>>> >>>>>> >>>>>> >>>> of=3D20 >>>> >>>> >>>> >>>>>> the interconnect that shelves bandwidth and hence resolution = >>>>>> >>>>>> >>>>>> >>> versus=3D20 >>> >>> >>> >>>>>> distance for the instrument. This is one of the big limitations of = >>>>>> >>>>>> >>>>>> >>> >>> >>>> a=3D20 >>>> >>>> >>>> >>>>>> TDR for channel evaluation. A through measurement with a TDT or = >>>>>> >>>>>> >>>>>> >>> VNA=3D20 >>> >>> >>> >>>>>> does not suffer that limitation, give true measure of S21 and so >>>>>> report=3D20 >>>>>> the real channel performance. Eric Bogatin spends some time on = >>>>>> >>>>>> >>>>>> >>> the=3D20 >>> >>> >>> >>>>>> issue of bandwidth versus interconnect length in his book. >>>>>> >>>>>> Regards, >>>>>> >>>>>> >>>>>> Steve. >>>>>> Stephen Zinck wrote: >>>>>> >>>>>> >>>>>> >>>>>>> Scott, >>>>>>> We may have some nomenclature issues here...=3D20 >>>>>>> >>>>>>> When I say "lossy interface to the capacitor" I mean with = >>>>>>> >>>>>>> >>>>>>> >>> impedance >>> >>> >>> >>>>>>> dis=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> 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=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> 1th hour. I don't recommend this, I just work within the customer's >>>>>> needs=3D >>>>>> /requirements. I make real world recommendations from simulation >>>>>> results =3D >>>>>> for designs where these discontinuities you mention are a fact of >>>>>> life. G=3D >>>>>> ranted my customers are not doing 5+ Gbit/s designs (right now ;-). >>>>>> >>>>>> >>>>>> >>>>>>> Above these data-rates, all you mention, capacitor transition = >>>>>>> >>>>>>> >>>>>>> >>> (pad, >>> >>> >>> >>>>>>> via=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> s, etc) are of the utmost importance. And I would absolutely agree >>>>>> that t=3D >>>>>> he more perfect you make these transitions, the less it matters = >>>>>> >>>>>> >>>>>> >>> where >>> >>> >>> >>>>>> the=3D >>>>>> y are placed... >>>>>> >>>>>> >>>>>> >>>>>>> So I do believe AC coupling capacitor position does matter, as you >>>>>>> stat=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> e, for the bulk of the designs occurring these days where component >>>>>> footp=3D >>>>>> 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=20 >>>>>>> >>>>>>> >>>>>>> >>>> <http://www.interconnectengineering.com> >>>> >>>> >>>> >>>>>>> ----- Original Message -----=3D20 >>>>>>> From: Scott McMorrow=3D20 >>>>>>> To: Stephen Zinck=3D20 >>>>>>> Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; >>>>>>> npatel@micro=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> n.com ; si-list@xxxxxxxxxxxxx=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> 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 =3D >>>>>>> >>>>>>> >>>>>>> >>>>>> to the capacitor, it shouldn't matter where you place it, given a >>>>>> purely =3D >>>>>> linear system. But the real world is lossy, even when one makes = >>>>>> >>>>>> >>>>>> >>> great >>> >>> >>> >>>>>> 3D =3D >>>>>> solved structures. Manufacturing and other tolerances tend to take >>>>>> the tr=3D >>>>>> ek towards perfection to task." >>>>>> >>>>>> >>>>>> >>>>>>> "Would either of you agree that AC coupling capacitor location = >>>>>>> >>>>>>> >>>>>>> >>> may >>> >>> >>> >>>>>>> ma=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> tter with a lossy interface to the capacitor?" >>>>>> >>>>>> >>>>>> >>>>>>> Insertion loss in a flat impedance linear lossy system will be >>>>>>> indepe=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> ndent of capacitor location. Run the math and see. The only time >>>>>> positi=3D >>>>>> on matters is in the face of discontinuities. In fact, given a = >>>>>> >>>>>> >>>>>> >>> low >>> >>> >>> >>>>>> loss=3D >>>>>> interconnect with discontinuities and a high loss interconnect with >>>>>> disc=3D >>>>>> ontinuities, the low loss system, with it's higher Q, will often = >>>>>> >>>>>> >>>>>> >>> have >>> >>> >>> >>>>>> wor=3D >>>>>> se behavior. >>>>>> >>>>>> >>>>>> >>>>>>> An improperly designed 0402 capacitor transition for a 50 ohm = >>>>>>> >>>>>>> >>>>>>> >>> line >>> >>> >>> >>>>>>> ca=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> n easily exhibit a discontinuity of 35 ohms for 50 ps. If attached >>>>>> to po=3D >>>>>> orly designed via transitions, the discontinuity will be even = >>>>>> >>>>>> >>>>>> >>> worse. =20 >>> >>> >>> >>>>>> Whe=3D >>>>>> n this is coupled closely to a high capacitance receiver input, a >>>>>> high ca=3D >>>>>> pacitance transmitter output, a low impedance via stub = >>>>>> >>>>>> >>>>>> >>> discontinuity, >>> >>> >>> >>>>>> or =3D >>>>>> a low impedance connector discontinuity, it can form a 1/2 wave >>>>>> resonant =3D >>>>>> circuit. This is most likely the problem you are seeing. =3D20 >>>>>> >>>>>> >>>>>> >>>>>>> If the interconnect has essentially flat impedance, position = >>>>>>> >>>>>>> >>>>>>> >>> does >>> >>> >>> >>>>>>> not=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> matter. If the capacitor transition is properly designed, position >>>>>> does=3D >>>>>> not matter. All of the data we have on this is proprietary at this >>>>>> time=3D >>>>>> =3D2E Our understanding of the physics has been verified by full = >>>>>> >>>>>> >>>>>> >>> wave >>> >>> >>> >>>>>> mode=3D >>>>>> 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=3DAE is the registered service mark of >>>>>>> Teraspeed Consulting Group LLC >>>>>>> >>>>>>> >>>>>>> Stephen Zinck wrote:=3D20 >>>>>>> Hi Scott and Steve, >>>>>>> >>>>>>> To answer both of your questions, it is the resulting Hspice >>>>>>> (with =3D >>>>>>> >>>>>>> >>>>>>> >>>>>> S-parameters) differential eye patterns, as viewed at the receiver >>>>>> die, t=3D >>>>>> hat were used to make a comparison of source versus destination AC >>>>>> coupli=3D >>>>>> ng capacitor locations. The system was excited with a string of = >>>>>> >>>>>> >>>>>> >>> ones, >>> >>> >>> >>>>>> fol=3D >>>>>> lowed by a single zero, followed by a string of ones.=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> I have not specifically designed a test board that varies the = >>>>>>> >>>>>>> >>>>>>> >>> AC >>> >>> >>> >>>>>>> co=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> upling capacitor location along a trace. >>>>>> >>>>>> >>>>>> >>>>>>> I understand the "shades of gray" here and agree that one = >>>>>>> >>>>>>> >>>>>>> >>> can't >>> >>> >>> >>>>>>> mak=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> e a "rule of thumb" generalization in our line of work these = >>>>>> >>>>>> >>>>>> >>> days.=3D20 >>> >>> >>> >>>>>>> I agree in theory with all you state. Assuming a lossless >>>>>>> interface=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> to the capacitor, it shouldn't matter where you place it, given a >>>>>> purely=3D >>>>>> linear system. But the real world is lossy, even when one makes = >>>>>> >>>>>> >>>>>> >>> great >>> >>> >>> >>>>>> 3D=3D >>>>>> solved structures. Manufacturing and other tolerances tend to take >>>>>> the t=3D >>>>>> rek towards perfection to task.=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> Do either of you have real world measured results, that you >>>>>>> could s=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> hare, that show no marked difference in received signal >>>>>> characteristics w=3D >>>>>> hen the AC coupling capacitor position is varied through a 30 inch >>>>>> backpl=3D >>>>>> ane system (or similar)? >>>>>> >>>>>> >>>>>> >>>>>>> I believe my experience with capacitor location may prove true >>>>>>> if t=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> he capacitor interface is lossy (which is the case). A lot of my >>>>>> customer=3D >>>>>> s are just looking for quick ways to maximize performance using >>>>>> standard =3D >>>>>> component packages and standard layout practices (in the end, I = >>>>>> >>>>>> >>>>>> >>> don't >>> >>> >>> >>>>>> lik=3D >>>>>> e to give anything away that is low lying fruit). Most of the time = >>>>>> >>>>>> >>>>>> >>> I >>> >>> >>> >>>>>> am d=3D >>>>>> oing my analysis after the board is in layout, where I have limited >>>>>> abili=3D >>>>>> ty to change the design (unless it is really broken). In a perfect >>>>>> world,=3D >>>>>> where I am involved early, the package optimization and layout >>>>>> structure=3D >>>>>> s can be optimized as you state, but only if the margins warrant it >>>>>> (syst=3D >>>>>> em performance issues are expected after initial "what-if" >>>>>> simulations ha=3D >>>>>> ve occurred). The right tool for the right job rules the day... >>>>>> >>>>>> >>>>>> >>>>>>> Would either of you agree that AC coupling capacitor location >>>>>>> may m=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> 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=20 >>>>>>> >>>>>>> >>>>>>> >>>> <http://www.interconnectengineering.com> >>>> >>>> >>>> >>>>>>> ----- Original Message -----=3D20 >>>>>>> From: Scott McMorrow=3D20 >>>>>>> To: Stephen Zinck=3D20 >>>>>>> Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; >>>>>>> npatel@m=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> icron.com ; si-list@xxxxxxxxxxxxx=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> 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 =3D >>>>>>> >>>>>>> >>>>>>> >>>>>> to linear system theory and measurements. =3D20 >>>>>> >>>>>> >>>>>> >>>>>>> I contend that the only difference an AC coupling capacitor >>>>>>> can p=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> ossibly have due to position in a linear interconnect is a result = >>>>>> >>>>>> >>>>>> >>> of >>> >>> >>> >>>>>> impe=3D >>>>>> dance mismatch. I contend that the capacitor will form a 1/2 wave >>>>>> resona=3D >>>>>> nt circuit with other interconnect discontinuities (connectors, = >>>>>> >>>>>> >>>>>> >>> vias >>> >>> >>> >>>>>> stub=3D >>>>>> s, packages, Tx die, Rx die ... etc) and that this interaction is >>>>>> system,=3D >>>>>> chip, connector and package design dependent. I contend that it is >>>>>> this=3D >>>>>> 1/2 resonance that can cause differences that can be measured, but >>>>>> that =3D >>>>>> there is no "rule of thumb", since the position and magnitude of >>>>>> disconti=3D >>>>>> nuities are different in every system. In some systems the = >>>>>> >>>>>> >>>>>> >>> receiver >>> >>> >>> >>>>>> cons=3D >>>>>> titutes a larger discontinuity than the transmitter. In other >>>>>> systems th=3D >>>>>> is is reversed. In yet other systems, connectors and vias = >>>>>> >>>>>> >>>>>> >>> represent >>> >>> >>> >>>>>> larg=3D >>>>>> er discontinuites than do either the transmitters or receivers. It >>>>>> all "j=3D >>>>>> ust depends". To state a specific rule is just plain incorrect. >>>>>> >>>>>> >>>>>> >>>>>>> I contend that once you remove the magic and myths = >>>>>>> >>>>>>> >>>>>>> >>> surrounding >>> >>> >>> >>>>>>> AC=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> coupling capacitors, analysis of the 3D structure shows that by >>>>>> reducing=3D >>>>>> the signal path discontinuity through the capacitor, you will >>>>>> necessaril=3D >>>>>> y improve performance. An AC coupling capacitor, with it's >>>>>> associated vi=3D >>>>>> a and pad transition design, can be viewed as a black box which has >>>>>> inser=3D >>>>>> tion loss and return loss, and can be modeled quite well using = >>>>>> >>>>>> >>>>>> >>> either >>> >>> >>> >>>>>> lum=3D >>>>>> ped element approximations or (my favorite) S-parameters. As such = >>>>>> >>>>>> >>>>>> >>> it >>> >>> >>> >>>>>> wil=3D >>>>>> l cascade in a simulation model just like any other linear element. = >>>>>> >>>>>> >>>>>> >>> =20 >>> >>> >>> >>>>>> If w=3D >>>>>> e start with a system with flat 50 ohm impedance from end to end, = >>>>>> >>>>>> >>>>>> >>> it >>> >>> >>> >>>>>> can =3D >>>>>> be easily shown that no matter what the position of the capacitor >>>>>> along t=3D >>>>>> he interconnect is, the insertion loss of the system is identical. = >>>>>> >>>>>> >>>>>> >>> >>> >>>>>> It is=3D >>>>>> only the return loss, as seen from each end that changes. >>>>>> >>>>>> >>>>>> >>>>>>> I've been designing AC coupling capacitor mounting = >>>>>>> >>>>>>> >>>>>>> >>> transitions >>> >>> >>> >>>>>>> pr=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> operly for quite a few years now and have some 0402 designs that = >>>>>> >>>>>> >>>>>> >>> keep >>> >>> >>> >>>>>> S12=3D >>>>>> above -0.2 dB up to 7.5 GHz, S12 below -20 dB @ 5 GHz, and below = >>>>>> >>>>>> >>>>>> >>> -15 >>> >>> >>> >>>>>> dB =3D >>>>>> @ 10 GHz. For all practical purposes, these designs are = >>>>>> >>>>>> >>>>>> >>> transparent >>> >>> >>> >>>>>> and =3D >>>>>> may be placed anywhere in an interconnect design where there is >>>>>> space, si=3D >>>>>> nce there is little resonance interaction with other devices and >>>>>> structur=3D >>>>>> 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=3DAE is the registered service mark of >>>>>>> Teraspeed Consulting Group LLC >>>>>>> =3D20 >>>>>>> >>>>>>> Stephen Zinck wrote:=3D20 >>>>>>> Hi Scott, >>>>>>> >>>>>>> My simulations show that the capacitor is best placed at = >>>>>>> >>>>>>> >>>>>>> >>> the >>> >>> >>> >>>>>>> re=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> 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=20 >>>>>>> >>>>>>> >>>>>>> >>>> <http://www.interconnectengineering.com> >>>> >>>> >>>> >>>>>>> ----- Original Message -----=3D20 >>>>>>> From: Scott McMorrow=3D20 >>>>>>> To: signalintegrity@xxxxxxxxxxx=3D20 >>>>>>> Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; >>>>>>> npat=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> el@xxxxxxxxxx ; si-list@xxxxxxxxxxxxx=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> 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=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> nances that are created by discontinuities and modal conversion >>>>>> (which ha=3D >>>>>> ve absolutely zero to do with signal rise time), there is no >>>>>> difference i=3D >>>>>> n the attenuation of a capacitor placed at the Tx as opposed at = >>>>>> >>>>>> >>>>>> >>> the >>> >>> >>> >>>>>> Rx. =3D >>>>>> W.R.T. the receiver, if it is "lost in the rise-time degradation of >>>>>> the =3D >>>>>> 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=3DAE is the registered service mark of >>>>>>> Teraspeed Consulting Group LLC >>>>>>> =3D20 >>>>>>> >>>>>>> Stephen Zinck wrote:=3D20 >>>>>>> Hi Jory, >>>>>>> >>>>>>> I have simulated this at length and concur with your experience = >>>>>>> >>>>>>> >>>>>>> >>> that >>> >>> >>> >>>>>>> th=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> e=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> capacitor is best placed at the receiver... >>>>>>> >>>>>>> In effect, the attenuation associated with the capacitor placement >>>>>>> at t=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> he=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> receiver (parasitics/pads/vias) is lost in the rise-time = >>>>>>> >>>>>>> >>>>>>> >>> degradation >>> >>> >>> >>>>>>> of=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> the=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> system. >>>>>>> The classic "don't break it until you have to" rule is = >>>>>>> >>>>>>> >>>>>>> >>> applicable... >>> >>> >>> >>>>>>> OK=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> this=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> 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=20 >>>>>>> >>>>>>> >>>>>>> >>>> <http://www.interconnectengineering.com> >>>> >>>> >>>> >>>>>>> ----- Original Message -----=3D20 >>>>>>> From: "Jory McKinley" <jory_mckinley@xxxxxxxxx> >>>>>>> To: <leeritchey@xxxxxxxxxxxxx>; <npatel@xxxxxxxxxx>; >>>>>>> <si-list@freelists=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> =3D2Eorg> >>>>>> >>>>>> >>>>>> >>>>>>> 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=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> ain)=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> in the lab some effects that appears to be location specific in = >>>>>>> >>>>>>> >>>>>>> >>> the=3D20 >>> >>> >>> >>>>>>> placement of the AC coupling caps at the rcvr. Now this may be = >>>>>>> >>>>>>> >>>>>>> >>> due >>> >>> >>> >>>>>>> in =3D >>>>>>> >>>>>>> >>>>>>> >>>>>> part=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> to the fact that I am using 50-ohm resistor termination in each = >>>>>>> >>>>>>> >>>>>>> >>> lead >>> >>> >>> >>>>>>> as=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> =3D20 >>>>>> >>>>>> >>>>>> >>>>>>> well and the combination (cap plus rcvr reflection) is giving = >>>>>>> >>>>>>> >>>>>>> >>> some=3D20 >>> >>> >>> >>>>>>> imbalance depending on distance. The best rcvr eye that I am = >>>>>>> >>>>>>> >>>>>>> >>> seeing >>> >>> >>> >>>>>>> is=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> =3D20 >>>>>> >>>>>> >>>>>> >>>>>>> when I can move the AC/term as close to the rcvr as I can. By the >>>>>>> way =3D >>>>>>> >>>>>>> >>>>>>> these are 5Gb/s signals. >>>>>>> If I have time I will try and isolate what I am seeing and even >>>>>>> simulat=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> e=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> 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=3D >>>>>>> >>>>>>> >>>>>>> other without, the loss vs. frequency is virtually identical at = >>>>>>> >>>>>>> >>>>>>> >>> leas >>> >>> >>> >>>>>>> ou=3D >>>>>>> >>>>>>> >>>>>>> >>>>>> t=3D20 >>>>>> >>>>>> >>>>>> >>>>>>> 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 >>> >>> >>> >>> >>> >>> >> -- >> Ronald Miller >> Ghz Data, Signal Integrity Consulting >> 7721 Sunset Ave. >> Newark CA 94560 >> tel 510-793-4744 >> cell 510-377-9380 >> fax 510-742-6686 >> www.ghzdata.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 >> >> >> >> > > > ------------------------------------------------------------------ > 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