[SI-LIST] Re: AC Coupled Signals

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
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>               
>>> ------------------------------------------------------------------
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>>>
>>>
>>>
>>>       
>> -- 
>> 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
>>
>>
>>
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-- 
Steve Weir
Teraspeed Consulting Group LLC 
121 North River Drive 
Narragansett, RI 02882 

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