[SI-LIST] Re: AC Coupled Signals

Scott,
We may have some nomenclature issues here... 

When I say "lossy interface to the capacitor" I mean with impedance 
discontinuities. 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 11th 
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. Granted 
my customers are not doing 5+ Gbit/s designs (right now ;-).

Above these data-rates, all you mention, capacitor transition (pad, vias, etc) 
are of the utmost importance. And I would absolutely agree that the more 
perfect you make these transitions, the less it matters where they are placed...

So I do believe AC coupling capacitor position does matter, as you state, for 
the bulk of the designs occurring these days where component footprint 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 ----- 
  From: Scott McMorrow 
  To: Stephen Zinck 
  Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; npatel@xxxxxxxxxx ; 
si-list@xxxxxxxxxxxxx 
  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 trek towards 
perfection to task."

  "Would either of you agree that AC coupling capacitor location may matter 
with a lossy interface to the capacitor?"


  Insertion loss in a flat impedance linear lossy system will be independent of 
capacitor location.  Run the math and see.  The only time position matters is 
in the face of discontinuities.   In fact, given a low loss interconnect with 
discontinuities and a high loss interconnect with discontinuities, the low loss 
system, with it's higher Q, will often have worse behavior.

  An improperly designed 0402 capacitor transition for a 50 ohm line can easily 
exhibit a discontinuity of 35 ohms for 50 ps.  If attached to poorly designed 
via transitions, the discontinuity will be even worse.  When this is coupled 
closely to a high capacitance receiver input, a high capacitance 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.  

  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.  Our understanding 
of the physics has been verified by full wave modeling, 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® is the registered service mark of
Teraspeed Consulting Group LLC


  Stephen Zinck wrote: 
    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, that 
were used to make a comparison of source versus destination AC coupling 
capacitor locations. The system was excited with a string of ones, followed by 
a single zero, followed by a string of ones. 

    I have not specifically designed a test board that varies the AC coupling 
capacitor location along a trace.

    I understand the "shades of gray" here and agree that one can't make a 
"rule of thumb" generalization in our line of work these days. 

    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 trek towards 
perfection to task. 

    Do either of you have real world measured results, that you could share, 
that show no marked difference in received signal characteristics when the AC 
coupling capacitor position is varied through a 30 inch backplane system (or 
similar)?

    I believe my experience with capacitor location may prove true if the 
capacitor interface is lossy (which is the case). A lot of my customers are 
just looking for quick ways to maximize performance using standard component 
packages and standard layout practices (in the end, I don't like to give 
anything away that is low lying fruit). Most of the time I am doing my analysis 
after the board is in layout, where I have limited ability to change the design 
(unless it is really broken). In a perfect world, where I am involved early, 
the package optimization and layout structures can be optimized as you state, 
but only if the margins warrant it (system performance issues are expected 
after initial "what-if" simulations have occurred). The right tool for the 
right job rules the day...

    Would either of you agree that AC coupling capacitor location may matter 
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 ----- 
      From: Scott McMorrow 
      To: Stephen Zinck 
      Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; 
npatel@xxxxxxxxxx ; si-list@xxxxxxxxxxxxx 
      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.  

      I contend that the only difference an AC coupling capacitor can possibly 
have due to position in a linear interconnect is a result of impedance 
mismatch.  I contend that the capacitor will form a 1/2 wave resonant circuit 
with other interconnect discontinuities (connectors, vias stubs, 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 discontinuities are different in every 
system.  In some systems the receiver constitutes a larger discontinuity than 
the transmitter.  In other systems this is reversed.  In yet other systems, 
connectors and vias represent larger discontinuites than do either the 
transmitters or receivers. It all "just 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 necessarily improve 
performance.  An AC coupling capacitor, with it's associated via and pad 
transition design, can be viewed as a black box which has insertion loss and 
return loss, and can be modeled quite well using either lumped element 
approximations or (my favorite) S-parameters.  As such it will cascade in a 
simulation model just like any other linear element.  If we 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 the 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 properly 
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, since there is little resonance 
interaction with other devices and structures.


      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® is the registered service mark of
Teraspeed Consulting Group LLC
    

      Stephen Zinck wrote: 
        Hi Scott,

        My simulations show that the capacitor is best placed at the receiver 
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 ----- 
          From: Scott McMorrow 
          To: signalintegrity@xxxxxxxxxxx 
          Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; 
npatel@xxxxxxxxxx ; si-list@xxxxxxxxxxxxx 
          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 resonances 
that are created by discontinuities and modal conversion (which have absolutely 
zero to do with signal rise time), there is no difference in 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® is the registered service mark of
Teraspeed Consulting Group LLC
    

          Stephen Zinck wrote: 
Hi Jory,

I have simulated this at length and concur with your experience that the 
capacitor is best placed at the receiver...

In effect, the attenuation associated with the capacitor placement at the 
receiver (parasitics/pads/vias) is lost in the rise-time degradation of the 
system.
The classic "don't break it until you have to" rule is applicable... OK this 
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 ----- 
From: "Jory McKinley" <jory_mckinley@xxxxxxxxx>
To: <leeritchey@xxxxxxxxxxxxx>; <npatel@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
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 domain) 
in the lab some effects that appears to be location specific in the 
placement of the AC coupling caps at the rcvr.  Now this may be due in part 
to the fact that I am using 50-ohm resistor termination in each lead as 
well and the combination (cap plus rcvr reflection) is giving some 
imbalance depending on distance.  The best rcvr eye that I am seeing is 
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 simulate 
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 out 
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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