[SI-LIST] Re: AC Coupled Signals

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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