[SI-LIST] Re: AC Coupled Signals

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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