[SI-LIST] Re: AC Coupled Signals
- From: "Stephen Zinck" <signalintegrity@xxxxxxxxxxx>
- To: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- Date: Tue, 25 Sep 2007 10:47:37 -0400
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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