[SI-LIST] Re: AC Coupled Signals

Stephen you seem to be treating lossy as nonlinear.  In my world, losses =

in passive components and structures are linear.  I would be very=20
curious to see a HSPICE deck that shows a difference between in S21 of a =

channel depending on the sequential order of elements that compose=20
same.  In linear theory that doesn't happen.  Can you share?

Regards,


Steve.
Stephen Zinck wrote:
> Hi Scott and Steve,
> To answer both of your questions, it is the resulting Hspice (with S-pa=
rameters) differential eye patterns, as viewed at the receiver die, that =
were used to make a comparison of source versus destination AC coupling c=
apacitor locations. The system was excited with a string of ones, followe=
d by a single zero, followed by a string of ones.=20
>
> I have not specifically designed a test board that varies the AC coupli=
ng 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.=20
>
> 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 lin=
ear system. But the real world is lossy, even when one makes great 3D sol=
ved structures. Manufacturing and other tolerances tend to take the trek =
towards perfection to task.=20
>
> 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 c=
apacitor interface is lossy (which is the case). A lot of my customers ar=
e just looking for quick ways to maximize performance using standard comp=
onent 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 t=
o change the design (unless it is really broken). In a perfect world, whe=
re I am involved early, the package optimization and layout structures ca=
n be optimized as you state, but only if the margins warrant it (system p=
erformance issues are expected after initial "what-if" simulations have o=
ccurred). The right tool for the right job rules the day...
>
> Would either of you agree that AC coupling capacitor location may matte=
r 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 -----=20
>   From: Scott McMorrow=20
>   To: Stephen Zinck=20
>   Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; npatel@micro=
n.com ; si-list@xxxxxxxxxxxxx=20
>   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 l=
inear system theory and measurements. =20
>
>   I contend that the only difference an AC coupling capacitor can possi=
bly have due to position in a linear interconnect is a result of impedanc=
e mismatch.  I contend that the capacitor will form a 1/2 wave resonant c=
ircuit with other interconnect discontinuities (connectors, vias stubs, p=
ackages, Tx die, Rx die ... etc) and that this interaction is system, chi=
p, connector and package design dependent.  I contend that it is this 1/2=
 resonance that can cause differences that can be measured, but that ther=
e is no "rule of thumb", since the position and magnitude of discontinuit=
ies are different in every system.  In some systems the receiver constitu=
tes a larger discontinuity than the transmitter.  In other systems this i=
s reversed.  In yet other systems, connectors and vias represent larger d=
iscontinuites 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 cou=
pling capacitors, analysis of the 3D structure shows that by reducing the=
 signal path discontinuity through the capacitor, you will necessarily im=
prove performance.  An AC coupling capacitor, with it's associated via an=
d 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 ca=
scade in a simulation model just like any other linear element.  If we st=
art with a system with flat 50 ohm impedance from end to end, it can be e=
asily shown that no matter what the position of the capacitor along the i=
nterconnect is, the insertion loss of the system is identical.  It is onl=
y the return loss, as seen from each end that changes.
>
>   I've been designing AC coupling capacitor mounting transitions proper=
ly for quite a few years now and have some 0402 designs that keep S12 abo=
ve -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=AE is the registered service mark of
> Teraspeed Consulting Group LLC
>
>
>   Stephen Zinck wrote:=20
>     Hi Scott,
>
>     My simulations show that the capacitor is best placed at the receiv=
er 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 -----=20
>       From: Scott McMorrow=20
>       To: signalintegrity@xxxxxxxxxxx=20
>       Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ; npatel@m=
icron.com ; si-list@xxxxxxxxxxxxx=20
>       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 resonanc=
es that are created by discontinuities and modal conversion (which have a=
bsolutely zero to do with signal rise time), there is no difference in th=
e attenuation of  a capacitor placed at the Tx as opposed at the Rx.  W.R=
=2ET. the receiver, if it is "lost in the rise-time degradation of the sy=
stem", 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=AE is the registered service mark of
> Teraspeed Consulting Group LLC
>    =20
>
>       Stephen Zinck wrote:=20
> Hi Jory,
>
> I have simulated this at length and concur with your experience that th=
e=20
> capacitor is best placed at the receiver...
>
> In effect, the attenuation associated with the capacitor placement at t=
he=20
> receiver (parasitics/pads/vias) is lost in the rise-time degradation of=
 the=20
> system.
> The classic "don't break it until you have to" rule is applicable... OK=
 this=20
> 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 -----=20
> From: "Jory McKinley" <jory_mckinley@xxxxxxxxx>
> To: <leeritchey@xxxxxxxxxxxxx>; <npatel@xxxxxxxxxx>; <si-list@freelists=
=2Eorg>
> 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=
ain)=20
> in the lab some effects that appears to be location specific in the=20
> placement of the AC coupling caps at the rcvr.  Now this may be due in =
part=20
> to the fact that I am using 50-ohm resistor termination in each lead as=
=20
> well and the combination (cap plus rcvr reflection) is giving some=20
> imbalance depending on distance.  The best rcvr eye that I am seeing is=
=20
> 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 simulat=
e=20
> 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 ou=
t=20
> 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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--=20
Steve Weir
Teraspeed Consulting Group LLC=20
121 North River Drive=20
Narragansett, RI 02882=20

California office
(408) 884-3985 Business
(707) 780-1951 Fax

Main office
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