[SI-LIST] Re: AC Coupled Signals
- From: steve weir <weirsi@xxxxxxxxxx>
- To: signalintegrity@xxxxxxxxxxx
- Date: Tue, 25 Sep 2007 08:45:31 -0700
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
(401) 284-1827 Business=20
(401) 284-1840 Fax=20
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This e-mail contains proprietary and confidential intellectual property o=
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