[SI-LIST] Re: AC Coupled Signals
- From: "Chris Cheng" <Chris.Cheng@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 25 Sep 2007 15:48:36 -0700
I think the point is at high data rate, such pad and cap design does not exist
and you have to deal with it. Or better yet, take advantage of using one
negative (cap pad) to compensate for another negative (imperfect receiver
load).
I certainly have seen imperfect receiver load and I certainly have seen
imperfect pad. I can not do much about the imperfect receiver short of
switching vendor. But I can do something about the pad and placement of it.
Real world problem needs real world solutions.
-----Original Message-----
From: ronald miller [mailto:ron@xxxxxxxxxxx]
Sent: Tuesday, September 25, 2007 3:34 PM
To: Chris Cheng
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: AC Coupled Signals
Chris
If your coupling cap is a problem, get a better cap and design the pads to have
no reflection.
If there are no reflections at the lowest data rate and at the highest data
rate, the position does
not matter.
Now, about the S11 and S22, it is much more intuitive and much easier to deal
with TDR and
reflection coefficients, or impedance than it is to deal with the network
analyzer data.
Although I am a microwave engineer, I have learned the hard way, and now I try
to dtay away from
the S-Parameters as models and for analysis because they are clumsy and
non-intuitive.
Ron
Chris Cheng wrote:
Let me try my hand on why position matter.
A normal passive channel is reciprocal. e.g. S12=3DS21 It only says the =
off diagonal elements are symmetic. It doesn't say the diagonal elements =
have to be equal. I believe this was the basis of Jeff Loyer's =
discussion a while ago.
The presence of the discontinuity affects the S11 and S22 dramatically =
different based on whether it is close to the Tx or Rx.
In the presences of imperfect loading on the Rx side, it is the =
interaction between the S22 and loading that matters.=20
Thus position makes a difference. i.e. we are tuning the S22 with the =
non-ideal loading.
QED
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[ mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Tuesday, September 25, 2007 1:35 PM
To: Jory McKinley
Cc: Stephen Zinck; Scott McMorrow; leeritchey@xxxxxxxxxxxxx;
npatel@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: AC Coupled Signals
Jory, I think this is good example of where intuitively appealing=20
misconceptions can seduce one into translating correlation into=20
causation. If you have more ringing in one case than another, it means=20
that you have set up a resonance that is more severe in the one case. =20
This can easily happen as a result of any number of things going on: =20
suboptimal silicon to package launch, suboptimal IC to PCB, via stubs,=20
connector transitions, etc, etc.
The very simple test is to take a VNA, a couple of sections of coax and=20
a DC block. Move the DC block between the transmit end, the junction of =
the two cables, and the receiver and look at the behavior of that net=20
channel. With good coax and connectors the channel performance will=20
change almost immeasureably. Now go and add a coax T on one side of the =
DC block. Move that whole thing around as a unit and again the channel=20
performance remains the same. Add a second coax T on the other side of=20
the DC block from the first, and again move the whole thing around. The =
results will still remain uniform. Now if you go and move one of those =
T's someplace else, then the pesky mole you're trying to whack moves and =
the resonance will pop up somewhere else. The bottom line is that it's=20
resonance that we need to fight and resonance doesn't know left from =
right.
Regards,
Steve.
Jory McKinley wrote:
To add to this, I will ask for release of lab data that I took that=20
shows RX_EYE clearly improves as the AC cap/term location is moved=20
closer to the RX. The data indicates that even though overall channel =
loss may not be affected, the 50ps edge rates we are sending through=20
the channel are affected (in terms of time domain ringing) by the AC=20
cap/term placement. This kind of feels right.
-Jory
=20
----- Original Message ----
From: steve weir <mailto:weirsi@xxxxxxxxxx> <weirsi@xxxxxxxxxx>
To: Stephen Zinck <mailto:signalintegrity@xxxxxxxxxxx>
<signalintegrity@xxxxxxxxxxx>
Cc: Scott McMorrow <mailto:scott@xxxxxxxxxxxxx> <scott@xxxxxxxxxxxxx>;
jory_mckinley@xxxxxxxxx;=20
leeritchey@xxxxxxxxxxxxx; npatel@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Sent: Tuesday, September 25, 2007 12:52:41 PM
Subject: Re: [SI-LIST] Re: AC Coupled Signals
Steve, as far as I know where we have agreement that capacitor =
location
can only affect performance where the combined capacitor and mount
presents a discontinuity and that discontinuity is located such that =
it
forms a resonant structure with another discontinuity in the channel. =
I
fail to see where we have moved any closer to supporting your premise
that locating a greater proportion of fixed loss before the capacitor
changes end to end loss than placing that same fixed loss behind it.
As for lab measurements, we have these as we have characterized many
links. We also have extensive simulations.
Regards,
Steve.
Stephen Zinck wrote:
Hi Steve,
I understand your point but I actually thought Scott and I were
getting close. I guess I still need him to explain his statement:
"The only time position matters is in the face of discontinuities."
because this runs counter to your assertion.
It would be good to have some concrete lab measurement results to =
back
either of our points up. I am sorry I don't have any.
We agree on TDR/VNA characteristics...
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=20
<http://www.interconnectengineering.com>
<http://www.interconnectengineering.com>
----- Original Message ----- From: "steve weir" <mailto:weirsi@xxxxxxxxxx>
<weirsi@xxxxxxxxxx>
To: <mailto:signalintegrity@xxxxxxxxxxx> <signalintegrity@xxxxxxxxxxx>
Cc: "Scott McMorrow" <mailto:scott@xxxxxxxxxxxxx> <scott@xxxxxxxxxxxxx>; =
<mailto:jory_mckinley@xxxxxxxxx> <jory_mckinley@xxxxxxxxx>;
<mailto:leeritchey@xxxxxxxxxxxxx> <leeritchey@xxxxxxxxxxxxx>;
<mailto:npatel@xxxxxxxxxx> <npatel@xxxxxxxxxx>; =
<mailto:si-list@xxxxxxxxxxxxx> <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, September 25, 2007 12:24 PM
Subject: [SI-LIST] Re: AC Coupled Signals
Stephen, OK so when you say "lossy" or "nonlinear" you mean=3D20
discontinuous. Discontinuities aggravate resonances based on
specific=3D20
structure material and geometries, in other words the distance on =
a=3D20
centimeter or millimeter scale between discontinuities. We =
have=3D20
essentially the same opportunities for channel discontinuities at =
and
in =3D
the vicinity of the transmitter as the receiver. So I still do not
see=3D20
a defensible basis for the offered position: that placing a =
capacitor
at =3D
one end of the line versus the other changes the end to end loss. =20
What=3D20
matters is if wherever I place one discontinuity that it sets up a
sharp =3D
resonance with another discontinuity. That can happen equally well
at=3D20
either end of the line.
If one looks at a channel with only a TDR I might understand =
the=3D20
erroneous perception that placing a discontinuity down the line =
is=3D20
better than up front. But that is an illusion. TDR resolution=20
falls=3D20
with interconnect distance. This ia a result of the inherent loss=20
of=3D20
the interconnect that shelves bandwidth and hence resolution =
versus=3D20
distance for the instrument. This is one of the big limitations of =
a=3D20
TDR for channel evaluation. A through measurement with a TDT or =
VNA=3D20
does not suffer that limitation, give true measure of S21 and so
report=3D20
the real channel performance. Eric Bogatin spends some time on =
the=3D20
issue of bandwidth versus interconnect length in his book.
Regards,
Steve.
Stephen Zinck wrote:
Scott,
We may have some nomenclature issues here...=3D20
When I say "lossy interface to the capacitor" I mean with =
impedance
dis=3D
continuities. So I think we are on a similar page given your =
statement:
"The only time position matters is in the face of =
discontinuities."
Again, most often, my role is to simulate the customers system at
the 1=3D
1th hour. I don't recommend this, I just work within the customer's
needs=3D
/requirements. I make real world recommendations from simulation
results =3D
for designs where these discontinuities you mention are a fact of
life. G=3D
ranted my customers are not doing 5+ Gbit/s designs (right now ;-).
Above these data-rates, all you mention, capacitor transition =
(pad,
via=3D
s, etc) are of the utmost importance. And I would absolutely agree
that t=3D
he more perfect you make these transitions, the less it matters =
where
the=3D
y are placed...
So I do believe AC coupling capacitor position does matter, as you
stat=3D
e, for the bulk of the designs occurring these days where component
footp=3D
rint and via optimization, etc. is NOT occurring...
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=20
<http://www.interconnectengineering.com>
<http://www.interconnectengineering.com>
----- Original Message -----=3D20
From: Scott McMorrow=3D20
To: Stephen Zinck=3D20
Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ;
npatel@micro=3D
n.com ; si-list@xxxxxxxxxxxxx=3D20
Sent: Tuesday, September 25, 2007 11:08 AM
Subject: Re: [SI-LIST] Re: AC Coupled Signals
Steven,
I would not agree with your following statements.
"I agree in theory with all you state. Assuming a lossless
interface =3D
to the capacitor, it shouldn't matter where you place it, given a
purely =3D
linear system. But the real world is lossy, even when one makes =
great
3D =3D
solved structures. Manufacturing and other tolerances tend to take
the tr=3D
ek towards perfection to task."
"Would either of you agree that AC coupling capacitor location =
may
ma=3D
tter with a lossy interface to the capacitor?"
Insertion loss in a flat impedance linear lossy system will be
indepe=3D
ndent of capacitor location. Run the math and see. The only time
positi=3D
on matters is in the face of discontinuities. In fact, given a =
low
loss=3D
interconnect with discontinuities and a high loss interconnect with
disc=3D
ontinuities, the low loss system, with it's higher Q, will often =
have
wor=3D
se behavior.
An improperly designed 0402 capacitor transition for a 50 ohm =
line
ca=3D
n easily exhibit a discontinuity of 35 ohms for 50 ps. If attached
to po=3D
orly designed via transitions, the discontinuity will be even =
worse. =20
Whe=3D
n this is coupled closely to a high capacitance receiver input, a
high ca=3D
pacitance transmitter output, a low impedance via stub =
discontinuity,
or =3D
a low impedance connector discontinuity, it can form a 1/2 wave
resonant =3D
circuit. This is most likely the problem you are seeing. =3D20
If the interconnect has essentially flat impedance, position =
does
not=3D
matter. If the capacitor transition is properly designed, position
does=3D
not matter. All of the data we have on this is proprietary at this
time=3D
=3D2E Our understanding of the physics has been verified by full =
wave
mode=3D
ling, simulation and measurement.
regards,
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=3DAE is the registered service mark of
Teraspeed Consulting Group LLC
Stephen Zinck wrote:=3D20
Hi Scott and Steve,
To answer both of your questions, it is the resulting Hspice
(with =3D
S-parameters) differential eye patterns, as viewed at the receiver
die, t=3D
hat were used to make a comparison of source versus destination AC
coupli=3D
ng capacitor locations. The system was excited with a string of =
ones,
fol=3D
lowed by a single zero, followed by a string of ones.=3D20
I have not specifically designed a test board that varies the =
AC
co=3D
upling capacitor location along a trace.
I understand the "shades of gray" here and agree that one =
can't
mak=3D
e a "rule of thumb" generalization in our line of work these =
days.=3D20
I agree in theory with all you state. Assuming a lossless
interface=3D
to the capacitor, it shouldn't matter where you place it, given a
purely=3D
linear system. But the real world is lossy, even when one makes =
great
3D=3D
solved structures. Manufacturing and other tolerances tend to take
the t=3D
rek towards perfection to task.=3D20
Do either of you have real world measured results, that you
could s=3D
hare, that show no marked difference in received signal
characteristics w=3D
hen the AC coupling capacitor position is varied through a 30 inch
backpl=3D
ane system (or similar)?
I believe my experience with capacitor location may prove true
if t=3D
he capacitor interface is lossy (which is the case). A lot of my
customer=3D
s are just looking for quick ways to maximize performance using
standard =3D
component packages and standard layout practices (in the end, I =
don't
lik=3D
e to give anything away that is low lying fruit). Most of the time =
I
am d=3D
oing my analysis after the board is in layout, where I have limited
abili=3D
ty to change the design (unless it is really broken). In a perfect
world,=3D
where I am involved early, the package optimization and layout
structure=3D
s can be optimized as you state, but only if the margins warrant it
(syst=3D
em performance issues are expected after initial "what-if"
simulations ha=3D
ve occurred). The right tool for the right job rules the day...
Would either of you agree that AC coupling capacitor location
may m=3D
atter 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=20
<http://www.interconnectengineering.com>
<http://www.interconnectengineering.com>
----- Original Message -----=3D20
From: Scott McMorrow=3D20
To: Stephen Zinck=3D20
Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ;
npatel@m=3D
icron.com ; si-list@xxxxxxxxxxxxx=3D20
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 =3D
to linear system theory and measurements. =3D20
I contend that the only difference an AC coupling capacitor
can p=3D
ossibly have due to position in a linear interconnect is a result =
of
impe=3D
dance mismatch. I contend that the capacitor will form a 1/2 wave
resona=3D
nt circuit with other interconnect discontinuities (connectors, =
vias
stub=3D
s, packages, Tx die, Rx die ... etc) and that this interaction is
system,=3D
chip, connector and package design dependent. I contend that it is
this=3D
1/2 resonance that can cause differences that can be measured, but
that =3D
there is no "rule of thumb", since the position and magnitude of
disconti=3D
nuities are different in every system. In some systems the =
receiver
cons=3D
titutes a larger discontinuity than the transmitter. In other
systems th=3D
is is reversed. In yet other systems, connectors and vias =
represent
larg=3D
er discontinuites than do either the transmitters or receivers. It
all "j=3D
ust depends". To state a specific rule is just plain incorrect.
I contend that once you remove the magic and myths =
surrounding
AC=3D
coupling capacitors, analysis of the 3D structure shows that by
reducing=3D
the signal path discontinuity through the capacitor, you will
necessaril=3D
y improve performance. An AC coupling capacitor, with it's
associated vi=3D
a and pad transition design, can be viewed as a black box which has
inser=3D
tion loss and return loss, and can be modeled quite well using =
either
lum=3D
ped element approximations or (my favorite) S-parameters. As such =
it
wil=3D
l cascade in a simulation model just like any other linear element. =
=20
If w=3D
e start with a system with flat 50 ohm impedance from end to end, =
it
can =3D
be easily shown that no matter what the position of the capacitor
along t=3D
he interconnect is, the insertion loss of the system is identical. =
It is=3D
only the return loss, as seen from each end that changes.
I've been designing AC coupling capacitor mounting =
transitions
pr=3D
operly for quite a few years now and have some 0402 designs that =
keep
S12=3D
above -0.2 dB up to 7.5 GHz, S12 below -20 dB @ 5 GHz, and below =
-15
dB =3D
@ 10 GHz. For all practical purposes, these designs are =
transparent
and =3D
may be placed anywhere in an interconnect design where there is
space, si=3D
nce there is little resonance interaction with other devices and
structur=3D
es.
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=3DAE is the registered service mark of
Teraspeed Consulting Group LLC
=3D20
Stephen Zinck wrote:=3D20
Hi Scott,
My simulations show that the capacitor is best placed at =
the
re=3D
ceiver 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=20
<http://www.interconnectengineering.com>
<http://www.interconnectengineering.com>
----- Original Message -----=3D20
From: Scott McMorrow=3D20
To: signalintegrity@xxxxxxxxxxx=3D20
Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ;
npat=3D
el@xxxxxxxxxx ; si-list@xxxxxxxxxxxxx=3D20
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
reso=3D
nances that are created by discontinuities and modal conversion
(which ha=3D
ve absolutely zero to do with signal rise time), there is no
difference i=3D
n the attenuation of a capacitor placed at the Tx as opposed at =
the
Rx. =3D
W.R.T. the receiver, if it is "lost in the rise-time degradation of
the =3D
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=3DAE is the registered service mark of
Teraspeed Consulting Group LLC
=3D20
Stephen Zinck wrote:=3D20
Hi Jory,
I have simulated this at length and concur with your experience =
that
th=3D
e=3D20
capacitor is best placed at the receiver...
In effect, the attenuation associated with the capacitor placement
at t=3D
he=3D20
receiver (parasitics/pads/vias) is lost in the rise-time =
degradation
of=3D
the=3D20
system.
The classic "don't break it until you have to" rule is =
applicable...
OK=3D
this=3D20
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=20
<http://www.interconnectengineering.com>
<http://www.interconnectengineering.com>
----- Original Message -----=3D20
From: "Jory McKinley" <mailto:jory_mckinley@xxxxxxxxx>
<jory_mckinley@xxxxxxxxx>
To: <mailto:leeritchey@xxxxxxxxxxxxx> <leeritchey@xxxxxxxxxxxxx>;
<mailto:npatel@xxxxxxxxxx> <npatel@xxxxxxxxxx>;
<si-list@freelists=3D
=3D2Eorg>
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=3D
ain)=3D20
in the lab some effects that appears to be location specific in =
the=3D20
placement of the AC coupling caps at the rcvr. Now this may be =
due
in =3D
part=3D20
to the fact that I am using 50-ohm resistor termination in each =
lead
as=3D
=3D20
well and the combination (cap plus rcvr reflection) is giving =
some=3D20
imbalance depending on distance. The best rcvr eye that I am =
seeing
is=3D
=3D20
when I can move the AC/term as close to the rcvr as I can. By the
way =3D
these are 5Gb/s signals.
If I have time I will try and isolate what I am seeing and even
simulat=3D
e=3D20
it, has anyone else seen or simulated this?
-Jory
----- Original Message ----
From: Lee Ritchey <mailto:leeritchey@xxxxxxxxxxxxx> <leeritchey@xxxxxxxxxxxxx>
To: "npatel@xxxxxxxxxx" <mailto:npatel@xxxxxxxxxx>
<mailto: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=3D
other without, the loss vs. frequency is virtually identical at =
leas
ou=3D
t=3D20
to
6 GHz. That would be 12 Mb/S.
Lee Ritchey
[Original Message]
From: <mailto:npatel@xxxxxxxxxx> <npatel@xxxxxxxxxx>
To: <mailto:si-list@xxxxxxxxxxxxx> <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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- References:
- [SI-LIST] Re: AC Coupled Signals
- From: ronald miller
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- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- » [SI-LIST] Re: AC Coupled Signals
- [SI-LIST] Re: AC Coupled Signals
- From: ronald miller