Correct. Which is why for correlation to measurements this is the way to go.
Measurement are artificial, and we need to replicate the same artificial
environment in modeling as we do in measurements. In this case, we'll see the
traveling crosstalk wave from excitation of the open circuit nets superimposed
on the measurement.
This is one of the reasons I hate microprobes for measurement. Since it is not
easy to terminate unused lines, the measurements do not match terminated
modeling 1-to-1, unless you correct for the open circuit conditions.
Connectorized measurements are more faithful to the transmission line
environment, since we can terminate unused connectors. But then we have the
issue of removing the connector from the measurement.
Either way, we are dealing with the law of Conservation of Misery. - Misery is
neither created nor destroyed. It is merely transferred from one place (or
person) to another.
Scott McMorrow, CTO Signal Integrity Group
Samtec
Office 401-284-1827 | +1-800-726-8329
www.samtec.com<http://www.samtec.com/>
From: Walter Katz [mailto:wkatz@xxxxxxxxxx]
Sent: Friday, July 21, 2017 11:50 AM
To: Scott McMorrow <Scott@xxxxxxxxxxxxx>; Arpad_Muranyi@xxxxxxxxxx; IBIS-ATM
<ibis-macro@xxxxxxxxxxxxx>; IBIS-Interconnect <ibis-interconn@xxxxxxxxxxxxx>
Subject: RE: [ibis-macro] Re: [ibis-interconn] Re: Question about references in
BIRD189, and related comments about AMI Flows
Scott,
One hopefully final question. If the s12 measurement is made with all other
pins not terminated, does not the s12 already include the self-induced
crosstalk of the aggressor connections?
Walter
Walter Katz
wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>
Phone 303.449-2308
Mobile 303.335-6156
From: Walter Katz [mailto:wkatz@xxxxxxxxxx]
Sent: Friday, July 21, 2017 10:01 AM
To: Scott@xxxxxxxxxxxxx<mailto:Scott@xxxxxxxxxxxxx>;
Arpad_Muranyi@xxxxxxxxxx<mailto:Arpad_Muranyi@xxxxxxxxxx>; IBIS-ATM
<ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>; IBIS-Interconnect
<ibis-interconn@xxxxxxxxxxxxx<mailto:ibis-interconn@xxxxxxxxxxxxx>>
Subject: RE: [ibis-macro] Re: [ibis-interconn] Re: Question about references in
BIRD189, and related comments about AMI Flows
Scott,
Thanks for the clarifications. Assuming most pins of a component are in fact
connected to something on the board, we need to be concerned about used
connections in the package that are adjacent (near) interconnect on the
package that are not connected to anything on the Board. What is the EDA tool
to do? What is the package model maker to do?
If a pin is marked NC in a component, and in fact, that pin has routing in the
package, then the most accurate thing to do is to include all these NC pins
along with the pins of interest in the terminals of the interconnect mode, and
then set the termination of all of the unused terminals to ~50 Ohms. An
alternative is to make the interconnect model for a specific pin include all of
the significant aggressor pins. The EDA tool could chose to terminate those
aggressor pins that have connections on the board with ~50 Ohms, and those pins
that do not have connections on the board (whether NC or not) with 1Meg Ohm.
Without this help from the interconnect model maker, the EDA tool can either
run with all 1 MegOhm terminations (pessimistic), or all ~50 Ohm terminations
(optimistic), and compare the results.
Walter
Walter Katz
wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>
Phone 303.449-2308
Mobile 303.335-6156
From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx>
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Scott McMorrow
Sent: Friday, July 21, 2017 8:00 AM
To: Arpad_Muranyi@xxxxxxxxxx<mailto:Arpad_Muranyi@xxxxxxxxxx>; IBIS-ATM
<ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>; IBIS-Interconnect
<ibis-interconn@xxxxxxxxxxxxx<mailto:ibis-interconn@xxxxxxxxxxxxx>>
Subject: [ibis-macro] Re: [ibis-interconn] Re: Question about references in
BIRD189, and related comments about AMI Flows
Arpad
Walt did not include peaking due to reflection doubling and resonance. That
can cause up to a 4X increase, or 4 mV. If the line being simulated has two
neighboring nets, then the error could be as much as 8 mV.
Finally, not all packages are well-designed for crosstalk isolation. My design
team shoots for -66 dB PSXT isolation. But we've seen much much worse in
packages we do not design. In our 112G PAM4 design work we are concerned with
errors of 100's of uV in our S-parameters. We like any noise error in the
impulse response to be less than -80 dB down, because of the stringent
signal-to-noise implications on PAM4 at extreme data rates.
Scott
Scott McMorrow, CTO Signal Integrity Group
Samtec
Office 401-284-1827 | +1-800-726-8329
www.samtec.com<http://www.samtec.com/>
From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx>
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Muranyi, Arpad
Sent: Thursday, July 20, 2017 11:17 PM
To: IBIS-ATM <ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>;
IBIS-Interconnect
<ibis-interconn@xxxxxxxxxxxxx<mailto:ibis-interconn@xxxxxxxxxxxxx>>
Subject: [ibis-macro] Re: [ibis-interconn] Re: Question about references in
BIRD189, and related comments about AMI Flows
Walter,
So are you saying that it really doesn't matter whether unused ports are
terminated with
50 Ohms or 2e+6 Ohms, because the signal that bleeds into a victim line through
crosstalk
is less than 1 mV either way (for 1 V signaling)?
In that case, why bother putting this unused termination resistance into
BIRD189 at all?
Thanks,
Arpad
=====================================================================
From:
ibis-interconn-bounce@xxxxxxxxxxxxx<mailto:ibis-interconn-bounce@xxxxxxxxxxxxx>
[mailto:ibis-interconn-bounce@xxxxxxxxxxxxx] On Behalf Of Walter Katz
Sent: Thursday, July 20, 2017 7:26 PM
To: Scott McMorrow <Scott@xxxxxxxxxxxxx<mailto:Scott@xxxxxxxxxxxxx>>; IBIS-ATM
<ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>; IBIS-Interconnect
<ibis-interconn@xxxxxxxxxxxxx<mailto:ibis-interconn@xxxxxxxxxxxxx>>
Subject: [ibis-interconn] Re: Question about references in BIRD189, and related
comments about AMI Flows
All,
I agree.
Now what is the difference in the voltage say at port 2, when an adjacent
connection in the package is terminated by 2 MegOhm or by an interconnect with
a nominal impedance of 50 +/- 10% Ohms?
Assuming the package was designed to minimize crosstalk (to be < 10mV (1%) on
1V signaling).
If the adjacent connections are terminated by a 2 MegOhm resistor
The 10mV signal will cause .1mv voltage (.01% x 10mV) at ports 1 and 2.
If the adjacent connections are connected to a 50 +/- 10% Ohm transmission line
Thw reflection will be <~ 1mV, and the voltage at ports 1 and 2 will cause a
.001mv voltage (.001% x 1mV) at ports 1 and 2.
Using 2MegOhm termination will give a much more pessimistic answer then the 50
Ohm terminations, but still <~ 1Mv.
The EDA tool can always choose 50Ohm, which can speed up simulations
dramatically, and giving an optimistic results by as much as 1Mv.
The EDA tool can give the User the option of doing it either way
Please evauate my assumptions, my arithmetic, and come to your own conclusion.
Walter
Walter Katz
wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>
Phone 303.449-2308
Mobile 303.335-6156
From: Scott McMorrow [mailto:Scott@xxxxxxxxxxxxx]
Sent: Thursday, July 20, 2017 7:14 PM
To: wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>; IBIS-ATM
<ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>; IBIS-Interconnect
<ibis-interconn@xxxxxxxxxxxxx<mailto:ibis-interconn@xxxxxxxxxxxxx>>
Subject: RE: [ibis-interconn] Re: Question about references in BIRD189, and
related comments about AMI Flows
Walter
Obviously, when you are making measurements on a package, all other ports are
open circuits. For correlation purposes, I will normally reduce the 100-port
s-parameter to a 2-port s-parameter using a 1 meg ohm termination impedance on
the unused ports.
For in-system performance modeling, all ports should be reduced using the
nominal interconnect impedance. This may or may not be the normalization
impedance recorded in the touchstone file.
Regards,
Scott
Scott McMorrow, CTO Signal Integrity Group
Samtec
Office 401-284-1827 | +1-800-726-8329
www.samtec.com<http://www.samtec.com/>
From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx>
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Walter Katz
Sent: Thursday, July 20, 2017 6:40 PM
To: IBIS-ATM <ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>;
IBIS-Interconnect
<ibis-interconn@xxxxxxxxxxxxx<mailto:ibis-interconn@xxxxxxxxxxxxx>>
Subject: [ibis-macro] Re: [ibis-interconn] Re: Question about references in
BIRD189, and related comments about AMI Flows
Arpad,
Consider a 50 pin package, that has an s100p. Port 1 is pin 1, port 2 is die
pad for pin # 1.
When measuring s12 with a 2 probe VNA, how are the other 98 ports usually
terminated?
Once we get this answer, then we can proceed to the next question.
Walter
Walter Katz
wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>
Phone 303.449-2308
Mobile 303.335-6156
From:
ibis-interconn-bounce@xxxxxxxxxxxxx<mailto:ibis-interconn-bounce@xxxxxxxxxxxxx>
[mailto:ibis-interconn-bounce@xxxxxxxxxxxxx] On Behalf Of Muranyi, Arpad
Sent: Thursday, July 20, 2017 4:04 PM
To: IBIS-ATM <ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>;
IBIS-Interconnect
<ibis-interconn@xxxxxxxxxxxxx<mailto:ibis-interconn@xxxxxxxxxxxxx>>
Subject: [ibis-interconn] Re: Question about references in BIRD189, and related
comments about AMI Flows
Walter,
Let's keep the AMI related topics out of this subject line, and discuss them in
a separate
thread...
So, regarding this discussion about BIRD189, I am sensing a slight
miscommunication or a
different perspective here. Let me try to explain what I mean.
Using the reference impedance in Touchstone files we can terminate the unused
ports
of a Touchstone file in such a way that there will be no reflections at those
unused ports.
This is usually done to generate waveforms with the used ports of the
Touchstone file
as if the adjacent unused ports did not exist. I understand there are
numerical techniques
which can be applied to Touchstone files to reduce them to a smaller number of
ports
using this concept.
Now, the other perspective that you seemed to describe in your earlier email is
different.
In this interpretation, an "unused port", for example a pin of a package, is
actually connected
to the printed circuit board, and perhaps there are no further traces on the
board attached
to that pin. This is often called a "No Connect" (NC). When a user runs their
simulations,
they may be interested in seeing the effects of this dangling pin, i.e. they
WANT to see
the reflections at that pin, and they WANT to see how these reflections
influence the
adjacent lines through crosstalk, etc... But I would not call this scenario
"unused ports",
because the ports are indeed in use for a good reason.
So I would suggest that we come to an agreement first on what we mean by
"unused ports",
and perhaps put that in BIRD189 to eliminate any confusions or
misinterpretations.
Thanks,
Arpad
=========================================================================
From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx>
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Walter Katz
Sent: Thursday, July 20, 2017 12:04 PM
To: Matvienko, Andrey
<andrey_matvienko@xxxxxxxxxx<mailto:andrey_matvienko@xxxxxxxxxx>>; IBIS-ATM
<ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>
Subject: [ibis-macro] Re: [ibis-interconn] Re: Question about references in
BIRD189, and related comments about AMI Flows
All,
The current statement in BIRD 189
If this parameter (Unused_port_termination)is not defined, the EDA tool may
connect terminals to terminations as needed to prevent numerical instability in
simulation (EDA tools are recommended to alert users when this occurs and
document the termination value used). Note that the terminals remain
technically open, and terminations connected by the EDA tool are intended to
approximate open-circuit conditions.
Can be confusing (as indicted by how Andrey interprets it, see below).
We all need to remember that IBIS is an ASCII representation of a model (I/O
model, interconnect model, Touchstone file) based on "measurements" made on the
I/O model or interconnect. "Measurement" here can be based on actual test bench
measurements, SPICE simulations, extraction, ...
If the component is sitting on a table in the "dead bug" position, then
technically all of the pins of the component are in open-circuit condition. As
soon as the package is inserted in a board (or the die is mounted in the
package) the model maker has no ideas how the pin (or die pad is terminated).
It is the User/EDA too who knows this.
We continually make the mistake of telling the EDA tool how to use IBIS models,
rather than just describing the format of the data describing the models and
how to generate this data. This is so aptly demonstrated by the recent debates
over BIRD 166 and BIRD 190.
I suggest the following change:
If this parameter (Unused_port_termination)is not defined, the EDA tool may
connect terminals to terminations as needed to prevent numerical instability in
simulation (EDA tools are recommended to alert users when this occurs and
document the termination value used). The terminations connected by the EDA
tool are determined by what the unused component pins are connected to.
Some other unfortunate statements about how AMI models shall be used are:
System simulations will commonly involve a transmitter (Tx) and a receiver (Rx)
executable model file, each of which may perform filtering in the AMI_Init
function, the AMI_GetWave function, or both (i.e., a "dual" algorithmic model).
In the case of a "dual" algorithmic model, the filtering functionality in the
AMI_Init and AMI_GetWave functions are each intended to be independent
representations of the device's equalization. Users of a dual model can elect
to use either the AMI_Init or AMI_GetWave filtering functionality, but not
combine both simultaneously.
While the primary purpose of the AMI_Init function is to perform the required
initialization steps, it may also include LTI signal processing algorithms.
Therefore, statistical simulations may be performed using the AMI_Init function
alone.
Even though time domain simulations may also be performed with the LTI AMI_Init
and/or LTI AMI_GetWave functions, AMI_GetWave functions containing non-LTI
algorithms may only be simulated in the time domain.
How does a User or EDA tool know that a models AMI_GetWave is LTI or non-LTI.
No AMI Model can be perfectly LTI, so who makes the judgment that a AMI Model
can be used as an LTI model? Even if a model is truly LTI, it is
straightforward for a model make to include an AMI_GetWave as a convenience for
EDA tools when doing time domain simulations with other models in the channel
that have an AMI_GetWave - this eliminated deconvolution steps.
Walter
Walter Katz
wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>
Phone 303.449-2308
Mobile 303.335-6156
From: Matvienko, Andrey [mailto:andrey_matvienko@xxxxxxxxxx]
Sent: Thursday, July 20, 2017 11:45 AM
To: Walter Katz <wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>>
Subject: RE: [ibis-interconn] Re: Question about references in BIRD189
HI Walter,
Thanks for comments. Yeah, with the current requirement currently put in BIRD
(requirement for open-circuit approximation for unused port) the only
reasonable thing is exactly "ignore all of the rules on how to deal with unused
terminals" :)
Btw I don't try to "blame" nobody, by some reason I myself noticed it only
yesterday, at that I have been reviewing this BIRD quite regularly as it has
been evolved. Yesterday I was literally shocked finding that we sort of
"enforce" open- circuit for unused ports in this BIRD, by default. We also have
been using "advanced package" features in our tools almost "forever" (our
solution have been based on extensions to IBIS external circuit) so we are very
well familiar with topics of terminating of unused ports and of course we will
continue doing it properly despite of direct "recommendation" to use
open-circuit in the BIRD.
Still I think that that controversial require would be removed from the spec...
Thanks,
Andrey
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