[ibis-macro] Re: CTLE and Wave Shape filtering?
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: msteinb@xxxxxxxxxx
- Date: Fri, 15 Oct 2010 10:56:43 -0400
Mike
Several comments. Below
Scott McMorrow
Teraspeed Consulting Group LLC
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On 10/14/2010 9:18 PM, Mike Steinberger wrote:
Scott-
Your conclusions are all correct. Thanks for the constructive insight.
Your follow-on questions bring out some important principles as well.
Consider that for either a driver or receiver, the combination of the
algorithmic and analog models should accomplish two goals:
1. The model voltage waveforms into or out of a matched load should
match the waveforms from the reference model (e.g., SPICE) under the
same conditions as closely as possible.
2. The impedance presented by the model to the interconnect network
should match the reference model's impedance under the same conditions
as closely as possible (i.e., need to present the correct reflection
coefficient to the interconnect network).
Thus, for a driver we make the analog model responsible for modeling
the output impedance and we do most of the wave shaping in the
algorithmic model. And yes, we do use recursive digital filters
extensively in driver algorithmic models for that purpose. The one
exception is that we typically try to set the output amplitude in the
analog model so that it is slightly more useful in an EDA tool that
supports IBIS models but not IBIS-AMI models.
This creates a bit of a fuzzy model boundary problem. Migrating all
analog wave shape control into the Algorithmic side of the model
guarantees that the IBIS Analog Model does not match the AMI model, and
the computed analog impulse response for the channel is wrong.
Reflection behavior for the Tx silicon will be incorrect, since
transmitter complex impedance is dependent upon not only the termination
resistance, but any pulse shaping that is performed in the termination
network. Since modern transmitters utilize peaking circuits like
T-coils between the distributed capacitance of the receiver and ESD
structures, you cannot move frequency response filtering of the output
waveform into the algorithmic model without seriously impacting the
broadband return loss of the transmitter. The best you can do is create
an approximate equivalent model that is good for a fixed line impedance.
Unfortunately, a package is anything but.
Lumping all wave shape control into the algorithmic section necessarily
violates your #2 goal.
You are also quite correct that IBIS-AMI modeling as it is currently
defined does not model common mode behavior. I submit that there is a
precedent in the sense that classic IBIS does such a poor job of
modeling differential mode transmission. This lack of direct modeling
of common mode behavior has two practical implications:
1. The model is really only defined for a single set of bias
conditions (e.g., common mode voltage). Therefore, well written
documentation for a model should state the bias conditions for which
the model is valid, and an intelligent user should make sure that the
bias conditions in their application match the bias conditions for the
model.
2. Drivers can generate significant amounts of common mode noise, and
that noise can be coupled into adjacent differential paths. Similarly,
differential receivers have only a finite amount of common mode
rejection and a finite common mode rejection range.
We have some ideas for addressing common mode behavior and others have
made suggestions to the IBIS-ATM subcommittee as well. To date,
however, no system developer has expressed interest in these problems.
If there is someone who thinks common mode modeling is important
enough to invest in, especially if they're a system developer, we'd be
happy to work with them.
I'm a system developer. I express interest. If I'm interested, my
customers and our common customers are interested. Common mode modeling
will make or break 25 Gbps and higher signaling systems. You cannot
correctly see packaging common mode issues if the modes are not being
excited, and the modeling is not extended into the power domain of the
package. Above 5 GHz, packages begin to exhibit electromagnetic
non-locality problems.
Mike S.
On 10/14/2010 05:15 PM, Scott McMorrow for Jim Bell wrote:
Mike,
Thanks, I understand the tradeoff that you've made. Essentially what
you are saying is that anything that is buffered from the channel by
a high impedance may be considered for AMI DLL modeling. From my
perspective a table of s-parameters could just as easily have been
used to model the CTLE. But given that at the time there was no
mechanism in IBIS to include s-parameters, I can understand why it
was placed in the algorithmic section.
How about on the driver side? You did not respond to the wave
shaping portion of the question. What is your stance on analog wave
shaping and filtering that are part of the driver, and are not
buffered from the channel? It appears that you are incorporating
these into the die S-parameter models. Is that correct? Does this
include any possible asymmetric driver behavior?
Finally, on the driver and the receiver side, how are common mode
effects integrated into the channel modeling? I'm guessing that
since the receiver is buried in the AMI DLL that no degradation due
to common mode offset is modeled, since the information has been lost
in the differential transformation. However, common mode inducing
skew and waveform asymmetry could certainly be incorporated into the
drive side and propagated through the transmission channels.
best regards,
Scott
On 10/14/2010 5:19 PM, Mike Steinberger wrote:
Scott-
In a paper we gave at DesignCon2008 (attached), we described the
modeling of a CTLE in the algorithmic model of a receiver. In this
paper, we happened to refer to the filter as a "peaking filter"
rather than a "CTLE". This approach is equally applicable to a
transmitter.
Since 2008, we have produced a number of IBIS-AMI receiver models
for a number of IP suppliers, and most of these receiver models
contained a CTLE. In each case, we have applied the techniques
described in the paper to the algorithmic model of the receiver, in
each case, we have achieved good correlation, and customers are
using these models to do real work.
I recommend against any attempt to incorporate the CTLE into the
analog model for two reasons:
1. The analog model will necessarily expose its internal structure
whereas the algorithmic model does not.
2. It's an awful lot easier to write these responses into the
algorithmic model than it is to design an analog circuit to produce
the desired response.
Have I addressed your question?
Thanks.
Mike Steinberger
On 10/14/2010 2:30 PM, Scott McMorrow wrote:
A question to the group.
Modern SERDES Tx and Rx circuits can employ both continuous time
linear equalizers (CTLE) at both the transmitter and receiver. In
addition, wave shape filtering is often employed to control the Tx
output spectrum. By definition, these circuits are in the analog
domain, and as a first approximation are linear time invariant (LTI).
How are these elements being handed with current AMI models within
the proposed comprehensive set of simulation flows, since IBIS Tx
drivers have only a partial ability to model wave shaping, and no
ability to model a CTLE? Are they currently being included as
additional external circuit models to be concatenated with the
remainder of the Analog channel?
Regards,
Scott
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