[ibis-macro] Re: CTLE and Wave Shape filtering?
- From: Mike Steinberger <msteinb@xxxxxxxxxx>
- To: Scott McMorrow for Jim Bell <jim@xxxxxxxxxxxxx>
- Date: Thu, 14 Oct 2010 20:18:55 -0500
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.
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.
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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