[ibis-macro] Re: CTLE and Wave Shape filtering?
- From: ckumar <ckumar@xxxxxxxxxxx>
- To: <msteinb@xxxxxxxxxx>
- Date: Fri, 15 Oct 2010 10:15:00 -0700
I agree with Scott
It is not a question of output impedance as a function of output voltage.
The issue is more of whether there is a high impedance connection which
prevents feed back.
At the output of the driver there is no such high impedance connection.
wave shaping if it happens at the output will be subject to strong feed
back from the rest of the channel. So the model has to be in the front end
analog circuit and not in the AMI model.
On Fri, 15 Oct 2010 11:39:02 -0500, Mike Steinberger <msteinb@xxxxxxxxxx>
wrote:
> Scott-
>
> This conversation is useful in that it addresses the assumptions that
> have been the basis for algorithmic modeling from the very beginning. I
> think it's useful to state those assumptions clearly and examine the
> conditions under which they're valid.
>
> Your statement
>
>> 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.
>
> only makes sense if you assume that the output impedance is a function
> of the output voltage. While this assumption is certainly true for a
> single ended driver, it turns out to be a second order effect for a
> balanced differential driver. We have demonstrated this with SPICE
> models for a number of different differential drivers from a number of
> different IP suppliers. The nonlinear effects on reflection coefficient
> are there, but they're so small that you really have to look for them to
> find them. In the case we examined in greatest detail, the effect at the
> driver output was less than 1% of the driver amplitude.
>
> This brings up another useful general principle: Nonlinear effects are
> more important for low loss channels (e.g., < 5 dB at r/2) than they are
> for higher loss channels (e.g., > 5 dB at r/2). Consider that the first
> order effect of a nonlinearity is to generate harmonics of the input
> signal, thus increasing the spectral density at higher frequencies. For
> a low loss channel, these higher frequency spectral components will be
> evident at the receiver input and will therefore affect the receiver's
> behavior. This is usually the case for a parallel interface (e.g.,
> DDR2), for example. For a lossy channel, however, the loss increases
> with frequency and so the channel attenuates the higher frequency
> spectral components (including those generated by any nonlinearities)
> more than it does the lower frequency spectral components (especially
> those generated by the linear response).
>
> In short, for high speed serial channels (i,e, those with high loss
> channels), a linear driver model produces results which are more than
> accurate enough to accurately predict link performance. Given this
> assumption of linearity, the output impedance and output wave shape of a
> driver are independent.
>
> I do agree with you that the boundary between the algorithmic model and
> the analog model is fuzzy; and in fact we have sometimes found it useful
> to move that boundary for a given model. The only responsibility that
> can be unequivocally assigned is that the analog model is solely
> responsible for modeling the impedance presented to the interconnect
> network. We generally assign the more complex behaviors such as
> equalizer behavior or wave shape to the algorithmic model, however,
> because the algorithmic model can easily, flexibly, and efficiently
> implement behaviors that would be extremely difficult to implement in an
> analog model.
>
> Mike S.
>> 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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