[ibis-macro] Re: CTLE and Wave Shape filtering?

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • Date: Thu, 14 Oct 2010 18:18:39 -0400

 sorry everyone.  I sent that from the wrong email account.

Scott McMorrow
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On 10/14/2010 6:15 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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