[ibis-macro] Re: another silly IBIS-AMI question about deriving the impulse response of a channel

  • From: Mike Steinberger <msteinb@xxxxxxxxxx>
  • To: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • Date: Fri, 28 May 2010 11:15:00 -0500

Scott-

I think what you're pointing out, quite correctly, is that there can be passive circuit elements which occur on the die but before the transition to the high impedance level typically used for most of the receiver. These circuit elements can include series elements as well as shunt elements. If these series circuit elements are left out of the network analysis, then the accuracy of the result will necessarily be compromised.

!->This statement applies regardless whether the analysis is performed in the time or frequency domain. If you leave circuit elements out, the accuracy of your analysis will be degraded.<-!

To date, IP vendors have not been inclined to provide equivalent circuits for their on-die termination networks, with or without passive series circuit elements; and the most general IBIS models do not include series circuit elements. If these on-die series circuit elements are to be included in the network analysis, therefore making more accurate analyses possible, then one of three things must occur:
1. The IP vendors supply equivalent circuits for their on-die networks. (Not happening)
2. The IP vendors supply encrypted models (SPICE or otherwise) and we all use those.
3. IBIS provides a more general mechanism for describing the circuit behavior of these networks while hiding the internal details of the network.

In the past, we've proposed on-die S parameters as a relatively simple and effective solution to this problem.

Mike S.

On 05/28/2010 08:25 AM, Scott McMorrow wrote:
Mike

The interesting thing about the compensation networks being used in transmitters and receivers today, is that the actual driver or receiver sits on a high impedance node formed by mutual inductance cancellation of the two 50 ohm line and termination ports.  A transient pulse response of the actual circuit will give the right answer.  But if, as I suspect many  do, frequency domain techniques are used to derive the impulse response, and  the network termination and receiver/driver are considered lumped at the same point, the resulting impulse response will be off by 10% and include additional pulse distortion, which is significant for a die.  The conclusion I made for all  impulse response generation methods is that the receiver/driver needs to be modeled at a minimum as a 3-port network per SE port, using a high impedance receiver/driver port whenever necessary, and the impedance deviation from 50 ohms needs to part of the modeling.  Otherwise, the resulting impulse response, or derived waveforms passed to the AMI DLL will be incorrect.   There are also some numerical issues that need to be addressed for s-parameters with mixed 50 ohm/10K ohm ports.

regards,

Scott



On 5/28/2010 4:52 AM, Mike Steinberger wrote:
Scott-

Excellent answer. Thanks for the information.

Mike S.

On 05/27/2010 06:25 PM, Scott McMorrow wrote:

I also wonder what you mean by "large errors". What is your numerical estimate of the magnitude of such errors? How did you perform that estimate? What data did you base your estimate on?

Mike S.


Numerical error estimate - approximately 9 to 10%, depending on compensation inductance
  • Lumping the receiver at the termination introduces about an additional 9% loss in eye height, along with pulse distortion.
Estimate performed using HSPICE transient simulation of receiver circuit
Published data on device input characteristics correlated to measurements




-- 
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
  


-- 
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC

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