[ibis-macro] Re: Question on dividing up the Tx behavior between the AMI and analog portions of the model

  • From: "Chen, Terry" <Terry.Chen@xxxxxxxxxx>
  • To: "twesterh@xxxxxxxxxx" <twesterh@xxxxxxxxxx>, 'IBIS-ATM' <ibis-macro@xxxxxxxxxxxxx>
  • Date: Thu, 8 Mar 2012 14:01:22 -0500

Hi Todd,

Yep, I am aware. I suppose I should elaborate further. I had initial 
correlation issues (especially when channel mismatch and reflects occurs) with 
it due to the same problem you mentioned. What I do now, in order to get the 
better correlation, is to factor out my on-die termination (ODT) and package 
parasitics as two separate S4P and use them in my channel simulator to 
represent my output impedance mismatch.

In fact, I started down this path, because I don't know of a good way to use 
IBIS analog "directives" to properly represent my ODT (it's a T-coil like 
structure intended to broadband the output impedance across a frequency range). 
Now, I don't want to hijack David's original thread, so I can start another 
thread and I can sketch out a rough outline of my ODT and we can all discuss 
what is the proper way to model it with IBIS.

Thanks,
Terry


From: ibis-macro-bounce@xxxxxxxxxxxxx [mailto:ibis-macro-bounce@xxxxxxxxxxxxx] 
On Behalf Of Todd Westerhoff
Sent: Thursday, March 08, 2012 1:52 PM
To: 'IBIS-ATM'
Subject: [ibis-macro] Re: Question on dividing up the Tx behavior between the 
AMI and analog portions of the model

Terry,

If you idealize either the TX analog driver or the RX termination network, you 
will miss the interaction that component has with the channel, and the ISI that 
results from it.  While you can model an analog output transfer function inside 
the algorithmic model, but you won't get *any* of the reflections that result 
from the discontinuities (e.g. capacitance) presented by the TX output or the 
ISI that results from that.

Depending on your measurement setup, it's easy to miss this ... or, for that 
matter, to misdiagnose much of the ISI as jitter.   Bottom line, IBIS-AMI 
assumes that the "analog channel" captures the combined behavior of the TX 
analog output - channel - RX termination network, and idealizing either the TX 
or RX analog models violates that assumption.

And - I repeat - it's easy to miss.  There are lots of cases where things look 
like they correlate (at least initially) when they actually don't.

My $0.02.

Todd.


Todd Westerhoff
VP, Software Products

Signal Integrity Software Inc. * www.sisoft.com<http://www.sisoft.com/>
6 Clock Tower Place * Suite 250 * Maynard, MA 01754
(978) 461-0449 x24  *  twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx>


"Three in the morning and I'm still awake,
So I picked up a pen and a page ... "
                                             -Sidewalk Prophets

From: ibis-macro-bounce@xxxxxxxxxxxxx [mailto:ibis-macro-bounce@xxxxxxxxxxxxx] 
On Behalf Of Chen, Terry
Sent: Thursday, March 08, 2012 1:33 PM
To: DBanas@xxxxxxxxxx; 'IBIS-ATM'
Subject: [ibis-macro] Re: Question on dividing up the Tx behavior between the 
AMI and analog portions of the model

Hi David,

Actually I am interested in other's response to this question as well...

But, for the TX Driver I am currently modeling, I am doing exactly what you 
have prescribed and using the IBIS-analog portion as effectively an ideal step 
function (by setting my ramp with extremely high rise/fall dv/dt) and letting 
the step response filter inside my AMI model to shape my output waveform. Now, 
I am not sure if this is the "right" or "ideal" way to do it, but I am getting 
a reasonably good correlation in my Re-driver model with the actual lab 
measurements (the max jitter mismatch is < 8ps).

I hope this is at least an useful data point for you.

Regards,
Terry

From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx> 
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx]<mailto:[mailto:ibis-macro-bounce@xxxxxxxxxxxxx]>
 On Behalf Of David Banas
Sent: Thursday, March 08, 2012 1:15 PM
To: 'IBIS-ATM'
Subject: [ibis-macro] Question on dividing up the Tx behavior between the AMI 
and analog portions of the model

Hi all,

Is it customary to split up the Tx behavior, such that the FFE is modeled in 
the AMI model and the pulse shaper in the analog model?
Or, is there a different dividing line that has been identified as "best 
practice".
(Or, am I completely off in the weeds?)

The context for this question: I just managed to get good correlation between 
our latest Tx AMI model and the HSPICE model.
And then I realized that, having dumped all of the behavior into the AMI model, 
I would need to put an ideal step function into the V-T curves of the analog 
IBIS model. And I wasn't sure that would be a good idea. (I'm guessing that 
that would reek havoc in most simulators; is that correct?)

Thanks,

David Banas
Sr. Member Technical Staff
Altera<http://www.altera.com/>
+1-408-544-7667 - desk

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