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

  • From: "Todd Westerhoff" <twesterh@xxxxxxxxxx>
  • To: "'IBIS-ATM'" <ibis-macro@xxxxxxxxxxxxx>
  • Date: Thu, 08 Mar 2012 13:52:09 -0500 (EST)

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. 

 

Description: cid:EAFF2D52-4B63-4A05-9D24-B96BE375B7E0@eau.wi.charter.com



Todd Westerhoff

VP, Software Products

 

Signal Integrity Software Inc. .  <http://www.sisoft.com/> www.sisoft.com

6 Clock Tower Place . Suite 250 . Maynard, MA 01754

(978) 461-0449 x24  .   <mailto:twesterh@xxxxxxxxxx> twesterh@xxxxxxxxxx

 

 

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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] 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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_ot_mi_mi_tx_a_311> ? Take one today!

 

 

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