[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 16:07:57 -0500 (EST)

Terry,

 

Thanks for your response - makes perfect sense to me.  I think providing a
broad-brush view of your ODT circuit is a great idea.

 

The challenge of modeling "broadband analog behavior" in IBIS is not new.
A number of vendors have been using "on-die S-parameters" to model the TX
analog and RX termination circuits in a frequency-dependent fashion.  The
problem is that you can't do that in a way that's compliant with IBIS 5.0,
so the syntax for pulling the .s4p data into the simulation model varies
from simulator to simulator.  Proposed BIRDs 116-118, 122 and 144 have all
addressed this in one way or another, but those items are still under
discussion.

 

There are regular meetings of the IBIS Advanced Technology Modeling
(IBIS-ATM) working group Tuesdays at 3PM EDT.  It would be great if Analog
would participate, even if only briefly, to help underscore the need to
address this issue.

 

I look forward to seeing what aspects of the circuit you'll be able to
share.

 

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

 

 

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

                                             -Sidewalk Prophets

 

From: Chen, Terry [mailto:Terry.Chen@xxxxxxxxxx] 
Sent: Thursday, March 08, 2012 2:01 PM
To: twesterh@xxxxxxxxxx; 'IBIS-ATM'
Subject: RE: [ibis-macro] Re: Question on dividing up the Tx behavior
between the AMI and analog portions of the model

 

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. 

 

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

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

 

 

"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] 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

 

Did you know Altera offers over 150 free online technical training courses
<http://www.altera.com/servlets/searchcourse?coursetype=Online&WT.mc_id=t9
_ot_mi_mi_tx_a_311> ? Take one today!

 

 

  _____  

Confidentiality Notice.
This message may contain information that is confidential or otherwise
protected from disclosure. If you are not the intended recipient, you are
hereby notified that any use, disclosure, dissemination, distribution, or
copying of this message, or any attachments, is strictly prohibited. If
you have received this message in error, please advise the sender by reply
e-mail, and delete the message and any attachments. Thank you.

GIF image

Other related posts:

  1. Home
  2. ibis-macro
  3. Archive
  4. 03-2012