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

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: twesterh@xxxxxxxxxx
  • Date: Thu, 8 Mar 2012 14:05:50 -0500

Good response Todd.

The following are things to look out for in correlation.

   - short channels with low loss and high return loss.
      - If you correlate with poorly terminated channels, or channels with
      built-in high-Q discontinuities, then the lack of correct analog modeling
      is readily seen.
   - In-package NEXT and FEXT is amplified by increased return loss.
      - Tx-Tx, Rx-Rx, and Tx-Rx crosstalk correlation can easily see 3 dB
      or more error when analog filtering is not modeled correctly.
   - Jitter will be affected significantly.
      - 8ps jitter mismatch may not seem like much, but that's 8% of a 10G
      channel, and 20% of a 25G channel.



On Thu, Mar 8, 2012 at 1:52 PM, Todd Westerhoff <twesterh@xxxxxxxxxx> wrote:

> 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. ****
>
> ** **
>
> [image: Description:
> cid:EAFF2D52-4B63-4A05-9D24-B96BE375B7E0@eau.wi.charter.com]
>
> ****
>
> *Todd Westerhoff*****
>
> VP, Software Products****
>
> ** **
>
> Signal Integrity Software Inc. • 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.****
>



-- 

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

GIF image

Other related posts:

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