[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: "scott@xxxxxxxxxxxxx" <scott@xxxxxxxxxxxxx>, "twesterh@xxxxxxxxxx" <twesterh@xxxxxxxxxx>
Date: Thu, 8 Mar 2012 14:23:40 -0500

Hi Scott,

Thanks for your suggestions. Please see my response to Todd for what I am 
currently doing to model TX output impedance mismatch. And, maybe the way I am 
currently doing it now is wrong (and I won't be surprised if it is, b/c I am 
still relatively new to this IBIS AMI modeling).

Now, I have another question for you. What sorts of jitter correlation can you 
normally expect with IBIS-AMI?

Although 8ps is not great, it doesn't seem unreasonable when compared to our 
semiconductor variation. I know that there are some weakness to the way I am 
modeling. For e.g., the TX output driver is actually highly non-linear and my 
step response modeling effectively linearizes it. Finally, I am also not 
modeling the non-linear effects of gain compression (when my TX FFE gain gets 
large) and the addition of some systematic jitter due to addition of internal 
offsets at each diff pair internally.

But I will go back into  the lab and see if I can correlate with worse channels 
with lots of discontinuities and poor terminations to verify my assertions. I 
will report back later and let you guys know my results.

Regards,
Terry



From: ibis-macro-bounce@xxxxxxxxxxxxx [mailto:ibis-macro-bounce@xxxxxxxxxxxxx] 
On Behalf Of Scott McMorrow
Sent: Thursday, March 08, 2012 2:06 PM
To: twesterh@xxxxxxxxxx
Cc: IBIS-ATM
Subject: [ibis-macro] Re: Question on dividing up the Tx behavior between the 
AMI and analog portions of the model

Good response Todd.

The following are things to look out for in correlation.
*         short channels with low loss and high return loss.
o    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.
o    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.
o    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<mailto: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.


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> 
[mailto: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<mailto: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

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<http://www.teraspeed.com/>



Teraspeed(r) is the registered service mark of

Teraspeed Consulting Group LLC

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

References:
- [ibis-macro] Question on dividing up the Tx behavior between the AMI and analog portions of the model
  - From: David Banas
- [ibis-macro] Re: Question on dividing up the Tx behavior between the AMI and analog portions of the model
  - From: Chen, Terry
- [ibis-macro] Re: Question on dividing up the Tx behavior between the AMI and analog portions of the model
  - From: Todd Westerhoff
- [ibis-macro] Re: Question on dividing up the Tx behavior between the AMI and analog portions of the model
  - From: Scott McMorrow

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

Other related posts: