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

  • From: ckumar <ckumar@xxxxxxxxxxx>
  • To: <Terry.Chen@xxxxxxxxxx>
  • Date: Thu, 08 Mar 2012 11:31:57 -0800

if it is possible to isolate just the wave shaping problem, it is a good
idea to just compare it against a time domain circuit simulation. If you
just straight go to lab, you may not be able to focus just one problem at a
time


On Thu, 8 Mar 2012 14:23:40 -0500, "Chen, Terry" <Terry.Chen@xxxxxxxxxx>
wrote:
> 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!
> 
> 
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> 
> --
> 
> 
> Scott McMorrow
> 
> Teraspeed Consulting Group LLC
> 
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