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

  • From: James Zhou <james.zhou@xxxxxxxxxx>
  • To: "ckumar@xxxxxxxxxxx" <ckumar@xxxxxxxxxxx>, "Terry.Chen@xxxxxxxxxx" <Terry.Chen@xxxxxxxxxx>
  • Date: Thu, 8 Mar 2012 15:52:37 -0800

One of the requirements for IBIS AMI Tx analog model is to have sufficiently 
high reverse isolation. Otherwise the reflection from the analog channel behind 
Tx analog driver would impact the AMI Tx output waveform. Since most of the 
existing proposals enforce reverse isolation in Tx analog model, using a near 
all pass analog filter (i.e. ideal step function) would result in "miss the 
interaction (reflections) between the parasitivs and the pkg/pcb channel". I 
agree with Kumar and others that the AMI output (Tx analog input) should be 
chosen at a location where the reflections from analog channels (after the Tx 
analog driver) are practically isolated by the analog Tx driver.

James Zhou
QLogic Corp.


-----Original Message-----
From: ibis-macro-bounce@xxxxxxxxxxxxx [mailto:ibis-macro-bounce@xxxxxxxxxxxxx] 
On Behalf Of ckumar
Sent: Thursday, March 08, 2012 11:32 AM
To: Terry.Chen@xxxxxxxxxx
Cc: scott@xxxxxxxxxxxxx; twesterh@xxxxxxxxxx; IBIS-ATM
Subject: [ibis-macro] Re: Question on dividing up the Tx behavior between the 
AMI and analog portions of the model

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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> --
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>
> Scott McMorrow
>
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