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

  • From: "Walter Katz" <wkatz@xxxxxxxxxx>
  • To: <james.zhou@xxxxxxxxxx>, <Terry.Chen@xxxxxxxxxx>, <DBanas@xxxxxxxxxx>, "'IBIS-ATM'" <ibis-macro@xxxxxxxxxxxxx>
  • Date: Mon, 12 Mar 2012 19:49:40 -0400 (EDT)

James,

 

The IBIS analog model is only useful to determine the Impulse Response of
a channel. You are absolutely correct that in reality a SerDes Tx analog
portion takes the output of the algorithmic section of an AMI model to
drive an analog model (e.g. an on-die S-parameter s4p, a simpler RC
circuit at described in BIRD 122, or an ISS subckt as defined in BIRD
116). IBIS AMI assumes an LTI channel, and IBIS-ISS defines all of the LTI
elements available in HSPICE.

 

This is the fundamental confusion of using [External Model] with IBIS-ISS
subckts describing the analog section. As written now, BIRD 116 identifies
the input of the Tx ISS subckt using the D_to_A statement, which
essentially defines a voltage swing and rise time - equivalent to Ramp.
The correct interpretation is that the D_to_A statement is only to define
the input to the Tx analog circuit, and is valid to determine the Impulse
Response of the channel.

 

Because of silicon drivers are in fact LTI, one can do the shaping of the
waveform in the algorithmic section, but one must be very careful to make
sure that return loss is properly accounted for.

 

Walter

 

From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of James Zhou
Sent: Monday, March 12, 2012 5:11 PM
To: Terry.Chen@xxxxxxxxxx; 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 and Terry,

 

Both of your emails mentioned "analog IBIS model" and "IBIS-analog
portion" represented by [Ramp] and/or [Rising/Falling Waveform] keywords
in IBIS file.  However, these "analog" IBIS models only take digital input
signals,  as stated in IBIS Specification 5.0, page 71-72 and section 6b.
The output of the "analog" IBIS model is not capable of tracking the
amplitude changes in the input (other than a rise/fall transition). It
would not make sense to feed the Tx AMI output to such digital inputs
based on IBIS Specification 5.0. 

 

If this approach of using [Ramp] and/or [Rising/falling Waveform] keywords
to represent "analog IBIS model" is adopted by IBIS AMI flow, some
clarification is needed on how to interpret and implement it. 

 

Regards,

James Zhou

QLogic Corp.

 

 

From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Chen, Terry
Sent: Thursday, March 08, 2012 10:33 AM
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!

 

 

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