[ibis-macro] Re: On impulse and step responses.

  • From: David Banas <DBanas@xxxxxxxxxx>
  • To: Todd Westerhoff <twesterh@xxxxxxxxxx>
  • Date: Wed, 26 Jun 2013 15:27:19 -0700

Almost; I'm not sure about #3. It seems some EDA tools are assuming that the 
proper units are "Volts/sec.", which is not how I read the spec., as per my 
previous response to you. Perhaps, we could get all the EDA tool vendors to 
comment?

-db


From: Todd Westerhoff [mailto:twesterh@xxxxxxxxxx]
Sent: Wednesday, June 26, 2013 1:37 PM
To: David Banas
Cc: ibis-macro@xxxxxxxxxxxxx
Subject: RE: On impulse and step responses.

Dave,

Thanks for your response.

From a specification standpoint, it seems to me that you're saying the 
following:


1.       The language in the current spec describing the impulse response 
matrix is imprecise and should be improved

2.       As we're dealing with a discrete time application, we should spell 
that our more precisely when describing the impulse response

3.       Given (1) and (2), the impulse responses currently being generated by 
EDA tools and the AMI models that use them are OK as-is and do not need to be 
changed.

Did I get that right?

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>

"I want to live like that"
                                             -Sidewalk Prophets

From: David Banas [mailto:DBanas@xxxxxxxxxx]
Sent: Wednesday, June 26, 2013 1:07 PM
To: Todd Westerhoff
Cc: ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: RE: On impulse and step responses.

Hi Todd,

There are two things being discussed, which is probably adding to the confusion.

Firstly, I'm challenging Mike's belief that the discrete time equivalent to the 
Dirac delta is the sequence, {<sample_rate>, 0, 0, ...}. I believe the discrete 
time equivalent to the Dirac delta is the sequence, {1, 0, 0, ...}. I think 
this one has probably been exhausted, unfortunately without resolution.

Secondly, the current spec. fails to name the units, which are to be assumed 
for the values passed into Init(), via the impulse_matrix parameter. This is 
the issue, which is less academic, more practical, and more worthy of the 
committee's time. Here is the current relevant language, excerpted from IBIS 
v5.1:

"impulse_matrix" points to a memory location where the collection of channel 
voltage impulse responses, ... The algorithmic model is expected to modify the 
impulse responses in place by applying a filtering behavior, for example, an 
equalization function, if modeled in the AMI_Init function. ...

(Note that my omissions of any original text are indicated by ellipses, and any 
emphasis is entirely mine.)

Now, the language "impulse response" is vague in signal processing parlance, as 
it can refer to either:

1.       The continuous time "impulse response function", or

2.       The discrete time "unit pulse response sequence",
both of which are more precise concepts.

In deciding which of the two interpretations, above, to accept, one notes the 
use of the language, "voltage" (i.e. - NOT "volts/sec."), as a qualifying 
preface to the term, "impulse response." Therefore, one could defensibly argue 
that `2' should be assumed, since the continuous time impulse response function 
must have units of "Volts/sec.", as has been pointed out now numerous times in 
this discussion.

Further confidence in choice '2' is gained, by noting that ours is a 
necessarily discrete time application. (It takes place entirely within the 
state space of a digital computer, and the interface between the model and the 
rest of the system is a discrete sequence of numbers.)

Finally, the language, "in place by applying a filtering behavior," suggests 
that the model should expect to be receiving values with units most natural to 
direct digital filter application to the unmodified input, which would be 
"Volts".

-db


From: Todd Westerhoff [mailto:twesterh@xxxxxxxxxx]
Sent: Wednesday, June 26, 2013 7:39 AM
To: David Banas
Cc: ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: On impulse and step responses.

Dave,

Can I ask you to confirm your initial question on this subject? I want to make 
sure I understand what we're saying before we get too deep into the math.

It seems to me that you've called into question the mathematics behind impulse 
responses being created for current IBIS-AMI models, and therefore how IBIS-AMI 
models must be written to process those impulse responses.  If there is indeed 
a problem with the math, it follows that the current standard would need to be 
either updated or extended, depending on how the details play out.

Is that what you're saying?

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>

"I want to live like that"
                                             -Sidewalk Prophets



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