Todd,
Thanks for that clarification. Yes this is with the assumption that
the IR is derived from a step response, but all other methods
would need to provide equivalent results...
Would everyone on this email list agree with Todd's interpretation?
Thanks,
Arpad
====================================================
From: Todd Westerhoff [mailto:twesterh@xxxxxxxxxx]
Sent: Monday, November 6, 2017 10:33 AM
To: Muranyi, Arpad <Arpad_Muranyi@xxxxxxxxxx>; ibis-macro@xxxxxxxxxxxxx
Subject: RE: [ibis-macro] Re: Question about "impulse response"
Arpad,
My interpretation is that the channel impulse response passed to Tx_Init will
be the result of differentiating the 800 mV differential step response, as you
say.
That, of course, is assuming that the impulse response is derived from a step
response and not through some other method ...
Todd.
Todd Westerhoff
VP, Semiconductor Relations
Signal Integrity Software Inc. * www.sisoft.com
6 Clock Tower Place * Suite 250 * Maynard, MA 01754
(978) 461-0449<978-461-0449> x124 * twesterh@xxxxxxxxxx
"I want to live like that"
-Sidewalk Prophets
From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx>
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Muranyi, Arpad
Sent: Thursday, November 2, 2017 2:27 PM
To: ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Re: Question about "impulse response"
Mike,
I am trying to apply what you are saying to IBIS-AMI. Let's use an example in
which
the differential Tx buffer has a 400 mV voltage swing on each of its outputs,
and as a
result, the difference step waveform at the Rx goes from -400 mV to +400 mV
(i.e.
the amplitude of the step waveform is 800 mV). The simulator will
differentiate this
step waveform and pass it into the AMI Tx Init function as the impulse response
of
the channel (or convolve with the output of the Tx GetWave function for the time
domain AMI simulation). Is this the correct interpretation of "impulse
response" in
the IBIS-AMI specification? Or should the 800 mV difference step waveform be
divided by 2 first before differentiating it to become the "impulse response"
for the
purposes if AMI simulations?
Thanks,
Arpad
===================================================================
From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx>
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Mike Steinberger
Sent: Thursday, November 2, 2017 9:02 AM
To: ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Re: Question about "impulse response"
Arpad-
If you want an NRZ signal with unit amplitude, the symbol voltages need to be
-0.5 and +0.5. The symbol voltages can then be multiplied by the signal
amplitude in the system being modeled to correctly represent the signal in that
system. Choosing -1.0 and +1.0 is simply an error to begin with, unless one
then multiplies by half the signal amplitude in the system being modeled.
My 2C.
Mike Steinberger
On 11/02/2017 12:27 AM, Muranyi, Arpad wrote:
Todd, Walter,
I understand that the impulse response represent the analog channel, including
the Tx and Rx buffers.
I also understand that the voltage swing the Tx can drive into the channel is
also captured by this
channel response, and for that reason we are not talking about "unit" ***
responses.
Let me try to explain my question in different words:
First, let's assume for the sake of this conversation that the channel impulse
response is generated
by taking the derivative of the channel step response waveform. With this
assumption we are
shifting the question to what is the definition of the channel's step response.
Let's assume that
a step response is going from logic '0' to logic '1'. The IBIS Tx buffer has a
digital stimulus internal
to the simulators which does just that.
Using that stimulus, the non-inverting output of Tx goes '0' to '1' and the
inverting output goes
'1' to '0'. However, in IBIS-AMI we are dealing with the difference waveform
at the Rx input, in
which case the states would be '-1' and '+1'. (We all know that the difference
waveform is usually
twice the magnitude of the two individual output waveforms of a differential
buffer). So if the
states of the difference step waveform obtained at the Rx correspond to '-1'
and '+1', while the
"text book" definition of a step response is '0' to '1', than we should
theoretically divide the channel's
difference step response waveform seen at the Rx input pins by 2 in order to
arrive to a "text book"
channel step response (which corresponds to the states '0' and '1') before we
differentiate that
step response to generate the channel's "impulse response".
But I am afraid that this is not what all EDA vendors do when they generate the
impulse response
for the AMI Tx Init function. I suspect (this needs to be verified) that most
simulators just take the
channel step response difference waveform at the Rx and take the derivative of
that to turn it into
a channel impulse response for the Tx AMI Init function.
This is why I wanted to find a definition in the IBIS-AMI specification for
what "impulse response"
is, but I didn't find the answer to my question. And since I don't like
customers saying things like
"but other simulators do this or that", I would like to make sure that we, EDA
vendors, are all in
agreement on this question.
This might also have an effect on how we deal with single ended channel
response waveforms.
Theoretically those should not be divided by 2, but then how would the EDA tool
know when is
the waveform single ended or differential in order to decide when the channel
response should
be divided by 2 or not... Or, should this be decided by looking at the [Diff
Pin] keyword and check
whether the [Model] is differential or not?
Thanks,
Arpad
=============================================================================
From: Todd Westerhoff [mailto:twesterh@xxxxxxxxxx]
Sent: Thursday, October 26, 2017 9:48 AM
To: wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>; Muranyi, Arpad
<Arpad_Muranyi@xxxxxxxxxx><mailto:Arpad_Muranyi@xxxxxxxxxx>;
ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: RE: [ibis-macro] Re: Question about "impulse response"
Arpad & Walter,
I think the thing that we need to be really clear about in IBIS-AMI is that the
impulse response represents the analog channel, *NOT* the passive channel. I'll
admit to only having done a quick search, but the only two places where I find
that point made are at the start of 10.2.2.3.1 and the start of 10.2.2.3.2
(both on page 177). Both of these are indirect statements; they require that
you understand what the analog channel is and how it's different from the
passive channel.
So - to the original question, I would say it's "under-defined" and could use a
bit of beefing up.
Todd.
Todd Westerhoff
VP, Semiconductor Relations
Signal Integrity Software Inc. * www.sisoft.com<http://www.sisoft.com>
6 Clock Tower Place * Suite 250 * Maynard, MA 01754
(978) 461-0449<978-461-0449> x124 * twesterh@xxxxxxxxxx
"I want to live like that"
-Sidewalk Prophets
From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx>
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Walter Katz
Sent: Wednesday, October 25, 2017 9:51 PM
To: Arpad_Muranyi@xxxxxxxxxx<mailto:Arpad_Muranyi@xxxxxxxxxx>;
ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Re: Question about "impulse response"
Arpad,
https://en.wikipedia.org/wiki/Impulse_response
https://www.google.com/search?q=impulse+response+is+derivative+of+step+response&spell=1&sa=X&ved=0ahUKEwjXioCTlo3XAhXqslQKHfknDr8QvwUIJSgA&biw=1468&bih=782
The impulse response of a channel has a clear definition in the industry. Why
does IBIS need one?
Walter
Walter Katz
wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>
978.461-0449 x 133
Mobile 303.335-6156
From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx>
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Muranyi, Arpad
Sent: Wednesday, October 25, 2017 9:01 PM
To: ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Question about "impulse response"
Hello Everyone,
I would like to find out what the definition of "impulse response" is in
the AMI specification. I thought we had something about that in the
spec, but I can't find anything now that I am looking for it. Am I just
not seeing the forest from the trees, or is it really not defined?
I thought we did have something along the lines of "derivative of a
step response", but I can't find anything...
Thanks,
Arpad
======================================================
