Marek,
Arpad and Todd both spoke truth, but they didn't get to the real answer that
you were looking for. In the simulators that I know about, the simulation to
characterize a channel uses a step function stimulus. The edge rate of the step
is what Arpad and Todd talked about. Generally, a step is much easier to
simulate than an impulse (dirac) function. We have done that for decades in
signal integrity. The important math here is that the impulse function is the
derivative of the step function. Therefore, in an LTI circuit, the impulse
response is the derivative of the step response. This is how most simulators
get the impulse response to pass into the IBIS-AMI model. I think you could
also calculate the impulse response through an IFFT of S parameters. In this
case, the response would be limited by the highest frequency in the S-parameter
data.
Regards,
Weston
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On Behalf Of
Westerhoff Todd
Sent: Wednesday, April 3, 2024 9:14 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: IBIS-AMI pulse shapes for channel characterization
I'm with Arpad on this one.
Remember that your channel models are normally S-parameters, which are
bandwidth limited in their own right. If your edge rate represents a higher
frequency than the channel model, then the simulated response will be based (at
least in part) in how the simulator extrapolates S-parameters to higher
frequencies. Extrapolation algorithms are tool-dependent, which creates another
whole set of issues.
It's true that the channel characterization stimulus should be a Dirac pulse,
in theory. In practice, it never has been.
Todd.
Todd Westerhoff
High-Speed Design Product Marketing | Electronic Board Systems (EBS) Siemens EDA
300 Nickerson Rd, Marlboro, MA 01752
todd.westerhoff@xxxxxxxxxxx | cell: 351-226-9037
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On Behalf Of
arpad.muranyi @ siemens . com
Sent: Wednesday, April 3, 2024 9:49 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: IBIS-AMI pulse shapes for channel characterization
Marek,
I can't recommend any books or papers on this subject, which doesn't mean that
there aren't any...
However, I would like to offer a simple answer to your question. While it is
true that SPICE-like simulators can't simulate an infinitely vertical pulse or
impulse, I don't think that it is really necessary in practice. Consider a
simple illustration that you could even try easily in your favorite simulator,
an RC circuit with an ideal pulse source with a finite edge (slope) as the
driver. The resistor is between the source and the capacitor, and you are
interested in the waveform at the capacitor.
This represents a "channel" in which the voltage source and resistor is the
transmitter (Tx) and the capacitor represents the losses in the traces
(channel) and the receiver's input capacitance.
How steep does the edge of the pulse of this ideal source need to be to obtain
the correct RC response waveform at the capacitor?
Mathematically it should be a vertical edge. But is it going to make a
difference if it is perfectly vertical, say if it has a 1 ps slope? Well, if
the RC constant is significantly larger than that 1 ps edge, say 1 ns, you will
probably not even notice that the driving edge is not vertical, or whether it
has a 1 ps or 2 ps slope. On the contrary, if the RC circuit have a 1 ps time
constant and your voltage source has a 1 ns slope, you would probably only see
the 1 ns slope at the capacitor, instead of the usual exponential RC waveform.
This handwaving argument applies to IBIS-AMI and similar channel
characterization simulations too. The conclusion is that you only need a
faster edge than the time constant of the channel you are measuring, but it
doesn't have to be a perfectly vertical edge as mathematics would like to have
it.
Does this help?
Thanks,
Arpad
===================================================================================================
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On Behalf Of
Marek Jableka
Sent: Wednesday, April 3, 2024 4:24 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] IBIS-AMI pulse shapes for channel characterization
Hi experts,
I am looking for some good handbooks, good papers or even a math description
for the pulses used to characterise the channel while running IBIS-AMI (SERDES)
simulations. In theory this is a Dirac pulse. In reality - using SPICE
simulations - dirac pulse is not possible to use from a numerical perspective -
I believe this needs to be closer to a triangular or maybe Gauss shape or ....
This is actually the question - from a numerical perspective, how the best
pulse looks like - anyone knows where to find such information?
Any hints are welcome :)
Regards,
Marek Jableka
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