[ibis-macro] another silly IBIS-AMI question about deriving the impulse response of a channel

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: IBIS-ATM <ibis-macro@xxxxxxxxxxxxx>
  • Date: Thu, 27 May 2010 18:03:43 -0400

Since the analog channel is required to be LTI from driver to receiver, I believe that in order to create a correct impulse response it is necessary to know what the termination impedance should be. In the case where a driver or a receiver is terminated at the end of the line, the termination impedance is obvious. So, for a 100 ohm differential system the differential impedance used for impulse response extraction is 100 ohms. Essentially the high-impedance receiver amplifier, or driver current sources are considered to be lumped at the termination network.

How are people modeling, extracting, and simulating drivers and receivers where the high-impedance ports are physically isolated from the termination, where the waveform in the algorithmic domain (driver/receiver) is substantially different than the waveform in the analog domain at the termination? I'm confused, since IBIS-AMI does not include the on-die termination network, except in the rudimentary and unsatisfying form of a *.ibs model, and that model is a lumped approximation. In my mind, in order to correctly extract an impulse response of a channel that will give a correct representation of the waveform at the receiver, one has to either perform a transient simulation of the full circuit from the driver to the receiver, or perform frequency domain modeling to impulse response transformations with known driver/receiver input impedance. Otherwise, the resulting impulse response is wrong.

For a hypothetical 50 ohm single-ended transmitter or receiver, the simplest circuit that can correctly model many transmitters and receivers is the following network:

1 --- Network --- 3

            Port 1 = 50 ohm
            Port 3 = 50 ohm
            Port 2 = some high impedance.
and Port 1 to Port 2 is the path used for computation of the impulse response

If the Network is lumped into:

1 --- Network --- 2

The resulting impulse response and waveform is different. How are people solving this problem?



Scott McMorrow
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