# [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.

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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.
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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
|
|
2

where:
Port 1 = 50 ohm
Port 3 = 50 ohm
Port 2 = some high impedance.
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and Port 1 to Port 2 is the path used for computation of the impulse response
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If the Network is lumped into:

1 --- Network --- 2

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

Scott

--
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882