All, I am enclosing the BIRD that defines AMI Parameters to support Touchstone Analog Buffer Models. I reviewed last week's meeting minutes. There was a request that I indicate whether the specification should define how the impulse response should be generated for a channel. The generation of the impulse response of a channel is well defined when the Tx and Rx buffers specify a Touchstone file in accordance with the enclosed BIRDs. Since the channel is LTI and these analog buffers are LTI there are many methods that can be used to generate the impulse response, and one can prove these methods should give identical results within numerical accuracy. The first method is to do a step response SPICE simulation using the differential voltage range specigied by Tx_R. The impulse response is the first derivative of the differential signal at the Rx Touchstone nodes specified on this BIRD. A second way is to convert the channel into an s4p Touchstone field. Using the S Parameter arithmetic James presented recently, all one needs to do is cascade the Tx s4p, channel s4p and Rx s4p into a single s4p, inverse Fourier transform this s4p, and voila, one has the impulse response of the channel. On can similarly describe various methods to generate the impulse response of a channel when the Tx and Rx are legacy models with linear IV curves. This is true because one can generate an s4p analog model for these Tx and Rx legacy model. Defining a method to generate the impulse response of a channel when the Tx and/or Rx have non-linear IV curves is problematic since the premise of AMI modeling implies Linear Time Invariant (LTI). Which is another way of saying a "Small Signal Interface". Since most implementations of the IBIS B element require a digital stimulus, and according to the common understanding that IBIS B elements (and external model) have digital stimulus, then an impulse response generate with a SPICE simulator using the IBIS B element can only generate a step response across the full swing of the Tx. Walter Walter Katz <mailto:wkatz@xxxxxxxxxx> wkatz@xxxxxxxxxx Phone 303.449-2308 Mobile 303.335-6156
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BIRD_AMI_Tstonefile.docx
Description: MS-Word 2007 document
