Techniques and considerations for verification of model causality

High data switching rates of today´s computer architecture continue to intensify the need for transmission line modeling accuracy. As these data rates exceed hundreds of megahertz and the physical complexity of the transmission channel increase, it may no longer be sufficient to ensure only the frequency dependency of channel models. Rather, the model developer must guarantee the model response is passive and causal. This is of particular interest given that non-causal models may not allow convergence or yield accurate channel loss within industry-standard tool suites, even though it´s response may yield reasonable correlation to measured scattering parameters. Therefore, model developers must understand how the model creation, checking and simulation tools work together to ensure validity of transient simulations. Commercially-available tools exist that provide numerical and visual interpretation of causality compliance (or violation) for any given touchstone model. It is critical, however, for the model developer to understand the verification process used within a chosen tool suite and how to interpret results. Moreover, model developers must know how model frequency content, step size, length and other parameters may impact a checking tool´s ability to accurately flag causality violations. As a result, the developer must have an in-depth understanding of the correlation between model content and causality error reporting, whether or not violations raise real concerns and if so, how they may impact the results obtained from the system-level simulation methodology. This paper will discuss the interaction between the model development process, the ability to verify model causality and the impact at the link or system-level as a result of non-causal models or inaccurate interpretation of causality-checking verification tools.