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52 DESIGN007 MAGAZINE I APRIL 2019 Fast and Accurate Transmission Line Modeling The ability to simulate complex PCB design has become a critical factor in the success of a project. Today's high-speed processors and SERDES interfaces coupled with sometimes unrealistic time-to-market requirements are pushing design teams toward more nimble development processes. However, there is no point in completing a design on time if it does not work! To ensure the prototype has first-pass suc- cess, designers need to perform accurate board-level simulation, throughout the entire design process. But the challenge is how do you quickly perform the simulation if all the models for the board are not available and if the simulation setup time is prohibitive? En- ter the I/O Buffer Information Specification (IBIS). This spec- ification is a fast and accurate behavioral method of model- ing input/output buffers based on I-V curve data derived from measurements or full circuit simulation. Figure 1 depicts the Ibis (bird). With its greedy, little black eyes and its long, trash- probing beak, the Ibis has readily adapted to its environ- ment. We Australians collo- quially call it "the bin chick- en" since its six-inch curved beak has been sculptured by evolution to effortlessly reach to the bottom of a Macca's (MacDonald's) fries container. Getting back to simulation, before the release of the IBIS specification, SPICE transistor-lev- el models were the only consistent method by which circuit models could be created. How- ever, transistor-level models are not well suit- ed to simulate an entire multilayer PCB con- taining several hundred nets and drivers. Also, semiconductor vendors that generate the mod- els for their integrated circuits do not readily give these models out since they can disclose proprietary information that is confidential, in- cluding circuit nodal connections and under- lying fabrication processes parameters. Mean- while, the IBIS model does not require propri- Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: The Ibis. (Source: Australian Geographic)