Analysis and Optimization of Fast Ethernet Terminations for Reduction of Differential-to-Common-Mode Conversion

Bob Smith (BS) termination is usually adopted to terminate twisted wired pairs in order to reduce common mode (CM) interference, as suggested by Ethernet board design guidelines compliant with the 100BASE-TX standard. However, there is not a robust proof of the effectiveness of the specific component values for this termination. This article proposes a detailed analysis and design technique for the termination components that allow for effectively suppressing CM disturbance at the Ethernet board transmission driver ports. The focus is on the analysis of the CM reflection and differential-to- CM conversion. To this end, an equivalent circuit of the system is derived and characterized at the transmission ports by a suitably-reduced mixed-mode S-parameters matrix. Parametric analysis and multiobjective optimization are used to optimize resistance value efficiently. The final results indicate the necessity of identifying a ground path for CM currents, and confirm the effectiveness of BS termination. Regarding the value of the involved resistors, it is proven that it should be defined based on the specific system configuration and application requirements, i.e., there is not a single optimal value applicable to all configurations. Accordingly, instead of a specific value, a range of optimal resistance values is proposed.