Effects of Mechanical Tolerances of USB 2.0 Cables on Skew and Radiated Emission

This paper analyzes skew due to unavoidable mechanical tolerances of a universal serial bus (USB) 2.0 cable. The contribution of the tolerances to the skew is determined via full-wave simulation of the cable cross section. The contributing factors and interaction terms are found using fractional factorial experimental design. Each factor's sensitivity to skew is investigated using Monte Carlo simulations. The design of experiments approach allows recognition of the relationship between the mechanical tolerances and skew. The skew-emission relationship, however, is found through a systematic approach. It was observed that adding about 20-30 ps net skew leads to about 4-5 dB increase in the emission above 350 MHz.