EMI Emissions from mismatches in High Speed Differential Signal Traces and Cables

The use of so-called differential signaling for high speed signals has become very common in today´s high speed system designs. While this signal strategy allows better data quality and signal integrity, there are some significant EMC and signal integrity issues that are not readily apparent. This paper demonstrates that a significant amount of common-mode current can be created when the two legs of the differential signal are skewed in time, or if the rise and fall times of the signals differ. These common mode currents can impact the EMC performance, causing significant levels of emissions unless careful design consideration is given to shielding and common-mode (ground) current return.