EMI common-mode current dependence on delay skew imbalance in high speed differential transmission lines operating at 1 gigabit/second data rates

Abstract

EMI-related common-mode currents in a high speed differential transmission line circuits can be generated by delay imbalance (skew) at a rate almost directly proportional to the amount of skew imbalance. Delay skew was shown to generate common-mode currents at a rate of four times that of slew rate skew. Radiated EMI levels were shown to follow increasing common-mode currents. Attention to delay skew imbalance should be an important design consideration at the chip level, the transmission line level and at the load in order to produce high speed differential circuits with low emission characteristics. While models using identified waveforms predict common-mode waveform harmonics that include only odd harmonics, measurements with real devices indicate the presence of all harmonics due to waveform asymmetries, such as dirty cycle distortion and rise/fall time asymmetries.