Experimental analysis of common mode currents on fibre channel cable shields due to skew imbalance of differential signals operating at 1.0625 Gb/s

The spectral nature of common mode currents induced on high speed differential cables operating at 1.0625 Gb/s was investigated using specially constructed shielded test boards. The source test board provided a source with a selectable amount of delay skew. The load test board provided a simple 150 Ohm differential load. The two boards were placed in separate shielded enclosures with a one meter fibre channel cable connecting them. The common-mode cable shield current and radiated emissions at 3 meters were measured as a function of delay skew. At the fundamental frequency of 531.25 MHz, the common-mode current and radiated emissions increased at a rate of approximately 9 dB/decade of skew. At skew values much lower than the rise time of the signal, the common-mode current increased nearly linearly with skew. The second harmonic was present on the cable shield due to duty cycle distortion and rise and fall time differences inherent in the driver transceiver