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

Differential signaling can produce a common mode signal on the signal ground or the inside of an interconnect cable if the two signals are not balanced. Fourier analysis of common mode time domain signals produced by differential signaling of a 1.0625 Gigabit (appropriate for the Fibre Channel protocol) continuous 1010...bit pattern showed that the common mode spectral components, particularly the fundamental 531.25 MHz component, are relatively insensitive to rise time but very dependent on delay skew. Slew rate skew was a minor contributor to the 531.25 MHz component compared to equivalent delay skew. When a 32-bit sequence approximating more realistic signals was analyzed, the frequency components occurred at 33 MHz (the repetition frequency of the 32-bit sequence) and multiples thereof. The major components were grouped around 531.25 MHz (half the bit rate) and the odd harmonics thereof (1.6, 2.7...GHz). Comparison of the 32-bit sequence with the continuous 010101 data stream showed that the peak magnitude of the 531.25 MHz components for the 010101 data stream was about 4 times that for the 32-bit sequence. Analysis of multiple 32-bit sequences showed that the peak amplitudes and the character of the spectrum remain the same, but the individual frequency components were narrower. Analysis of a pseudo random bit sequence showed that the peak amplitudes were slightly lower than those produced by the multiple 32-bit sequences.