Common-mode current harmonics on differential pair cable shields operating in a regime as high as 2.125 Gb/s

The spectral harmonics of common-mode currents induced on high-speed differential cables operating at 1.0625 and 2.125 Gb/s were investigated using an analytical model and an experiment. Modeling of a differential clock type signal using waveforms with exponential rise and fall and adjustable delay skew and duty cycle allowed prediction of the spectral content of the resulting common-mode voltage waveform. A shielded test board was constructed to allow generation of a differential signal with selectable amounts of delay skew. Using an IC driver on the board as a differential source, common-mode current was measured at the source end of a long cable that was at the far end for both the differential and the common-modes. Modeled and measured results show that the fundamental frequency of the clock type waveform and the higher harmonics increase with increasing delay skew.