Effects of termination networks on signal-induced EMI from the shields of Fibre Channel cables operating in the Gb/s regime

Differential transmission lines support both odd (differential) and even (common) modes of excitation. Imbalance in the differential signal creates a common-mode signal resulting in EMI. A common practice in differential signaling is to match terminate the differential pair with a single resistor. A termination network that provides a good match to both the differential and common-mode impedances should reduce common-mode reflections and therefore common-mode shield currents and radiated electric fields. Three different termination networks (differential, Pi and Tee) were built and studied with a network analyzer. In addition, the networks were shielded and driven through a shielded cable using a battery-powered transceiver that was placed in a shielded enclosure. The radiated electric field was measured at a 3 meter distance in a fully anechoic chamber. The radiated electric field below 500 MHz was found to be more than 4 dB higher for a purely differential termination, when compared to either the Pi or Tee. Above 500 MHz, parasitics appear to dominate the common-mode impedance of the termination network, degrading the impedance match.