Method of Partial Shielding for Controlling Cable Radiated Emissions

High frequency common mode noise or a differential signal flowing on non-transmission lines, most often, does not reach the load and is lost as radiation and reflection within a fraction of a wavelength. The gradual reflection of common mode noise current or radiation along the cable can be determined using current and near field probes. Therefore radiated emissions from cables can be minimized by partially shielding a fraction of the cable length toward the source end where common mode noise or radiation is predominant. This paper describes partial cable shielding theory and practical considerations. The author has implemented partial shielding on several product lines. Radiated emission data on a product with unshielded cables and with partial shielding are presented and results discussed.