An analysis of normal-mode noise caused by braided shield current flowing on coaxial cable attached by a ferrite core

Since coaxial cables have excellent electromagnetic shielding characteristics, they are widely used as interconnecting cables for digital information equipment. However, since braided lines are used in the shield layer of most coaxial cables, if an RF current flows, a noise voltage (normal mode noise) is induced between the core wire and the braided wire by electromagnetic coupling. Electromagnetic noise is also radiated from the common mode current of the braided shield. It has been considered that ferrite core loading is effective for reduction of the electromagnetic noise, and such loading is frequently used. In this article, in order to study whether ferrite core loading is effective in reducing normal mode noise, the normal mode noise generated by the braided shield current is analyzed with respect to the core loading location. Next, a formula for the noise-reduction effect due to ferrite core loading is derived and its validity is demonstrated experimentally. (C) 1998 Scripta Technica, Electron Comm Jpn Pt 1, 82(2): 1-10, 1999.