PURPOSE:To provide good particle characteristics over a wide band to a common mode noise current which conducts through a transmission line thereby to be able to apply it to a high speed pulse transmission line by forming a densely concentrated winding on the short axis magnetic path of symmetrical magnetic paths, and forming a rough winding on a part except the part on which the concentrated winding is wound. CONSTITUTION:A closed magnetic path core 12 has a long axis X-X and a short axis Y-Y, and is so formed symmetrically with respect to the axis Y-Y that a distance between opposed magnetic paths becomes maximum near the axis Y-Y. Windings 13, 13 are formed in a multilayer or concentrically wound at that part of the length l0 of the core 12 where the distance between the opposing magnetic paths is maximum. Windings 15, 15' are roughly wound uniformly oppositely to the parts of lengths l1, l2 longer than l0 of the core 12, and the winding directions of the windings wound between input/output terminals 10-11 and 10'-11' are so formed so that magnetic fluxes generated in the core 12 are in the same direction of a common mode current flowing in phase from one terminal and strengthen each other.
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