A mode-switching transformer with a 1-to-4 impedance ratio having a first planar winding formed in a first conductive level from a first differential mode terminal outside of the winding; a second planar winding formed in a second conductive level from a second differential mode terminal outside of the winding; a via of interconnection of the central ends of the first and second windings intended to be connected to ground; and at least one third planar winding in one of the two conductive levels, interdigited with the first or the second winding from a first common-mode terminal outside of the winding, the internal end of the third winding being connected to the via for direct grounding.