Planar balun and multi-layer circuit board

A planar balun formed on a multi-layer circuit board, comprising:

• a first winding with at least one turn, which is formed in a first conductive layer, and has a first lead and a second lead serving as a first balanced end and a second balanced end of the balun respectively;
• a first peripheral grounding pad formed at a periphery of the first winding, the first peripheral grounding pad matching an outer contour of the first winding;
• a second winding with at least one turn, which is formed in a second conductive layer separated from the first conductive layer by at least a first insulating layer, and has a third lead and a fourth lead, wherein the third lead is connected to a ground potential, and the fourth lead serves as an unbalanced end of the balun;
• a second peripheral grounding pad formed at a periphery of the second winding, the second peripheral grounding pad matching an outer contour of the second winding;
• and
• a first balancing capacitor, which is connected between a selected portion near a center of the first winding and the ground potential.