Common mode filter

A common mode filter, comprising:

• a circuit board, comprising a first signal layer, a second signal layer and a third signal layer;
• a ground element, disposed in the second signal layer, and comprising a ground portion and a first ground line electrically connected to each other, wherein the ground portion includes a slot, and the first ground line is disposed in the slot and extends from a bottom portion of the slot to an opening of the slot;
• a transmission line pair, disposed in the first signal layer, wherein an orthogonal projection of the transmission line pair on the first signal layer is partially overlapped with an orthogonal projection of the ground portion on the first signal layer, and the orthogonal projection of the transmission line pair on the first signal layer is not overlapped with an orthogonal projection of the first ground line on the first signal layer;
• and
• an extension element, disposed in the third signal layer, wherein the extension element is electrically connected to the first ground line through at least one conductive via,
• wherein an orthogonal projection of the ground element on the first signal layer is symmetrical with respect to a central line of the transmission line pair.