A filter is disposed on a base board. The filter includes a first portion, a second portion, a ground portion, a first coupling portion and a second coupling portion. The first portion is disposed on a first layer in the base board to input signals. The second portion is disposed on the first layer to output signals. The ground portion is disposed on a second layer in the base board. The first coupling portion is disposed on the first layer. The first coupling portion is electrically coupled to the first portion and the second portion. The first coupling portion is electrically coupled to the ground portion through via holes. The second coupling portion is disposed on the first layer. The second coupling portion is electrically coupled to the first portion and the second portion. The second coupling portion is electrically coupled to the ground portion through the via holes.