This paper proposes two frequency based methods for parallel active filtering of current harmonics in utility grids. Active filter (AF) reference prediction and AF current control are the two key-points to obtain high performances. This paper deals with the AF reference prediction. The two proposed methods are based on the Fourier Series and its implementation on DSP. They belong to two prediction structures: 1) the open-loop structure which predicts the reference currents from the load currents; 2) the closed-loop structure which predicts the reference currents from the grid currents. Fourier Series based methods are very flexible and have several interesting properties. These two methods have been successfully implemented and tested. The latter method cancels permanent errors and results in better performances. Both methods are very flexible.