The phase-scrambled frequency spectrum (181) is inverse Fourier transformed (182) to produce the final time domain waveform (183) which is used to generate the electric field (177) which excites the ions in an ion cyclotron resonance cell (101). In the iterative method of the invention, the desired frequency spectrum (181) is phase scrambled such that all frequencies are not in phase in any point in time, an inverse Fourier transform (182) is performed on the phase scrambled frequency spectrum (181) and the result multipled by a window function (185). The time domain waveform (183) is forward Fourier transformed (187) to produce an output spectrum which is compared to a reference spectrum (188) to provide correction factors which are used to predistort the magnitude of the final frequency spectrum (189) the steps are repeated until the output frequency spectrum (187) is sufficiently close to the reference spectrum (188) whereafter the time domain waveform (183) corresponding to that output frequency spectrum (187) is applied as the excitation signal.