A method of processing a plurality of image charge/current signals representative of trapped ions undergoing oscillatory motion, e.g. for use in an ion trap mass spectrometer. The method includes producing a linear combination of the plurality of image charge/current signals using a plurality of predetermined coefficients, the predetermined coefficients having been selected so as to suppress at least one harmonic component of the image charge/current signals within the linear combination of the plurality of image charge/current signals.