Apparatus and method for processing an image-charge/current signal for an ion(s) undergoing oscillatory motion within an ion analyser apparatus. The method comprises: obtaining a recording of the image-charge/current signal (20a-20e) in the time domain. Then, by a signal processing unit, a value for the period (T) of a periodic signal component is determined within the recorded signal. Subsequently, the recorded signal is segmented into a number of successive time segments [0;T] of duration corresponding to the period (T). These lime segments are then co-registered in a first time dimension (t1) defining the period (T). The co-registered time segments are then separated along a second time dimension (t2) transverse to the first time dimension (t1). This generates a stack of time segments collectively defining a 2-dimensional (2D) function. The 2D function varies both across the stack in the first time dimension and along the stack in the second time dimension.