In a drift tube partitioned into a plurality of cascaded drift tube segments each followed by an ion elimination region, a method of separating ions as a function of ion mobility includes repeatedly, and alternating between at least two different time durations, establishing electric drift fields in the drift tube segments and in some of the ion elimination regions while establishing electric repulsive fields in others of the ion elimination regions such that ions having a predefined mobility or range of mobilities are transmitted through the drift tube at one or more frequencies which include one or more overtones of a fundamental frequency at which ions having the predefined mobility or range of mobilities are transmitted through the drift tube with the electric drift fields and electric repulsive fields repeatedly established with uniform time durations.