A voltage supply system for supplying an RF voltage to an RF resonant load comprising an ion-optical component of a mass spectrometer is disclosed. The system comprises a Direct Digital Synthesizer (“DDS”) arranged and adapted to output an RF voltage. The voltage supply system is arranged and adapted: (i) to vary the frequency of the RF voltage output by the Direct Digital Synthesizer, (ii) to determine a first resonant frequency of the RF resonant load comprising the ion-optical component, and (iii) to determine whether or not the generation of an RF voltage at the first resonant frequency by the Direct Digital Synthesizer would also result in the generation of a spur frequency close to the first resonant frequency. If it is determined that a spur frequency would be generated close to the first resonant frequency then the voltage supply system is further arranged and adapted: (iv) to consult a look-up table comprising one or more preferred frequencies, and (v) to direct the Direct Digital Synthesizer to generate an RF voltage at a second frequency which corresponds with one of the preferred frequencies from the look-up table, wherein the second frequency is different to said first resonant frequency.