In the first part, a fast and accurate method to compute the harmonic spectra of PWM modulators is presented. This method allows to compute all measurements based on the harmonics values such as the THD or FWTHD. It is especially advantageous for multilevel modulators. In the second part a new modulation method that achieves very good performances for the single-phase multilevel inverters is presented. Finally, several modulation methods are compared with the proposed measurement tool.