1) An ac adjustable-frequency drive. 2) A particular section of an ac drive. This section uses the dc voltage from a previous circuit stage (intermediate dc circuit) to produce a pulse-width-modulated or stepped ac current or voltage waveform that has characteristics similar to the desired sine-wave frequency. 3) A circuit whose output signal is the inverse of its input (a positive-going pulse is inverted to a negative-going pulse, and vise versa).