Method of correcting a gain and phase imbalance of an analogue modulator (32) for multiple channels (CHi) of a multi-carrier transmission signal, the method comprising the steps of determining a gain imbalance correction factor (GCFi) and phase imbalance correction factor (PCFi) for each channel individually and applying said correction factors (GCFi, PCFi) to the corresponding channel (CHi) individually, before the multi-carrier synthesis of the channels is done, whereas step a) for each one of the multiple channels (CHi) is performed in a time-multiplexed manner with step a) for the other ones of the multiple channels (CHi).