A key challenge with RF GaN technology is its slowly varying behaviour introducing distortions within the envelope of the transmitted signal at time constants ranging from less than a microsecond to tens of milliseconds. The interaction between such a highly dynamic device behaviour and the modulated RF signal poses a characterisation challenge that limits the understanding and modelling of this technology. This presentation focused on an emerging measurement solution that is based on DC and RF-pulsed n-tone waveforms with the capability to capture time-varying signal distortions at power levels less than -60dBc and a time resolution of few nanoseconds.