The power from 60 GHz gyrotrons was squarewave modulated to produce periodic temperature perturbations in a Doublet III discharge. The modulated component of the resulting soft-X-ray signals was isolated by digital processing and synchronous detection using the modulation as reference. The phase shift of the signals as a function of radius agrees well with a model calculation based on the heat balance equation, allowing thermal diffusivity to be determined and demonstrating the efficacy of this technique.