  We present simple XSPEC models for fitting excess variance spectra of AGN.
Using a simple Monte-Carlo approach, we simulate a range of spectra
corresponding to physical parameters varying, then calculate the resulting
variance spectra. Starting from a variable power-law, we build up a set of
models corresponding to the different physical processes that can affect the
final excess variance spectrum.
  We show that the complex excess variance spectrum of IRAS 13224-3809 can be
well described by such an intrinsic variability model, where the power-law
variability is damped by relativistic reflection and enhanced by an ultra fast
outflow. The reflection flux is correlated with that of the power-law, but not
perfectly. We argue that this correlation is stronger at high frequencies,
where reverberation lags are detected, while excess variance spectra are
typically dominated by low frequency variability.
