  We have constructed models of quasi-periodic variability of X-ray emission
from accreting compact objects. Assuming a general scenario of a propagation
model of variability, with inverse Compton upscatering as the emission
mechanism, we have considered a number of cases for the periodic modulation:
modulation of the plasma heating rate, cooling rate by external soft photons,
and the amplitude of the reprocessed component. We have computed various
observational characteristics which can be compared to good quality data.These
include Fourier-frequency resolved spectra and results of cross-correlation
analysis between light-curves at different energies. Each model of modulation
predicts specific observational signatures, which help in identifying the
physical processes driving QPO emission in accreting sources.
