Left: periodogram of the frequency variations of 4U1608-522 as derived from the MLEs of the QPO frequency on a timescale of 16 s. The best power-law fit is shown with a dashed line (index 1.8 ± 0.2). Right: broadening of the QPO with increasing integration time of the fitted PDS. As expected from a random walk, the broadening goes like . The best fit is shown with a dot-dashed line. Such analysis enables us to derive the intrinsic QPO width to be 3.2 Hz, corresponding to a quality factor of 259 ± 5 at a frequency of 829 Hz.