  We present a detailed cross-correlation (CCF) and power spectrum re-analysis
of the X-ray light curves of the bright Seyfert 1 galaxy NGC5548 obtained with
EXOSAT. The 0.05-2 keV and 1-4 keV light curves are cross--correlated with the
1-9 keV and 4-9 keV light curves respectively. We discuss how spurious time
lags can be introduced by systematic effects related to detector swapping as
well as the switching on and off of the instruments. We also find strong
evidence that one of the ME detectors was not working normally during the
second part of the March 1986 observation. When these effects are taken into
account, the CCF peaks are in all cases consistent with the absence of delays
between X-ray variations at different energies. This is unlike the results
found by several authors based on the same data. The power spectra of the 1-9
keV light curves are calculated and a detailed search for quasi periodic
oscillations (QPOs) carried out on these spectra by using a new technique for
the detection of periodic (or quasi-periodic) signals even in the presence of
source noise variability. No significant peaks are found above the 95%
confidence detection threshold, except during the second part of the March 1986
observation, most probably as a consequence of the ME detector malfunctioning.
We discuss and compare our results with those of Papadakis & Lawrence (1993a).
