  We present X-ray intraday variability and power spectral density (PSD)
analyses of the longest 23 pointed XMM-Newton observations of the blazar 3C 273
that were taken during 2000 -- 2021. These good time intervals contained
between 5 and 24.6 hours of data. Variability has been estimated in three
energy bands: 0.2 -- 2 keV (soft), 2 -- 10 keV (hard), and 0.2 -- 10 keV
(total). Nine of the 23 observations exhibited some variability, though no
major variations exceeding 5 per cent were detected. Typical timescales for
variability were $\sim 1$ ks. For those variable light curves we find that a
power-law model provides good fits to each PSD, with most of the slopes between
$-1.7$ and $-2.8$. Although no variations of hardness ratio could be measured
in any individual observation, an anti-correlation in flux and hardness ratio
is found in long term data that indicates a harder when brighter trend. Our
flux and spectral analyses indicate that both particle acceleration and
synchrotron cooling processes make an important contribution to the emission
from this blazar.
