  We present an analysis of the RXTE observations of the recently discovered
Galactic microquasar XTE J1748--288 during its 1998 outburst. The spectral
evolution of the source during the outburst can be considered a sequence of
qualitatively distinct states. During the first observations, corresponding to
the maximum of X-ray flux, the spectrum of the source consisted of a dominating
hard power law component and a soft thermal component, which can be described
by the model of multicolor disk emission. The hard component contributed >80%
to the X-ray luminosity in the 3-25 keV energy band. Overall two-component
spectral shape is an attribute of Very High state (VHS) observed previously in
BHC, but the domination of hard component is unusual. Later on, as the X-ray
source faded, its energy spectrum qualitatively changed, showing High (HS) and
then Low (LS) states, both typical for black hole binaries. As the energy
spectrum changed, the fast variability also evolved dramatically. Initially the
power density spectrum was formed by a dominating band-limited noise component,
QPO features at 20-30 Hz and at ~0.5 Hz, and a very low frequency noise
component. After a significant decrease of the contribution of the hard
spectral component the amplitude of the fractional variability decreased by an
order of magnitude and the PDS spectrum adopted a power-law shape with a broad
QPO peak around 0.03 Hz. When the system switched to the LS, the PDS shape
changed again and the QPOs have not been detected since. When the source was
observed in VHS, a clear correlation between QPO parameters and X-ray flux was
seen.
