  We present an analysis of the spectral properties of the extragalactic radio
sources in the nearly-complete VSA sample at 33GHz. Data from different surveys
are used to study source spectra between 1.4 and 33GHz. We find that, in
general, spectra can not be well described by a single power law in the range
of frequencies considered. In particular, most of the VSA sources that are
steep between 1.4 and 5GHz, show a spectral flattening at \nu>5GHz. We identify
20 objects (19% of the sample) clearly characterized by an upturn spectrum,
i.e., a spectrum falling at low frequencies (\nu<5GHz) and inverted at higher
frequencies. Spectra with high-frequency flattening or upturn shape are
supposed to occur when the emission from the AGN compact core begins to
dominate over the component from extended lobes. This picture fits well with
the AGN unified scheme, for objects observed at intermediate viewing angles of
the AGN jet. Finally, we discuss implications that this class of sources can
have on future CMB observations at high resolution.
