  We investigate the spectral characteristics of 51 candidate High Frequency
Peakers (HFPs), from the ``bright'' HFP sample, in order to determine the
nature of each object, and to obtain a smaller sample of genuine young radio
sources. Simultaneous multi-frequency VLA observations carried out at various
epochs have been used to detect flux density and spectral shape variability in
order to pinpoint contaminant objects, since young radio sources are not
expected to be significantly variable on such a short time-scale. From the
analysis of the spectral variability we find 13 contaminant objects, 11
quasars, 1 BL Lac, and 1 unidentified object, which we have rejected from the
sample of candidate young radio sources. The 6 years elapsed between the first
and latest observing run are not enough to detect any substantial evolution of
the overall spectrum of genuine, non variable, young radio sources. If we also
consider the pc-scale information, we find that the total radio spectrum we
observe is the result of the superposition of the spectra of different regions
(lobes, hot-spots, core, jets), instead of a single homogeneous radio
component. This indicates that the radio source structure plays a relevant role
in determining the spectral shape also in the rather common case in which the
morphology appears unresolved even on high-resolution scales.
