  The low frequency tail of the CMB spectrum, down along the radio range (~1
GHz), may carry weak spectral distortions which are fingerprints of processes
occurred during different epochs of the thermal history of the Universe, from
z~3\times 10^6 to reionization. TRIS and ARCADE2 are the most recent
experiments dedicated to the exploration of this chapter of CMB cosmology. The
level of instrumental accuracy they reached in the determination of the
absolute sky temperature is such that the removal of galactic and
extra-galactic contamination is the true bottleneck towards the recovery of the
cosmological signal. This will be certainly the case also for future
experiments in the radio domain. Here we present an update of a study
originally done to recognize the contribution of unresolved extra-galactic
radio sources to the sky brightness measured by TRIS. Despite the specific
context which originated our analysis, this is a study of general interest,
improved by the inclusion of all the source counts available up-to-date from
150 MHz to 8.4 GHz.
