  Blazars are jet-dominated extragalactic objects characterized by the emission
of strongly variable non-thermal radiation across the entire electromagnetic
spectrum. Therefore, the study of blazars (and in general of radio loud AGN)
through the use of multi-frequency simultaneous data is essential in order to
understand the physical processes that take place in these objects. With
Planck, Fermi and Swift simultaneously on orbit, complemented with other space
and ground-based observatories, it is possible to assemble high-quality
multi-frequency simultaneous broad-band spectra of large and statistically
well-defined samples of radio-loud AGN. In particular, four samples of sources
have been selected. The first three samples are flux limited in the high energy
part of the electromagnetic spectrum: the soft X-ray (0.1-2 keV) sample
includes 43 sources from the Rosat All Sky Survey Bright Source Catalog, the
hard X-ray (15-150 keV) sample includes 34 sources from the Swift-BAT 54 months
source catalog and the gamma-ray sample includes 50 sources from the Fermi-LAT
3 months Bright AGN Source List. The fourth sample is radio flux limited,
including 104 bright northern and equatorial radio-loud AGN (most of which have
been monitored at Metsahovi Radio Observatory for many years) with average
radio flux density at 37 GHz greater than 1 Jy. We present the methods applied
and the results of the analysis performed using Fermi-LAT data for all sources
in the four different samples of AGN.
