  We study the variability properties at radio frequencies of the jets thought
to be typical of Active Galactic Nuclei, i.e. bulk Lorentz factor \Gamma ~ 10,
and incoherent synchrotron emission. We assume that the radiating electrons are
accelerated at shocks within the jet, and that these shocks have a suitable
(conical) geometry. In our framework, we can reproduce the variability pattern
of Intra Day Variables (20% in less than 1 day) as long as the observed
brightness temperature is T_B < 3x10^{17} K. The only stringent condition is an
injection timescale of the perturbation shorter than the variability timescale;
the geometric condition on the viewing direction is not especially severe, and
agrees with the observed occurrence of the phenomenon. For higher values of T_B
coherent processes are probably necessary.
