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 \approx 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 } < 3 ~ { } 10 ^ { 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 .