— The paper presents a simple stochastic model of a True Random Number Generator, which extracts randomness from the tracking jitter of a phase-locked loop. The existence of such a model is a necessary condition in the security certification process. The proposed model can be used to test, in real time, the proper behavior of the generator and thus to guarantee its robustness against cryptographic attacks. The model is validated on real data, which have been obtained using Altera Stratix Nios and Altera Stratix DSP Professional boards.