This paper deals with the propagation of RF disturbances injected inside an integrated circuit. To increase our knowledge about this topic, an on-chip voltage sensor is implemented inside a test vehicle to quantify the conducted RF disturbances injected on the power supply of its I/Os. The experiment results give information about coupling paths taken by conducted disturbances carried into I/Os. I/O immunity thresholds computed from standard external immunity tests (IEC 62132-4) are compared to the one deduced from internal measurements. Moreover, in view of developing simulation tools for IC's immunity prediction, a test vehicle model is proposed. Comparison between simulation results, internal and external measurements raises some discussions about the accuracy of immunity measurement set-ups.