Developing integrated circuit immunity models has become one of the major concerns of integrated circuits suppliers to predict whether a chip will pass susceptibility tests before fabrication and avoid redesign process. This paper presents the development process of the susceptibility model an integrated phase-locked loop to harmonic disturbances up to 1 GHz. The model construction is based on basic circuit information and S parameter measurements. An evaluation of the model accuracy is ensured by the characterization of internal voltage fluctuations with an on-chip sensor.