In this paper, the effect of an Electromagnetic Interference (EMI) modulated in amplitude on an Operational Amplifier (Op. Amp.), first as a standalone circuit and then integrated in a microwave Phase-Locked Loop (PLL) is studied. In both cases, the EMI is injected through a magnetic near field probe. First, a CW sinusoidal EMI is injected on the input tracks of the Op. Amp. alone in order to determine resonance frequencies. Then, these resonance frequencies are used as carriers to inject amplitude modulated EMI. Then, the Op. Amp. is integrated into a PLL. The susceptibility of the PLL when an EMI modulated in amplitude is injected on the Op. Amp. is carried out. Parameters of the modulated signal leading to the most sensitive perturbation effects are presented.