Ring Oscillators (ROs) are widely used in various electronic systems, contributing to their functionality, security, and reliability. Therefore, the characterization of the robustness of RO-based designs against fault attacks such as ElectroMagnetic Fault Injection (EMFI) is a real concern. In this paper, we study the impact of electromagnetic (EM) pulses on ROs implemented in FPGAs. We show that the induced harmonic response depends on the placement and routing of the inverters for different parameters of the pulse. Such a characterization can help developing RO-based structures optimized either for better robustness against attacks or on the opposite for higher sensitivity in order to implement on-chip detectors.