A study on mechanism of radiated emissions from fiber optic module

Experiments were performed to evaluate of the radiated emissions from an optical transmission circuit on commonly used compact fiber optic modules and to clarify the mechanism of radiated emissions. Far electric field strength measurements showed radiated emissions having a clear peak at a frequency of high-order harmonic of signal pulse and a radiated emission level at this peak exceeding specifications. Further near magnetic field distribution measurements showed that the interference source lies between the LED and the LED driver. Measured results were also compared with electromagnetic field calculations based on a high-frequency equivalent circuit. It was found that the mechanism of radiated emission consists of a peaking effect on deformed pulse waves that carry the source of interference and a resonance effect in the optical transmission circuit