How to rectify conducted emission from I/O modules?

To rectify excessive conducted emissions from I/O modules, a systematic approach is needed to pinpoint the noise source and coupling path. First, use a spectrum analyzer with a line impedance stabilization network to measure the frequencies exceeding the limits. A common noise source is the switching power supply; check if its input filtering is sufficient. Consider increasing the inductance of the common-mode inductor or replacing it with a high-performance integrated filter. Check the power decoupling of other high-frequency circuits within the module, such as the clock and data buses. Noise may couple to the power port through common-ground impedance; therefore, check the PCB grounding system to ensure low impedance power return paths.

Apply ferrite cores of different materials to the power lines to observe which frequency bands are effective, and then add targeted filtering components. For narrowband noise at specific frequencies, it may be clock harmonics; consider spreading the clock source. Remedial measures may include: increasing the capacitance of the X capacitor, increasing the number of turns of the common-mode inductor, adding additional decoupling capacitors to the chip's power pins, and optimizing the grounding design. Retest after each modification, observing the changes in the conducted emission curve until all frequencies are below the limits with sufficient margin.