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During EMI testing, I’m encountering issues with a board I designed. The board includes a SIM7600 module, STM and ESP32 microcontrollers. It is powered from a 12 V source and uses three AOZ6663DI DC-DC buck converters to generate 3.3 V, 3.8 V, and 5.0 V rails for the board. There is also a battery on the board, charged using an LT3650-8.2 circuit.

I’ve placed 0-ohm resistors in many locations on the board to allow sections to be disconnected or filtered if needed. All power rails are decoupled using MLCC capacitors ranging from 100 nF to 100 µF (there are no electrolytic capacitors on the board). The GND is connected at a single point to an M3 mounting hole through a 1 nF, 2 kV capacitor, but this point is not yet connected to the metal enclosure housing the board.

The issue is shown in the attached images. I have some ideas on how to start troubleshooting it, but I would appreciate other perspectives since I don’t have much experience in this area and this is the first board I’m testing for EMI compliance.enter image description here

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  • \$\begingroup\$ The first thing is to identify what is generating those frequencies. A sniffer probe usually helps. Are these frequencies harmonics of the cpu clock etc? Once you identify the source, only then can you consider strategies. It could be the resonant frequency of the inductor(s) or may be specific tracks or pcb structures. Apart from the scans, you’ve given us little to go on. Once you’ve identified the source, then we can discuss cures. \$\endgroup\$ Commented Jul 11 at 12:38
  • \$\begingroup\$ The source for these kind of problems are often either poor layout or poor EMI protection measures (in that order). Either you find the source with a sniffer or you disclose your schematic and layout so we can review it. \$\endgroup\$ Commented Jul 11 at 13:17
  • \$\begingroup\$ The test range, from 30 MHz to 1 GHz (EN 61000-6-3), covers radiated emissions. To rule out a radiation problem with the 12V power cable, you can wrap it in aluminum foil. If the emission test is still poor, the problem lies with the PCB routing. We need the diagram + routing diagram to answer. \$\endgroup\$ Commented Jul 11 at 13:47

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