Last week we did a pre-certification test with a device and it failed.
Description of the device:
- It is powered from Li-ion battery. It integrates a BQ24090DGQT which is a Li-ion battery charger, the charging current comes from an external charger that we connect to a micro-USB port (like a smartphone)
- It has a MCU which drives high-current LEDs and low-current LEDs, and a DC motor.
We did some tests and we had the following conclusions:
- The problem does not come from a specific device (another one had the same curves).
- The problem does not come from powering the LEDs and the motor because we simply had the device connected to the charger without launching the firmware.
- We managed to suppress the noise at 30 MHz by rolling up aluminium foil around the charger and the USB cable, still the noise at 50 MHz remained
I am using a spectrum analyzer (SSA3032X from Siglent) with near-field probes and I thought I would be able to find the same noise at 50 MHz but after inspecting all over the device, nothing! Here are the specs of the probe I am using :
And here is my setup :
- Is it normal that these noises at 40 dBm are not detected by the near-field probes?
- I have very little experience with EMC curves. By looking at the curves, do you have ideas on what could be the cause of this?
- If I cannot detect the noise with the near-field probes, is there a way to check it without passing EMC tests?