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.

The horizontal and vertical curves of the test were quite good when the device is on battery enter image description here

When we connected the charger to the micro-USB connector, the vertical curves showed noise at 30 MHz and 50 MHz: enter image description here

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 :

enter image description here

And here is my setup :

enter image description here So I have three questions:

  • 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?


  • \$\begingroup\$ Near-field probes should pick that up. \$\endgroup\$
    – Andy aka
    Mar 28 at 12:00
  • \$\begingroup\$ Check which frequency span your probe is specified for? Mine say >1MHz. We can't really answer without knowing what probe you are using. \$\endgroup\$
    – Lundin
    Mar 28 at 12:01
  • \$\begingroup\$ Do you know what's happening below 30 MHz? \$\endgroup\$
    – SteveSh
    Mar 28 at 12:13
  • \$\begingroup\$ A picture or cartoon of your setup would help. Is the USB charger in the same room/enclosure as your DUT (device under test)? \$\endgroup\$
    – SteveSh
    Mar 28 at 12:14
  • \$\begingroup\$ I have added two pictures : specs of the probe and setup configuration. The probe should detect signals at 50 MHz. I do not know what was happenning below 30 MHz when I did the pre-certification tests. Yes the USB charger was in the same room as my DUT. If I inspect the cable and the charger with the near-field probe, it does not detect anything... But I know that the probe IS working because it detects spikes on the device of another project. \$\endgroup\$ Mar 28 at 12:48

1 Answer 1


It sounds like you are measuring the noise of the external charger, not your equipment; you'd be better off using a USB battery pack as your supply, since it won't contribute any extra noise to the tests.

With regard to near-field measurements, we used a few turns of wire wound into a small coil, and soldered it to an SMA connector; it made a very good search coil for finding interference similar to yours.

You can probably find calculations on the correct dimensions for a 50 MHz search coil; we just guessed, and it worked sufficiently well for our purposes.


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