We are having an unusual issue where we are failing emissions at 67.8Mhz or the 5th harmonic of the NFC frequency. We are also close to the limit for the 3rd and 4th harmonic as well.
It is unusual because it does not fail in all operating modes. It is failing only when our sensors (devices being charged via NFC) are fully charged, so they are not "taking in" energy anymore. The specific standard for which we are failing is CISPR11B (graph below).
Below is a schematic of our circuit for the ST25R3916:
Below is a schematic of the matching circuit and PCB coil antenna:
We are trying to understand why the failure only occurs when it appears that the sensors are no longer taking charge.
Any help would be appreciated!
