ST25R3916 NFC Emissions Issue

[km_sibel]

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!

[Nick_Bradley · Accepted solution]

Thank You for reaching out with Your detailed description of the issue. Based on the information provided, here are some insights and potential solutions to address the emissions problem You have:

• When the sensors are fully charged, the load on the NFC transmitter is significantly reduced. This change in load can affect the impedance matching and the behavior of the transmitter circuit, leading to increased harmonic emissions. Could You please clarify why the reader continues to operate after the sensors are fully charged? Is continuous communication necessary, or can the reader be turned off or put into a low-power state when charging is complete?

• The matching circuit might be optimized for a specific load condition (e.g., during charging). When the load changes, the matching might not be optimal, leading to increased emissions at certain frequencies. Since You are already using voltage-controlled capacitors, it is worth considering how You can leverage them to dynamically adjust the matching circuit when the sensors are fully charged.

• The layout of the PCB and the design of the coil antenna can influence harmonic emissions. Parasitic elements and coupling can exacerbate these emissions. Additionally, we are not aware of the PCB stack-up or the number of layers used in Your design (e.g., 2, 4, or maybe 6 layers?).

• Improve decoupling of the power supply to reduce noise coupling into the NFC transmitter. Use high-quality capacitors (e.g., X7R capacitors) to ensure consistent performance in Your NFC transmitter circuit.

I hope these insights help You diagnose and mitigate the issue. If You need further assistance or specific recommendations, please feel free to reach out.