Hi STM teams , i have developed a custom NFC board using the STM32L476RCT6 and the ST25R3916B NFC reader, based on the reference design of STEVAL-D25R16B (https://www.st.com/resource/en/schematic_pack/steval-25r3916b-schematic.pdf). I am using an external sticker antenna (Manufacturer Part Number: V2089-013-A-01).
Currently, the board detects only the STEVAL-SMARTAG2 NFC tag card (https://www.st.com/resource/en/schematic_pack/steval-smartag2-schematic.pdf), while other NFC cards are not being detected.
However, when I connect the custom NFC board to the DK antenna, it successfully detects the STEVAL-SMARTAG2 card as well as all other NFC cards. so i think issue may be sticker antenna
Could anyone suggest a suitable sticker antenna or recommend an alternative external antenna solution?
I have attached pictures of the sticker antenna, the STEVAL-SMARTAG2 card, and the other NFC tags for reference.
Hello Giridharan,
unfortunately the smith chart you have shared is not ideal for antenna matching.
M1 should be set to 1MHz => correctly done
M2 is optional and should be set to 13.56MHz => correctly done
M3 should be set to the self resonance point => I cannot identify M3
The M3 should be set where the Impedance is almost 0:
Using M3 and M1 parameter you can follow the steps in AN5276 and input the parameters in the antenna matching tool. The AMT then provides you the correct matching values for your antenna.
If the self resonance is very low, you have to play with the input values of AMT. It is possible that not all combinations of EMI cut-off, inductance value, target matching impedance... result in a valid mathematical solution.
If you change between the antennas (EVAL kit default antenna and other antenna) also the matching components need to be updated.
Please let us know, if you succeeded.
br Travis
Hello Giridharan,
the datasheet is not very clear to me.
They are stating a self resonance frequency around 38MHz (Marker is set to the wrong frequency, so it is maybe more 36MHz.
Out of the datapoints (guessed self resonance frequency and Q-Factor) i was estimating the following matching.
I strongly recommend to check the resulting matching using a VNA.
For sure some finetuning as mentioned in AN5276 Rev6. Chapter 6.3 is required.
BR Travis
