Reading NFC tags that are in phone case

[FurkanSenol]

Hello,

We are able to detect and read NFC tags with ST25R3920, but we have a little problem. When we are trying to read an NFC tag that is in a phone case (credit card, ID card etc.) we can't get a reading at all. Our first guess was that mobile phone is creating a field that interferes with NFC. Could there be something wrong or different in our software to get a reading like this? Should I be doing different configurations on registers like Antenna calibration, External Field Detector Threshold? What are your suggestions for this case?

Thanks in advance!

Best regards,

Furkan

[Brian TIDAL]

Hi,

my guess is that the phone case has an anti RFID card holder. Just remove the phone from its case and try to read the tag in the phone case: if you can't read it, this means that the phone case has an anti RFID shield.

Rgds

BT

[FurkanSenol]

Hello Brian,

We can read the tag when we remove the phone from it's case. In fact we can read the tag with X-NUCLEO-NFC06A1 shield on NUCLEO-L476RG board when phone is in the case. We just can't read it with our custom board. I should have mentioned this earlier, sorry for the lack of information.

Could it be caused by antenna design faults?

[Brian TIDAL]

Hi,

ST provides several application notes and tools to help customer to design and to verify their board.

Application notes:

• AN5276 Antenna design for ST25R3916/16B, ST25R3917/17B, ST25R3918, and ST25R3920/20B devices
• AN5240 Layout recommendations for the design of boards with the ST25R3916/16B, 17/17B, 18, and 20/20B devices
• AN5592 ST25R single ended antenna matching
• AN5322 Automatic antenna tuning (AAT) for ST25R3916/16B and ST25R3920/20B devices

Tools

• eDesignSuite
• Antenna Matching Tool for ST25R NFC/HF reader family

Chapter 7 of AN5276 provides guidelines for Design verification. This can be an entry point for your investigation.

Let us know whether more support on this issue is needed.

Rgds

BT

[FurkanSenol]

Hello Brian,

Thanks for all the information! I will get help from hardware team and try to diagnose problem. I will post here if we find the problem and the solution to fix it.

Best regards,

Furkan

[Travis Palmer · Accepted solution]

Hello Furkan,

Because of the close proximity of the card and phone, the phone is changing the resonance frequency of the card.

As example in free air the card is tuned to 14.2MHz. When beeing put in the case of the resonance increases to something like 19MHz. This effect is due to metal parts inside the phone.

This is affecting the power transfer between reader and card. The card will receive less power and can therefore not be read successfully.

Also the readers antenna geometry can play a role here.

Since you said, that the NFC06A1 can read the card in this condition, it should be also possible to read it with your device.

AAT (antenna tuning) can be a great help here, but can also make it a bit more complicated.

I would recommend to measure the smith chart of the reader in free air and 1cm distance to the card in the case.

If you are using AAT (like on the default NFC06A1) you have to power the board and disable the driver (Register 27h set to 0xXF => bit d_res =0xF).

If no AAT is used, the board can simply left unpowered.

The above mentioned application notes can be a great help to increase the output power of your device.

Please feel free to share your custom antenna design (including antenna parameters) as well as the above mentioned smith charts.

br Travis