NFC forum matching in ST25R3911B_NFC05A1 evaluation board and antenna feed location

[Tomoki Prugger]

Dear All,

we are developing a prototype board based on ST25R3911B_NFC05A1 evauation board and we have 3 questions:

1) What is the NFC forum matching circuit? Does it have any constraints in the layout?

2) Can moving the antenna feed up by 10-15mm (see below image) be a problem for antenna performance?

3) Does supplying ST25R3911b VDD with 3.3V instead of 5V reduce reading/writing distance? With the demo (supplied at 5V) we can achieve 4-5cm and we should maintain that distance with 3.3V supply.

Thank you very much for your support.

Best regards,

[Ulysses HERNIOSUS]

Hi Tomoki,

ad 1) The NFC Forum matching are just components for your convenience. Please refer to UM2252 4.2.3

ad 2) I think typically we recommend symmetric layouts, maybe @Travis Palmer can give a few hints

ad 3) When moving from 5V supply to 3.3V supply the outputted NFC field reduces. Depending on the tag this will likely cause reduction of operating range.

Regards, Ulysses

[Tomoki Prugger]

Dear @Ulysses HERNIOSUS,

thank you for your kind reply and feedback.

1) From UM2252 4.2.3, unfortunately it is not clear how this circuit is designed and any layout contraints. Do you perhaps have any additional information?

2) is the performance drastically reduced with this asymmetric layout? Hopefully @Travis Palmer can help too!

Thank you and best regards,

[Travis Palmer · Accepted solution]

Hello Tomoki,

1.) The default matching pupulated on the NFC05A1 is a VHBR matching. It means it supports up to 3.4Mbit (which also the that has to support and there are very little cards that do support it). But due to the higher bandwidth the Q-Factor is reduced which affects the output power. The NFC Forum matching has a higher Q-Factor and therefore higher output power / read range. Dependent on the bit rate you want to support, the Q-Factor could also be further increased (for example for 106kbps only). Please consult the antenna matching appnote for further details.

2.) I do not expect any impact when moving the antenna taps to the side is ok from my POV.

You would need to watch out if the antenna is done symmetrically (Like the ST25R3916-DISCO, which is having 3 terminals. Ant1, Ant2, GND). 

3.) Yes there will be an impact when changing to 3.3V. You can simply find out by changing the supply on the NFC05A1 as shown in the attached picture.

Please let us know, if you have additional questions.

Br Travis