How to use ST25R3911B for long reading distance(40cm)

[Hzhao.2186]

Hello,

I am new to NFC solutions and want to design a product that can read tags for 40cm distance with ST25R3911B , could you give me some suggestions ?

And how to adjust the antenna driver output resistance (register 0x27) to match the impedance,also what is the suitable value for the capacitive voltage divider?

Thank you very much for your help.

Best regards

Sky

[Hzhao.2186]

Hi Travis,

Thanks for the reply.

Do you mean maybe need add an additional amplifier between IC's RFO pins and antenna matching network?

And another question is for tag (ST25DV64K),What values do you recommend for the inductance and impedance of the coil?

I didn't see any matching elements on the circuit board for ST25DV tag.

Thanks�?

[JL. Lebon]

​Hello Sky,

Regarding ST25DV64K, the calculation of antenna inductance is explain in the AN2972 that you can find at this place: https://www.st.com/content/ccc/resource/technical/document/application_note/bc/ac/13/fe/69/fb/49/8a/CD00232630.pdf/files/CD00232630.pdf/jcr:content/translations/en.CD00232630.pdf

The ST25DV has an internal tuning capacitance of 28.5pF. If you design your antenna correctly you can have an antenna + tag resonating at 13.56MHz without external tuning components. It is still possible to add an additional external tuning capacitance in parallel to fine tune the resonance.

In order to increase the reading range on the ST25DV side, you can play on the level of energy received by the tag.

To increase the level of energy, the main factor is the antenna size, and the coupling factor with reader's antenna. In your case, considering the size and the distance of the reader's antenna, a large antenna on tag side should improve the distance (but I understand you need a small antenna ?),

Another thing you can try  is to power the ST25DV with it's VCC pin (if this is possible in your case). By doing this, some internal blocks of the ST25DV are powered by the wired VCC instead of being powered by the RF field, and the global power consumption of the device on RF is lowered a bit. This may slightly improve the reading range (but not a lot) if the limit was the powering of the tag.

Best regards.

[Hzhao.2186]

Hi Lebon，

Thanks for your help.

Yes，the tag antenna need a small antenna in my case，and need to receive four directional signals. Could you give me some suggestion about the shape of the tag antenna？

Now i'm studying AN2972， one question is how to define capacitance value which needed to fine tune the resonance？

Thanks�?​

[Travis Palmer]

Hello Sky,

Adding a push pull driver stage, connected to a higher supply voltage (12V or even higher) could help here to transfer more power to the tag.

BR Travis

[Hzhao.2186]

Hello Travis，

Recently I use the ST25R3916 board(1.7W) and try to find the power or sensitivity limited issue. I use the pick-up coil connected to the oscilloscope to test ,

Results as below:

Tag can be read area:(two different waveform)

When I increase the distance to move tag out of the detecting area, results as below: (one waveform)

From the results,is it the reader's power limited leads the communication stopped ?

Thanks!

[Travis Palmer]

​Hello Sky,

Since the tag does not respond anymore, it is very likely that the tag has not enough power to operate.

Therefore it is power limited.

For this measurement i would recommend not ot use the "Polling" tab of the Disco Gui.

If not already done, please go "ISO 15693" technology tab and execute Configure + Inventory. It only uses one slot and the PICC response will be right after the reader inventory command.

Please let me know, if you have futher questions!

br Travis

[Hzhao.2186]

Hi Travis,

I have another question, why the V_RFI should not exceed 3Vpp?

Thanks a lot!