NFC Antenna Frequency

[Salokmonr]

Hi Nordic Team,

Good week

We like to confirm a few technical details about the NFC-A tag interface on the nRF52840:

1. What is the source of the 13.56 MHz carrier used for the NFC-A tag interface?
   – Is the 13.56 MHz generated internally by the SoC, or does it rely on an external circuit to produce the carrier field?

2. What is the frequency tolerance of the NFC-A interface as implemented in the nRF52840?
   – Is there a specified range (e.g., ±7 kHz per ISO/IEC 14443-2), or does the SoC itself have an internal frequency specification for NFC operation?

3. What is the role of the external LC network (antenna coil and capacitors) connected to NFC1/NFC2 pins?
   – Does it simply act as a resonant matching network at 13.56 MHz, or does it play a part in generating or shaping the carrier signal amplitude?

I’d appreciate any references to official documentation that clarify these aspects. We are in the process of certifying the sensor  with NCC certification, and have been asked about this topic

Thank you,
Ronen

[ketiljo]

What is the source of the 13.56 MHz carrier used for the NFC-A tag interface?
– Is the 13.56 MHz generated internally by the SoC, or does it rely on an external circuit to produce the carrier field?

The field is produced by the reader (external)

What is the frequency tolerance of the NFC-A interface as implemented in the nRF52840?
– Is there a specified range (e.g., ±7 kHz per ISO/IEC 14443-2), or does the SoC itself have an internal frequency specification for NFC operation?

Frequency tolerance is limited by the bandwidth of the antenna. 

– Does it simply act as a resonant matching network at 13.56 MHz, or does it play a part in generating or shaping the carrier signal amplitude?

It's only a resonant circuit.

[Salokmonr]

Rgarding  frequency tolerance , so to get it clearly , the nRF52840 when he send back information the frequnecy 13.56MHz and tollerance is conrold by the external NFC antnna we design ?

[ketiljo]

Yes. The reader or initiator is generating a magnetic field that is being modulated by the tag. The frequency can in theory be anything, but to adhere to the standards, the tag antenna resonates at 13.56 MHz. 

The nRF52840 NFC peripheral operates the same way as passive NFC tag, like the ones you can find in key cards etc.