Trying to understand NFC ASK Modulation

Thanks for the replies.

I was inspired and curious to learn how Nordic Semiconductor do it on the nRF52 devices. Their documentation says that they emulate a NFC-A tag. Hence throwing this out to the audience to see if any other experts can offer insight.

https://infocenter.nordicsemi.com/topic/ug_nrf52840_dk/UG/dk/hw_nfc_if.html

(Check product specification and block diagram. Also under peripherals section NFCT):

https://infocenter.nordicsemi.com/topic/struct_nrf52/struct/nrf52840.html

Within the product spec is this diagram, which explains things better (to experts rather than novices like me):

When browsing on the Internet I came across this project on Hackster.io: https://www.hackster.io/sandeep-mistry/create-a-usb-microphone-with-the-raspberry-pi-pico-cc9bd5

Under the section "How does PDM work" is a block diagram, which with a little stretch of the imagination is quite similar to the block diagrams shown on the Nordic Semiconductor website about how the nRF52 handles ASKM. Both use DMA, both use 2 pins (one for clock and one for data) and both rely on modulation techniques - obviously pulse dense is different to amplitude shift modulation but that's just the method of application.

So, I was thinking over night that once a phone or NFC controller device activates the NFC field (I was planning to use a standard off-the-shelf NFC antenna attached to a Pico) then with some simple resonant circuitry the Raspberry Pico PIO Interrupt option could be used to sinc the clock signal and then read Manchester encoded data or send Manchester encoded data via the data pin.

Anyway those are my brain burps so far.

PS. I can across this document, which provides some good info too: scdn.rohde-schwarz.com/.../1MA182_5E_NFC_WHITE_PAPER.pdf

Receiving the signal is different, there's the 13.56 MHz receiver in the block diagram, and that's missing from the Pico, it requires some hardware.

if it is just ASK then normal AM demodulation can be straightforward, it may still need some amplification or limiting, and a bit of circuitry (known as a slicer) is needed to create the logic signal before you feed it into the Pico. It won't be great, this is very crude, but might work. Another approach (for any demodulation scheme) could be using SDR and DSP techniques but that too requires additional circuitry (for an example, see "sound card SDR", I wrote a project on this a while back too, most examples use the PC to do the DSP work, but it could be a microcontroller, although most people would choose a microcontroller with the right maths acceleration features, which I don't believe the Pico's RP2040 has, but I have not checked; it might still be fast enough but I don't know).

shabaz said:

Another approach (for any demodulation scheme) could be using SDR and DSP techniques

From what I can gather it is just ASK so basic circuit just capacitors/inductors and if need to amplification then amp and associated components required? Anyway, I will read your sound card SDR example as I had thought that maybe this was the way to do it.

shabaz said:

Another approach (for any demodulation scheme) could be using SDR and DSP techniques

From what I can gather it is just ASK so basic circuit just capacitors/inductors and if need to amplification then amp and associated components required? Anyway, I will read your sound card SDR example as I had thought that maybe this was the way to do it.

Exactly that, for ASK it could be as simple as the inductor and capacitor as your tuned circuit, and an op-amp buffer for instance (high GBW) for a bit of gain, it might not even need to be a lot of gain since the signal is close-by and large, and a diode to perform the demodulation, and a comparator circuit to extract the on/off nature of the signal that was modulated by the transmitter side.