Trying to understand NFC ASK Modulation

Hi Colin,

If all you require is ASK, then the circuit would be just a gate or switch (if the 13.56 MHz signal is not coming from the Pico). If you're generating the 13.56 MHz signal from the Pico somehow, then for ASK it just needs to be switched on/off in software. 

I've no idea if this would work though, I've never studied NFC at this low-level, and don't know if it requires bidirectional comms or anything else. Also, the output signal would be very low, since the Pico GPIO can't provide a lot of power, so then you'd need an amplifier.

The capacitor(s)/inductor usually form a resonant circuit, but it wouldn't ordinarily be connected to a GPIO. The resonant circuit is part of an oscillator, which is usually created with (say) a transistor of some sort, i.e. it's a different topology that isn't contained within the Pico, and not normally directly drivable by the Pico either, instead it becomes part of a frequency synth circuit (which isn't all that easy to build without using a dedicated chip). In your case, if you're trying to use a Pico for this, then you may as well connect an amplifier to the Pico, and drive the inductor with that. I think it's unlikely to work though (but I don't know for sure! I could be totally wrong, since I know ~zero about this), I'm guessing NFC protocol is more complex.

This will be harder than you think !

A long time ago (2006) I did work designing antennae for MIFARE (same thing, more or less) and it's quite tricky.

NXP still are fairly active in this field and you can learn a lot from reading their app notes and data sheets, Here's a start:

https://www.nxp.com/products/rfid-nfc/nfc-hf/nfc-readers/nfc-cortex-m0-all-in-one-microcontroller-with-optional-contact-interface-for-access-control:PN7462

The antenna was two turns on a pcb (inner layers)  with earth shielding layers on both sides of the board and multiple vias to stich them togethher. There as a multi component passive matching network with an adjustable tuning cap. The pcb needed non standard layer stackup to work and we were discussing fine tuning the design of coil, copper thickness and pcb layer thicknesses. All backed up by impedance measurements with a home made network analyser (computer + sig gen + scope + active analogue interface pcb.

Sorry, can't post pics because customer might object (although they might not still exist  - actually they do, just checked and the two guys I worked with then are still there !).

MK

This will be harder than you think !

A long time ago (2006) I did work designing antennae for MIFARE (same thing, more or less) and it's quite tricky.

NXP still are fairly active in this field and you can learn a lot from reading their app notes and data sheets, Here's a start:

https://www.nxp.com/products/rfid-nfc/nfc-hf/nfc-readers/nfc-cortex-m0-all-in-one-microcontroller-with-optional-contact-interface-for-access-control:PN7462

The antenna was two turns on a pcb (inner layers)  with earth shielding layers on both sides of the board and multiple vias to stich them togethher. There as a multi component passive matching network with an adjustable tuning cap. The pcb needed non standard layer stackup to work and we were discussing fine tuning the design of coil, copper thickness and pcb layer thicknesses. All backed up by impedance measurements with a home made network analyser (computer + sig gen + scope + active analogue interface pcb.

Sorry, can't post pics because customer might object (although they might not still exist  - actually they do, just checked and the two guys I worked with then are still there !).

MK

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.

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. 

It is possible for sure, but its not really a project I would recommend taking on without an oscilloscope.

So if your looking for an excuse to purchase an MXO4 scope, id say go for it ;) 

I am Rx/Tx USB-PD with the pio on a PI pico right now which is differential Manchester, so that could provide some inspiration  

Agree, unfortunately this is a project that's very unlikely to work, because there's several hurdles here; not just the hardware design and building of the receiver, but also the decoding complexity, it won't be a nice friendly stream of usable data all the time either, there will be noise to deal with so it needs some signal processing as well (can be partially in hardware and partially in software). It's all a major project, not a weekend thing. 

If the Pico can be replaced with a PC and sound card, then that could make life easier to better understand the challenge and implement on the PC first perhaps, rather than coding for the Pico initially. But the signal would need to be mixed down to see it (e.g. with any SDR project hardware).

Yes that was in the back of my mind as I don't have a suitable scope... more like hoping those who have already purchased an MX04 scope, or equivalent, may be looking for another excuse (i.e. project) to get some more play time with their expensive toy.