Showing posts with label smart card reader. Show all posts
Showing posts with label smart card reader. Show all posts

Sunday, May 12, 2019

Seeing NFC with Oscilloscope





Seeing NFC with Oscilloscope


In the last post (https://michaellogusz.blogspot.com/2019/03/wireless-power-transmission-to-make.html) I showed a smart card reader.

This smart card reader sends and receives 13.56Mhz signals for Near Field Communication. To test it I put a little LED chip that lights up by harvesting 13.56Mhz energy.

I wanted to see these waves on my oscilloscope. I found one super simple way to sniff NFC signals and another that was almost as easy.






The first thing I did was plug my RFID smart card reader thing into a cell phone AC wall charger outlet. Luckily, that made the card reader constantly send out a continuous NFC signal. This way I didn't have to hook it up to a computer and keep clicking "read" to get a signal. Nice!

Next I turned my oscilloscope probe into a signal tracer antenna sniffer loop. How did I do this? I clipped the alligator ground clip to the tip of the probe. That's it! Oh, I also set the probe to 10x instead of 1x. That's not a big deal, but the readout on screen looked nicer, because I had the scope set to 10x for no particular reason I can recall.







Anyway, then on my Rigol oscilloscope I clicked "Clear" button and all of a sudden all 4 channels came on. I shut them off one-by-one except for channel 1. I must have had some weird settings leftover from last time I was playing with it, because after seeing nonsense information all of a sudden I was treated to this:



Big waves with tiny little waves jiggling around inside them. If you look at the center of the photo you'll see toward the bottom "Freq = 13.7MHz". This would constantly go from 13.56MHz to slightly above and below. There was my Near Field Communication signal from the card reader right on screen.



Then I thought, what if I used an actual antenna made for NFC--instead of my home made loop? So, I fished around in a junk drawer and phone a broken phone I found in the garbage that had an NFC antenna. This antenna is pictured below. It's actually technically not an antenna: it's an NFC and wireless charging induction coil.





As you can see by my drawings I took a multimeter and did continuity tests. It turns out the little copper connection boxes (which I arbitrarily numbered 1-7) aren't all connected.

Boxes 1, 2 and 4 connect to each other.
Boxes 3 and 7 connect to each other.


So, I hooked my oscilloscope probe tip and ground clip up: one went to 1 or 2 or 4; and the other went to 3 or 7.








When I slammed this induction sniffer onto the card reader I got a way cooler looking set of waves and the frequency readout at the bottom of the oscilloscope screen read 13.56MHz most of the time.









In this way I sort of proved that the smart card reader was putting out 13.5MHz waves, but also that the phone induction coil wasn't just self-resonating at 13.5Mhz: because my crappy sniffer loop in the first part of this experiment also gave me 13.5MHz.

If you have an oscilloscope that's 50-100Mhz you can do this too. However, older scopes that don't go up that high (I have many scopes that are under 10Mhz) won't show you 13.5MHz waves. I think you're supposed to have an oscilloscope that is rated about 5 times the signal you're trying to see. Theoretically, my oscilloscope would be just under this...but it still worked.

If you want to do this the correct way, you'd by a $9000 spectrum analyzer or one of those cool hand-held "RF Explorer" boxes that go up to 6GHz (GIG! not Meg) so like 6000MHz. RF Explorer has units that are weirdly advertised and named so you think you're getting a certain range of frequency, but it's not really that range. For this I think they have a model that's only $180 that would do the trick, for a little more you can get a 2.4GHz model to play with Wifi signals (but I don't think that model can go down to 13MHz. The RF Explorer units range from $120-$500 and up.

The best thing they have is a signal generator that can go up to 6GHz. I thought about getting one of those for doing this same stuff with WiFi signals, but my oscilloscope can't go up to the Gig range (only MHz). Then I came up with an idea: the router I used for my last post about lectennas puts out 2.4 and 5Ghz signals: just plug an ethernet cable into one port and then into another of it's output ports: create a loop back into itself: this creates a packet storm and plenty of WiFI waves flooding the room. This might also be a way to stress-test a WiFi system, lol: no anechoic chamber and spectrum generators needed.

Anyway, after clearing the settings the frequency stopped bouncing around from 60-100Hz and went to 13.5MHz, and cool waves appeared. Not sure why that didn't happen until I hit "clear" on the oscilloscope. Keep fiddling with buttons and sometimes stuff just works out!