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Receiving NFC signals from contactless card - 13.56MHz HF

Posted: Fri Feb 19, 2021 10:58 am
by blackwiz4rd
Hi everyone,
I specifically need to receive the signal coming from an NFC communication. The carrier wave is at 13.56MHz. However, I need to demodulate the card signal, whose subcarrier is at 13.56MHz+/-847.5MHz = 14.4075MHz, 12.7125Mhz.
The problem is there seems to be a lot of noise at 14.4MHz coming from the dongle as stated here as well: http://superkuh.com/rtlsdr.html.
I have tried the experimental drivers and no-hardware mod without success in demodulating the card consistently (sometimes I had luck and got a good signal). As the matter of facts, I performed HF hardware mod and I can correctly receive the reader consistently but not the card, whose signal is weak/noisy.
There is someone on youtube who managed to demodulate both the card and reader by using the experimental drivers and a home made antenna but I tried two dongles, using the same acquisition chain, without success (noeelec smart and a chinese version):
- https://www.youtube.com/watch?v=wGBXTrt1_vk&t=323s: NFC decoding with Universal Radio Hacker
- https://www.youtube.com/watch?v=2PxpHsa724w: reader and card signal acquisition
- https://www.youtube.com/watch?v=KHfLxwMC5So&t=334s: reader signal acquisition

My questions are:
I have read that a baloon of 4:1 between the antenna (I will post an image of my setup) and the RTL2382U could help acquiring HF signals with direct sampling but not in my case (maybe I did it wrong).
Is there a way to avoid receiving noise from the dongle at 14.4MHz, which could help in demodulating the card? I have seen that changing the crystal oscillator (28.8MHz) is not possible and I believe noise comes from it.

Things which yield to the same result:
  • home made antenna or DPL-FANT antenna (13.56MHz antenna)
  • wire between q pin and antenna core
  • capacitor of value 100nF between q pin and antenna core
  • baloon 4:1 between q pin and antenna
I have attached a signal acquisition example in which the reader can be demodulated and the card can't. I was centered around 13.56MHz (and if I shift the frequency nearby, the signal of the card is even worse).

Any suggestion is welcome.
Thanks a lot!

Re: Receiving NFC signals from contactless card - 13.56MHz HF

Posted: Fri Mar 05, 2021 5:08 am
by rtlsdrblog
14.4 MHz is the Nyquist zone cutoff point, so things can get a bit funky around that frequency. This might be a case where an upconverter with an amplifier is required.

You could also try disconnecting the VHF/UHF path by removing C13 (next to D6 by the MCX connector). That might stop some signal loss. But you won't be able to use the dongle in quadrature sampling mode anymore.

Re: Receiving NFC signals from contactless card - 13.56MHz HF

Posted: Tue Mar 09, 2021 9:24 am
by blackwiz4rd
rtlsdrblog wrote:
Fri Mar 05, 2021 5:08 am
14.4 MHz is the Nyquist zone cutoff point, so things can get a bit funky around that frequency. This might be a case where an upconverter with an amplifier is required.

You could also try disconnecting the VHF/UHF path by removing C13 (next to D6 by the MCX connector). That might stop some signal loss. But you won't be able to use the dongle in quadrature sampling mode anymore.
Thanks for your answer. I do understand your point. Should the direct sampling be enabled when using the upconverter? I had tried the Ham it Up without the direct sampling mode (some time ago) and the signal looked as if there was no upconverter so it did not make significant difference. In fact, if not using direct sampling, with or without the upconverter, a sinusoidal wave with high wavelength adds to the signal, probably due to the noise by the tuner.
Which upconverter do you suggest?
I have removed C13 as suggested, however it does not seem to make significant difference (even though there seems to be less noise from a gqrx capture).


My current setup looks as in picture (103M capacitor from Q- to GND, 104M from Q+ to antenna core, 16V 100uF capacitor from Vcc to GND). I have removed the thoroid because it was not very professional.
By the way, is there a specific wire to use for the connections which could give better results?
Thanks a lot.

Re: Receiving NFC signals from contactless card - 13.56MHz HF

Posted: Wed Apr 14, 2021 10:49 am
by rtlsdrblog
blackwiz4rd wrote:
Tue Mar 09, 2021 9:24 am
rtlsdrblog wrote:
Fri Mar 05, 2021 5:08 am
14.4 MHz is the Nyquist zone cutoff point, so things can get a bit funky around that frequency. This might be a case where an upconverter with an amplifier is required.

You could also try disconnecting the VHF/UHF path by removing C13 (next to D6 by the MCX connector). That might stop some signal loss. But you won't be able to use the dongle in quadrature sampling mode anymore.
Thanks for your answer. I do understand your point. Should the direct sampling be enabled when using the upconverter? I had tried the Ham it Up without the direct sampling mode (some time ago) and the signal looked as if there was no upconverter so it did not make significant difference. In fact, if not using direct sampling, with or without the upconverter, a sinusoidal wave with high wavelength adds to the signal, probably due to the noise by the tuner.
Which upconverter do you suggest?
I have removed C13 as suggested, however it does not seem to make significant difference (even though there seems to be less noise from a gqrx capture).


My current setup looks as in picture (103M capacitor from Q- to GND, 104M from Q+ to antenna core, 16V 100uF capacitor from Vcc to GND). I have removed the thoroid because it was not very professional.
By the way, is there a specific wire to use for the connections which could give better results?
Thanks a lot.
You shouldn't use direct sampling with the upconverter.