RE: [Discuss-gnuradio] Problems with the usrp_rx_cfile.py, usrp_fft.py and usrp_oscope.py and RFID signals

[Paul Mathews]

I am a student and I am doing my thesis project on the RFID protocols. At
the present time, with the USRP, I am trying to catch the signal between a
reader and a tag RFID. As reader I am using an OpendPCD (using the firmware
for the Mifare Classic
protocol) and as card a Mifare Classic Card.

Because of I am a very beginner of USRP and all the rest, I am using the
programs usrp_fft.py and usrp_oscope.py and the usrp_rx_cfile.py program
with another program called Baudline to analyse the spectrum.

My problem is that I can easily see in the frequency domain, using usrp_fft,
the carrier at 13.561Mhz, but not the subcarrier that should be a 847KHz
modulation of the carrier (probably covered by noise) even if I try to use a
baseRX daughterboard instead of the LFRX. Moreover, in the time domain (with
the usrp_oscope), I cannot recognize anything interesting.

As it isn't enough, with the usrp_rx_cfile and the program baudline I cannot
recognize neither the carrier: my opinion is that it is caused by a wrongly
interpretation of the bit-length of the samples (and for this I should need
a confirmation that the output samples are 16 bit-length). That program
supports these formats: alaw ulaw s8 u8 le16 be16 le32f be32f. Does anybody
know wich is the format I should use?

Probably I'm using wrongly the hardware and the software I have...

Could somebody suggest me a solution? Any kind of help will be absolutely
well appreciated!

thank you,

Marco

RFID modulation can be either AM or FM. For AM, modulation depth can be
anywhere between 10% and 100%. If your system is AM and your receiver is
overdriven, you will not see anything like 10% modulation depth. Try
increasing the distance between transmitter (RFID reader) and receiver
antenna, thus weakening the signal, and see if the modulation sidebands
appear. Using usrp_oscope.py, you should be able to clearly see the
modulation notches if you have an AM system, although it can be difficult to
trigger properly.
Note that the RFID transmissions have a low duty factor, so that the
modulation content represents a small portion of the total energy. So,
analyzing the RFID transmissions sometimes means you must first find a way
to synchronize your fft sampling to the packet bursts.
Paul Mathews