Re: RFID using LFTX/LFRX (Was [Discuss-gnuradio] GNU Radio mention inDEFCON subway presentation)

[Philip M. Lanese]

ChoJin,

Commercial 13.56 MHz RFID systems don't normally use a receiver in the
traditional sense to recover the tag data.

RFID manufacturers catering to the retail industry consider Tag Readers to be a
necessary give-away that allows them to sell millions of tags, so cost is always
driven towards the cheap end.

The tag operates in the magnetic field close to the reader transmitter antenna
and switches its antenna in and out of "resonance" at 13.56 MHz with the logic 1
/ logic 0 data stream output by the tag after an internal capacitor is charged
by the 13.56 RF energy and powers the internal logic circuits on, thereby
causing the reader antenna to hopefully have lower / higher Return Loss
corresponding to the tag digital data stream.

Think of using an o'scope to measure the voltage across a resistor that is in
series with the primary winding of a transformer while you open and short the
secondary winding.

Commercial systems measure the rectified output of a return loss measurement
circuit in series with the reader antenna to sense the data stream from the tag.
Most of the work is put into the design of the reader antenna, which is very
small in terms of wavelength, to increase the difference in the amplitude of the
millivolt level of the R/L output produced by the very small tag antenna  being
"in" and "out" of resonance.

Phil, K3IB


----- Original Message -----
From: "ChoJin" <address@hidden>

>
> On Sep 7, 2008, at 6:58 AM, Michael Ossmann wrote:
>
> > On Sun, Sep 07, 2008 at 02:02:35AM +0200, ChoJin wrote:
> >>
> >> I tried a very simple test:
> >> - two loops of wire, one for the LFTX (A slot, antenna TXA) and one
> >> for
> >>  the LFRX (slot B, antenna RXB)
> >> - generating the sinus wave using usrp_siggen.py -f 0 --sine -w
> >> 13.56e6
> >> - trying to detect this sinus back using usrp_fft/usrp_oscope
> >>
> >> I put my two loops of wires as close as possible from each other
> >> but I
> >> just can't detect my sinus wave back at all.
> >
> > I think the -w option is outside of your transmit bandwidth.  Try:
> >
> > # usrp_siggen.py -f 13.5e6 --sine -w 60e3
> >
> > Also, are you sure that you are receiving on the correct slot and
> > antenna port?
>
> Thanks, it actually worked,
> I had to use -f 13.50 --sine -w 60e3 to get my signal at 13.56Mhz
> though.
>
> I can now see my signal, but I don't get any sideband channels.
> So either my loop of wires is not correct (I actually did it "randomly",
> do you have an exact formula for the number of loops vs. diameter?)
> or I'll have to really implement the pulses instead of a simple sine
> signal.
>
> By the way, I'm a little bit confused about the meaning of the antenna
> slots
> on the LF*X boards. Why do we have two slots for the antenna?
> I'm currently using the antenna slot A for the LFTX plugged in the
> slot A
> and the antenna slot B for the LFRX plugged in the slot B, but I'm not
> sure
> that's what I was supposed to do.
>
> --
> Best Regards,
> ChoJin