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

[Eric Blossom]

On Sun, Sep 07, 2008 at 10:16:04AM -0400, Philip M. Lanese wrote:
> 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


Here's a link to the "Open RFID reader":

  http://www.openpcd.org/

See also

  http://www.openpcd.org/rfiddump.0.html


It's been used with GNU Radio...

Eric