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RFID
TECHNOLOGY
RFID
technology is based on the concept of magnetic
coupling, which is the principle that current
flowing in one circuit can induce current flow
in another circuit through a magnetic field generated
in the space between the circuits. In passive
RFID, there are two major components: the reader
and the mobile tag. The reader has two main functions:
the first is to transmit a carrier signal, and
the second is to receive a response from any tags
in proximity of the reader. A tag needs to receive
the carrier signal, modify it in some way corresponding
to the data on the card, and retransmit the modified
response back to the reader.
In
modern passive RFID devices, the tag consists
of a small integrated circuit (that performs the
modulation) and an antenna. The benefit of passive
RFID is that it requires no internal power source;
the circuit on the tag is actually powered by
the carrier signal. Thus, the carrier signal transmitted
from the reader must be considerably large so
that the response can be read even from the card.
Most
passive RFID devices operate in one of three frequency
bands: low frequency (125 kHz), high frequency
(13.56 MHz), and ultra-high frequency (400 to
930 MHz). Within these bands, there are various
ways to modulate the signal so the reader can
easily decipher the data. After some research,
we determined that Cornell uses HID Global’s DuoProx
II identification cards (see Photo 1).
The
datasheets on HID’s web site gave us some general
information about the cards such as the band of
operation. The Cornell ID cards operate at the
same frequency as most passive RFID security cards:
the low-frequency band at 125 kHz. A Google search
revealed that this specific HID card uses frequency-shift-keying
(FSK) modulation. In FSK, the modulating signal
switches between two different frequencies (12.5
and 15.625 kHz in our case) that represent either
a logical one or zero. This allows an electromagnetic
signal to hold data simply as a string of bits.
The modulated signal is then multiplied by the
carrier signal, overlaying the signal with binary
data from the tag and producing an output signal
that looks like what you see in Photo 2.
|
(Click
here to enlarge)
|
Photo
2—The FSK-modulated carrier signal indicates
a response from the tag. Resonance from the
LC circuit creates the extremely high peak-to-peak
amplitude of the signal. All of the data from
this signal is found in the modulating envelope. |
One
of the challenges associated with any type of
modulation is that there has to be a way to unambiguously
extract this binary data from the FSK modulated
signal. We decided to implement this part of our
reader mostly in hardware.
