by Louis E. Frenzel
The Latest in Networking and Wireless Technologies
Radio Frequency Identification (RFID)
Wireless technology comes in
all shapes and sizes these
days. Radio Frequency
Identification or RFID is one of them.
You have probably seen the initials
RFID and didn’t really know what
they meant. Yet, you may already be
If you have a company identification badge where you work and you
have to wave it in front of a box to get
in, you are using RFID. If you use an
E-Z Pass on your windshield to pay your
tolls, or use you ExxonMobil SpeedPass
to buy gasoline, you are using RFID. If
you shop at Walmart, you have experienced RFID. If you are in the military,
you may be wearing RFID or using it to
keep track of your equipment.
In any case, RFID is an electronic
method of identifying objects and
things with an electronic code, then
transmitting it wirelessly when asked.
It is just like the bar code that appears
on almost every object we buy and
use, but you don’t have to have a
laser reader to scan it. You can read
the RFID code by radio many feet
away. Here is an introduction to this
rather hot wireless topic of the day.
How Does it Work?
The main component of an RFID
system is the tag. Also called a smart
card or a transponder, this is a tiny
silicon chip containing a small
memory containing a unique electronic product code.
The chip is usually bonded to a flat
piece of plastic containing a loop antenna. The chip is typically only a few millimeters on a side, but the copper
antenna loop on the plastic can be several inches wide. See Figure 1. The tag
is then pasted to a box, pallet, carton,
or other object just like a mailing label.
The other part of the system is
the reader or interrogator. This is a
radio transmitter/receiver designed to
interrogate the tag and read its stored
code. Figure 2 shows the complete
system. Here’s what happens.
To read the tag, the transmitter in
the reader powers up and transmits a
signal to the tag. Unlike standard radio,
in RFID, the signal transmitted is not
the usual electromagnetic signal. The
electromagnetic signal occurs only
when it travels more than about one
wavelength from the antenna. This is
called the far field. At a distance less
than one wavelength (λ) where a wavelength is λ = 300/fMHz meters where fMHz
is the operating frequency.
The tag only sees the magnetic
field from the reader antenna. This is
called the near field. In fact, the reader
antenna acts like the primary winding
of a transformer. The magnetic field it
produces cuts across the tag antenna
that acts like the secondary winding of
a transformer. The voltage induced into
the tag antenna is sent to a rectifier and
a filter on the tag chip where it develops a low DC voltage that is used to
power the circuits in the tag. Neat,
huh? The tag doesn’t have its own
power source. Instead, it uses the RF
power from the reader and converts it
to DC to operate the circuits.
When the tag powers up, it transmits the special electronic code in
the memory back to the reader. The
memory is an electrically erasable
read-only memory (EEPROM). The
code is written into the memory by
the manufacturer. The serial data is
used to modulate the reader signal.
What the tag does is modify the
impedance of the antenna in such a
way that the reader detects a loading
and unloading of the circuits. It is the
same as putting a heavier load on a
transformer and that, in turn, causes a
change in the amplitude of the primary
signal. The result is a form of amplitude modulation known as amplitude
shift keying (ASK). The form of ASK is
referred to as backscatter modulation.
Back at the reader, a peak detector
— usually a type of voltage-doubling
diode rectifier circuit — demodulates
the ASK and the slicer shapes it up into
a clean serial data signal that is then
usually transmitted back to a computer
via an RS-232, RS-422, USB, or other
interface. The computer then identifies
the tag and goes on to perform whatever the application requires.
Some More Technical
A major factor in the use of RFID
is the frequency of operation. The
most common ones are 125 kHz,
13. 56 MHz, and 915 MHz. Sometimes
2.4 GHz is used. In any case, these frequencies are those blessed by the
Federal Communications Commission
Figure 1. A typical RFID tag for 13. 56 MHz.
The chip is too small to see, but you can make
out the fine copper loop pattern that is the
antenna. Courtesy of Texas Instruments.