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Photo: Infineon
If you have an E-tag for the tollway, a micro-chipped pet or a latemodel car with an immobiliser key, then you’re already using radio
frequency identification (RFID) technology. But this is only the start.
Over the next few years RFID technology will start to replace bar
code labelling systems. It might even be used to identify people! The
implications are enormous. So what is RFID and how does it work?
R
adio Frequency Identification
(RFID) has been around in one
form or another since World
War II. Although it has been used
in niche industrial sectors for many
years, the increasing desire for greater
efficiencies in supply logistics have
really pushed the development and
use of this technology.
An RFID system consists of a reader
and transponders. Transponders (derived from the words “transmitter”
and “responder”) are attached to the
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items to be identified. They are often
called “tags”.
Just like a bar code, a transponder
tag carries data about its host. When
interrogated by a reader, it responds
with that data over a radio frequency
link. The transponder could be really
simple, like those in clothing price
tags, consisting of just an antenna
and diode. When irradiated, the diode
By PETER SMITH
rectifies the incoming carrier and the
frequency-doubled signal is radiated
back to the reader which responds
with an alarm if you try to leave the
store without paying for the product.
These days, the generic term “RFID”
is used to describe an entire range of
dedicated short-range communication
(DSRC) systems.
This article does not attempt to
describe all RFID devices and technologies. Instead, we will focus exclusively on RFIDs used in identity
July 2003 7
Fig.1: a basic
RFID setup consists of a reader
(or interrogator)
and transponder.
Low frequency
systems rely on
inductive coupling to provide
transponder
power.
tagging and closely associated areas.
Let’s begin by dividing the subject
into two broad categories: active and
passive transponders.
Passive Transponders
Passive transponders do not have
an in-built power source; they are
powered entirely from the magnetic/
electric field of the reader’s antenna.
This energy is used to power on-board
electronics as well as to transmit data
back to the reader.
Because of the close coupling
requirements of the reader and programmer, reading distance is limited.
It varies from a few centimetres to
several metres, depending on the
transmission frequency, power level
and other factors that we’ll examine
shortly.
Passive tags come in a huge variety
of shapes and sizes, depending on
their application. They can be made
to withstand extremely harsh environments. Without a battery to run flat,
Active transponders are battery-powered and are generally designed for communication over greater
distances than their passive counterparts. On-board power allows higher
data rates and better noise immunity
but active transponders are bigger, cost
more and have a finite life.
The E-tag for Sydney’s tollways is a
good example of an active tag. Similar
systems in Europe operate in the microwave spectrum, which implies very
high data transfer rates. In fact, the
European systems allow you to speed
through the tollgates at up to 160km/h
and are still able to successfully bill
you for the trip!
be limited to 20 characters, whereas
tag memories can hold 512 bits (or lots
more) of data.
Importantly, the memory on some
tags can be both read and written many
times over, allowing “on-the-fly” data
updates. Simpler tags that contain
“WORM” (write once read many times)
memory are also in use.
Unlike bar coding schemes, “smart”
tags include computational electronics, enabling encrypted, high security
information exchange. This can be seen
in action in the new contactless credit
cards and “electronic purse” systems
already in use throughout Europe.
The invisible medium of radio also
means that tags do not need to be “lineof-sight” to be read. With help from
the on-board electronics, it also allows
multiple tags (within reader range) to
be read “simultaneously”.
Imagine how all this might ultimately change your shopping experience.
You could fill your trolley and wheel it
directly out of the supermarket. Invisible readers at the exits would scan all
of your items and charge your “smart”
credit card while it’s still in your wallet
(or purse)! No waiting at the checkouts
– wouldn’t that be great?
RFID advantages
How passive systems work
The fact that RFID is “contactless”
is only part of its attraction. RFID tags
carry much more data than bar codes.
For example, a typical bar code might
Passive tags usually consist of just
a single IC and an antenna (coil). Currently, most passive tags operate below
100MHz and rely on the magnetic field
they can last indefinitely.
Active transponders
Fig.2: block diagram of a typical low frequency reader. All high-level functions, such as data encryption/
decryption, collision detection and host communication are performed by the microcontroller.
8 Silicon Chip
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Fig.3: a typical low frequency
transponder. The transistor
across the coil loads (or
“damps”) the reader’s magnetic
field to transmit data from
memory. In most
implementations, a single IC
performs all of these functions.
produced by the reader for both power
and communication.
The reader generates a carrier signal
and this induces a voltage across the
coil of the tag. This voltage is rectified
and filtered to become the power supply for the IC. Some tags also divide
down the carrier signal and use it as
the clock for on-board logic, whereas
others generate their own clock signal.
Tag transmission
Essentially, tag data transmission is
achieved by switching a low resistance
across the antenna coil. Loading the
coil in this way causes a corresponding
dip in the peak voltage across the reader’s coil. In other words, the change
in voltage across the tag’s coil is
reflected back to the reader’s
coil. This is often referred to
as “backscatter”.
The serial data
stream from ROM
(and/or EEPROM/FRAM) memory
does not directly drive the coil-loading
switch. Instead, the switch is driven
by a low-frequency clock source. This
effectively superimposes a weaker
“subcarrier” on the main carrier signal.
Modulating this subcarrier performs
actual data transmission.
Without going into lengthy technical discussions, we can tell you that
the modulation method may be ASK
(amplitude shift keying), PSK (phase
shift keying) or FSK (frequency shift
keying). Serial data is typically Biphase, Manchester or Miller-encoded
before transmission.
stages is cleaned up with a Schmitt
trigger and pumped into a digital
logic block, where the original data is
reconstructed through a demodulation
and/or decoding process.
Typically, all of these functions are
performed by a single IC, supported by
a few external (passive) components
and perhaps an antenna power amplifier. Higher level functions, such as
data encryption/decryption, collision
detection and host interfacing are usually performed by a microcontroller,
which is interfaced to the reader IC
via a simple serial or parallel interface.
Reader reception
For two-way (read/write) systems,
the reader must also be able to transmit
data to the tag (to update the EEPROM/
FRAM). This is typically achieved by
amplitude, pulse-width or pulse-position modulation of the carrier signal.
In its simplest form, transmission
to the tag is performed by switching
the carrier signal on and off (100%
amplitude modulation). A “gap detect” circuit in the tag serialises and
demodulates the “gaps” and “no gaps”
to reconstruct the original data.
Once a complete data frame is received, it is checked for validity (using
a CRC polynomial). If sufficient power
is available, it is then committed to
memory.
In some systems, the carrier is not
switched on and off but is modulated
at a particular “depth” (about 10%).
This makes more power available for
In order to receive tag data transmissions, the reader’s antenna
signal is first processed by
analog front-end circuitry. Its main functions are to remove
the carrier signal
and then amplify
the (much) smaller
sub-carrier.
The resultant signal
from the envelope detection, filtering and amplifying
A much larger-than-life computerrendered image of TI’s DST+ (Digital Signture
Transponder Plus) module. These are embedded
into vehicle keys to provide sophisticated fraud
prevention information. The long ferrite rod
coil and transponder IC are clearly visible.
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Reader to tag transmission
July 2003 9
tag use, extending range and enables
smaller tag antennas to be used.
Frequencies and antennas
A collection of 14.35MHz tags and labels with TI’s “Tag-it” transponders hidden
inside. Photo: Texas Instruments
This is what’s inside the tags and labels. In bare format, the transponders are
referred to as “inlays”. Photo: Texas Instruments
The most common frequencies
in use for passive RFID systems are
125kHz - 134.2kHz and 13.56MHz,
with a few operating up in the 900MHz
and 2.45GHz regions. The frequency
of operation has a very big impact
on system design, configuration and
cost, and it’s all to do with “near” and
“far” fields.
Antennas radiating an electromagnetic field generate what is known as
“near” and “far” field components.
Most passive transponders rely on
inductive coupling, so they utilise
the “near” field component. The
“near” field signal decays as the cube
of distance (1/r3) from the antenna,
whereas the “far” field signal decays
as the square of the distance (1/r2) from
the antenna.
As you can see, the use of inductive
coupling and “near” field severely limits the reading distance. However, this
can be desirable, as it allows engineers
to tightly control the radiating pattern
and reach of the reader’s field. To
borrow from our earlier supermarket
example, it is possible to ensure that
shoppers are only charged for what is
in their trolley (and in their pockets!).
Low frequencies and small tag sizes
are two other important reasons for
using the “near” field. For example,
consider the size of conventional
¼-wave dipoles for 125kHz (or even
14.35MHz) that would be needed for
“far” field communication. These
would need to be 600m and 5.23m
long, respectively; much too big for integration into a pea-sized transponder
or credit card!
For inductive coupling, the antenna (we use the term loosely) must be
resonant at the chosen carrier frequency. This is achieved by adding
some parallel capacitance (for the
transponder) or series capacitance
(for the reader) to a known value of
antenna inductance.
Size does matter
Reader size varies according to application. Miniature units with built-in
antennas are available, whereas store-front models need walk-through antenna
loops. Here are two semi-portable (14.35MHz) readers from TI. As indicated in
the foreground, these models are designed for ID card use. Photo: Texas Inst.
10 Silicon Chip
Reader and transponder antenna
size is a critical factor in “near” field
systems. As the tag size is generally
fixed (in credit card form factor, for
example), the reader side becomes the
variable. Many manufacturers quote
a “rule of thumb” reading distance
roughly equivalent to the diameter of
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the reader’s antenna.
Identification Numbers.
However, it’s important to
240,000 books and 60,000 CDs
note that factors such as antenna
and DVDs in Vienna’s new main
orientation, radiated power and
library have been equipped with
environmental conditions all
RFID transponders. Self-service terhave significant effects on reading
minals in the library make checkout
distance.
completely painless.
For 125/134.2kHz systems, the
Mobil has teamed up with Texas
antennas (OK, the coils!) are conInstruments to create a hybrid acstructed with many turns of wire,
tive & passive transponder system
often wound on ferrite cores to
for petrol purchase. Based on TI’s
reduce size. Transponder coils can
TIRIS system, it enables thousands
be as small as a cm or two, making
of motorists in the US to fill up
them ideal for animal tagging (imwithout the need for cash or even
plants) and car security systems.
a card. Transponders in both the
Data transfer speed is typically
car and the driver’s key ring make
between 2 - 10kb/s.
a positive ID as soon as the vehicle
pulls up to the pump. Now all they
By contrast, 14.35MHz tranneed is a robot to fill the tank…
sponder antennas require less than Close up of a Tag-it inlay. The tiny black dot
10 turns (the readers may have only is the transponder IC, with the antenna coil
occupying most of the remaining space. These Where to from here?
one turn), which is easily printed as
a foil pattern for tag inlays or etched inlays are small and highly flexible and can
Despite all this activity, there are
directly onto PC boards. This fre- be attached to almost anything. Photo: Texas
still some wrinkles to be ironed out
Instruments
quency is widely used for credit
before you’ll see RFID in use in
cards, identity tags, anti-theft layour local supermarket. The lack
bels and bar code replacements. Data
of international RF standards (bands
The company now has the ability to
transfer speed at this frequency is up track the tagged garments even after
and power levels) is frustrating develto 100kb/s.
purchase, which is proving to be a opment. In addition, the cost per tag
is still prohibitive for use on low-cost
somewhat controversial ability.
UHF/microwave systems
products.
The London public transportation
Passive systems that operate in the
The bean counters tell us that tags
system is installing a smart ticketing
900MHz and 2.45GHz regions are also system that uses contactless smart must be priced at less than 1% of the
in use. The considerably shorter wave- cards. This is reputedly the largest products they’re attached to. Recent
length of these frequencies allows the
project of its kind to date, with 80,000 reports indicate prices as low as 10c
use of dipole antennas (usually 1/8staff already issued with Philips apiece but that’s still too expensive
wave) and the “far” field emissions
for the frozen peas and baked beans.
MIFARE cards.
of the reader.
On-going research into organic
Michelin engineers have develReader range is considerable longer oped RFID transponders that can be
semiconductors might prove to be the
(>3 metres) than for lower frequency
embedded into their tires, to store in- ultimate answer. Using this emerging
systems. However, microwave fre- formation such as maximum inflation technology, it may soon be possible to
quencies are highly directional and pressure, tire size, etc. It also allows “print” transducers just as we currentSC
readily absorbed by organic tissue, tyres to be associated with Vehicle
ly print barcode labels!
which makes them unsuitable for
many applications.
High frequency tags also require
precision manufacturing and more
expensive electronics than their
lower-frequency counterparts but
they can support data rates of 2Mb/s
or more.
RFID in the news
High profile manufacturers and retailers like Proctor & Gamble, Gillette,
Wal-Mart and Tesco are currently
trialing RFID technology. They’re
employing “smart” shelves that keep
track of stock using transponder tags.
When stock levels drop too low, the
shelves automatically notify staff.
Benetton have embraced the technology, sewing Philips I.CODE tags
into thousands of their retail products.
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125/134.2kHz transponder modules can be manufactured in almost any shape
and size, as demonstrated by this collection. Photo: Texas Instruments
July 2003 11
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