RFID Building Blocks
RFID Tags.
Packaging
Every object to be
identified in an RFID system will need to have a tag attached to
it. Tags are manufactured in a wide variety of packaging formats
designed for different applications and environments. The basic
assembly process
(see Fig 4.)
consists of first a substrate material (Paper, PVC, PET...),
upon which an antenna made from one of many different
conductive materials including Silver ink, Aluminum and copper
is deposited. Next the Tag chip itself is connected to the
antenna, using techniques such as wire bonding or flip chip
(see Fig 4.).
Finally a protective overlay made
from materials such as PVC lamination, Epoxy Resin or Adhesive
Paper, is optionally added to allow the tag to support some of
the physical conditions found in many applications like
abrasion, impact and corrosion.
Fig 2. Basic Tag Assembly
Flip chip
connection
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• PVC
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• Copper
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• PVC
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• PET •
PAPER |
• ALU •
Conductive Ink |
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• Epoxy
Resin • Adhesive |
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Paper
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Wire |
Chip bumps |
• |
……….
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Surface |
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Examples of different formats |
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Credit card size flexible labels with adhesive backs
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Tokens and coins
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Embedded tags – injection molded into plastic products such as
cases
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Wrist band tags
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Hard tags with epoxy case
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Key fobs
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Tags designed specially for
Palettes and cases
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Paper tags
Tag Cost
The type of materials
and assembly methods used to package tags impact directly on the
final cost ( around 30%), and to some extent on the
communication performance. In the supply chain, the cost of
tags is one of the main considerations for mass adoption, with
the 5 cent tag being the much talked about target. How to
achieve this figure is currently one of the great debates.
Traditionally chip die size has always been the key focus, and
IC companies have managed to get die sizes (chip area) down to
around 0.3mm2 for UHF chips, resulting in a manufacturing cost
of about 1-2 cents depending on the Silicon process. This leaves
3 cents for the rest! which is where the real challenge now
seems to lie. There are solutions in the pipeline from companies
like Alien Technology and Philips Semiconductors, whom have both
developed new chip assembly techniques which when used with the
very large volumes (billions of tags) expected in the supply
chain promises to optimize costs to the levels required in order
to reach the 5 cent goal.
Fig 3. HF (13.56 MHz) Tag examples
Paper labels
with conductive silver ink antennas Flexible label with an
aluminum antenna
Courtesy of ASK
Courtesy of Inside Contactless
Fig 4. UHF (860 – 930 MHz) tag examples
Courtesy of MATRICS
Courtesy of IPICO
Coil on chip Tags
(image courtesy of Maxell)
A new and fascinating
development for tags is based on having the antenna deposited
directly onto the tag chips surface. Although the communication
distance is limited to around 3mm, the result is a microscopic
tag which can be concealed for example in bank notes, as
proposed recently by Hitachi-Maxell. Both Maxell and Inside
contactless have developed working versions of these tags at
UHF, HF (Maxell) and HF (Inside).
Tag IC’s
Fig 5. Basic Tag IC architecture
RFID tag IC’s are designed and manufactured using some of the
most advanced and smallest geometry silicon processes available.
The result is impressive, when you consider that the size of a
UHF tag chip is around 0.3 mm2 i.e. about the size of the square
below 
In terms of computational power, RFID tags are quite dumb,
containing only basic logic and state machines capable of
decoding simple instructions. This does not mean that they are
simple to design! In fact very real challenges exist such as,
achieving very low power consumption, managing noisy RF signals
and keeping within strict emission regulations. Other important
circuits allow the chip to transfer power from the reader signal
field, and convert it via a rectifier into a supply voltage. The
chip clock is also normally extracted from the reader signal.
Most RFID tags contain a certain amount of NVM (Non volatile
Memory) like EEPROM in order to store data.
The amount of data stored depends on the chip specification, and
can range from just simple Identifier numbers of around 96 bits
to more information about the product with up to 32 Kbits.
However, greater data capacity and storage (memory size) leads
to larger chip sizes, and hence more expensive tags. In 1999 The
AUTO-ID center (now BRT Global) based at the MIT (Massachusetts
Institute of Technology) in the US, together with a number of
leading companies, developed the idea of an unique electronic
identifier code called the BRT (Electronic Product Code) . The
BRT is similar in concept to the UPC (Universal Product Code)
used in barcodes today. Having just a simple code of up to 256
bits would lead to smaller chip size, and hence lower tag costs,
which is recognized as the key factor for wide spread adoption
of RFID in the supply chain. Tags that store just an ID number
are often called ¨ License Plate Tags ¨
Tag Classes
One of the main ways
of categorizing RFID tags is by their capability to read and
write data. This leads to the following 4 classes. BRT global
has also defined five classes which are similar to the ones
below.
CLASS 0 – READ ONLY. – Factory programmed
These are the simplest type of tags, where the data, which is
usually a simple ID number, (BRT) is written only once into the
tag during manufacture. The memory is then disabled from any
further updates. Class 0 is also used to define a category of
tags called EAS (electronic article surveillance) or anti-theft
devices, which have no ID, and only announce their presence when
passing through an antenna field.
CLASS 1 – WRITE ONCE READ ONLY (WORM) – Factory or User
programmed
In this case the tag is manufactured with no data written into
the memory . Data can then either be written by the tag
manufacturer or by the user – one time. Following this no
further writes are allowed and the tag can only be read. Tags of
this type usually act as simple Identifiers
CLASS 2 – READ WRITE
This is the most flexible type of tag, where users have access
to read and write data into the tags memory. They are typically
used as data loggers, and therefore contain more memory space
than what is needed for just a simple ID number.
CLASS 3 – READ WRITE – with on board sensors
These tags contain on-board sensors for recording parameters
like temperature, pressure, and motion, which can be recorded by
writing into the tags memory. As sensor readings must be taken
in the absence of a reader, the tags are either semi-passive or
active.
CLASS 4 – READ WRITE – with integrated transmitters.
These are like miniature radio devices which can communicate
with other tags and devices without the presence of a reader.
This means that they are completely active with their own
battery power source.
Table 2. Different tag classes
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Class |
Known as |
Memory |
Power Source |
Application |
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0 |
EAS |
BRT
[1] |
None |
BRT |
Passive |
Ant-theft |
ID |
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1 |
BRT |
Read -Only |
Any |
Identification |
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2 |
BRT |
Read-Write |
Any |
Data logging |
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3 |
Sensor Tags |
Read-Write |
Semi-Passive/Active |
Sensors |
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4 |
Smart Dust |
Read-Write |
Active |
Ad Hoc networking |
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[1] The section on
BRT standards evolution shows that the BRT class 0 is likely to
evolve to a read-write
Selecting a
tag
Choosing the right tag for a
particular RFID application is an important consideration, and
should take into account many of the factors listed below:
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Size and form factor –
where does the tag have to fit?�
How close will tags be to each other�
Durability –
will the tag need to have a strong outer protection against
regular wear and tear.
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Is the tag re-usable
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Resistance to harsh environments
(corrosive, steam...)�
Polarization – what will be the tags orientation with respect to
the reader field�
Exposure to different temperature ranges�
Communication distance�
Influence of materials such as metal and liquids�
Environment
(Electrical noise, other radio devices and equipment)�
Operating Frequency (LF, HF or UHF)�
Supported Communication Standards and protocols (ISO, BRT)
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Regional Regulations (US, Europe and Asia)
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Will the tag data need to store more than just an ID number like an
BRT
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Anti-collision -how many tags in the field at the same time and how quickly must
they be
detected.
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How fast will tags move through the reader field
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Reader support –which reader products are able to
read the tag
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Does the tag need to have security –
Data protection by
encryption
Fig 6. How passive tags are defined
Active and passive tags
Fig 6, shows that the
first basic choice when considering a tag is between either
passive, semi-passive or active. Passive tags can be read at a
distance of up to 4 - 5m using the UHF frequency band, whilst
the other types of tags (semi-passive and active) can achieve
much greater distances of up to 100m for semi-passive, and
several kilometers for Active . This large difference in
communication performance can be explained by the following;
�• Passive tags use the reader field as a source of energy for
the chip and for communication from and to the reader. The
available power from the reader field, not only reduces very
rapidly with distance ,but is also controlled by strict
regulations, resulting in a limited communication distance of 4
- 5m when using the UHF frequency band (860 Mhz – 930 Mhz).
�• Semi-Passive (battery assisted backscatter) tags have built
in batteries and therefore do not require energy from the reader
field to power the chip. This allows them to function with much
lower signal power levels, resulting in greater distances of up
to 100 meters. Distance is limited mainly due to the fact that
tag does not have an integrated transmitter, and is still
obliged to use the reader field to communicate back to the
reader.
�• Active tags are battery powered devices that have an active
transmitter onboard. Unlike passive tags, active tags generate
RF energy and apply it to the antenna. This autonomy from the
reader means that they can communicate at distances of over
several kilometers.
This paper focuses on passive tags. The experience gained by
different companies running various trails and evaluations has
so far shown, that out of the different RFID frequencies
LF,HF,UHF and microwave (see fig 1). HF and UHF are the best
suited to the supply chain. Furthermore, it is expected that UHF
due to its superior read range, will become the dominant
frequency. This does not mean however that LF and microwave will
not be used in certain cases.
LF, HF Tags
Tags at these frequencies use inductive coupling between two
coils (reader antenna and tag antenna - see fig 7) in order to
supply energy to the tag and send information. The coils
themselves are actually tuned LC circuits, which when set to the
right frequency (ex; 13.56 MHz), will maximize the energy
transfer from reader to tag. The higher the frequency the less
turns required (13.56 MHz typically uses 3 to 5 turns).
Communication from reader to tag occurs by the reader modulating
(changing) its field amplitude in accordance with the digital
information to be transmitted (base band signal). The result is
the well known technique called AM or Amplitude Modulation. The
tags receiver circuit is able to detect the modulated field, and
decode the original information from it. However, whilst the
reader has the power to transmit and modulate its field, a
passive tag does not. How is communication therefore achieved
back from tag to reader?.
The answer lies in the
inductive coupling. Just as in a transformer when the secondary
coil (tag antenna) changes the load and the result is seen in
the Primary (reader antenna).The tag chip accomplishes this same
effect by changing its antenna impedance via an internal
circuit, which is modulated at the same frequency as the reader
signal. In fact its a little more complicated than this because,
if the information is contained in the same frequency as the
reader, then it will be swamped by it, and not easily detected
due to the weak coupling between the reader and tag. To solve
this problem, the real information is often instead modulated in
the side-bands of a higher sub- carrier frequency which is more
easily detected by the reader.
Fig 8. Creation of two higher frequency
side-bands
fc =13.56 MHz0 dB
Reader Carrier signal
Signal
Modulation
Information in subcarrier side bands
fc =13.348 MHz fc
=13.772 MHz
-80 dB
f
UHF
tags
Passive tags operating
at the UHF and higher frequencies use similar modulation
techniques (AM) as lower frequency tags, and also receive their
power from the reader field. What is different however, is the
way that energy is transferred, and the design of the antennas
required to capture it. We have already mentioned that this is
achieved using the far field ,which is in fact the region
in Electromagnetic Theory where the electric and magnetic field
components of a conductor (antenna) break away, and propagate
into free space as a combined wave. At this point, there is no
further possibility of inductive coupling like in HF systems,
because the magnetic field is no longer linked to the antenna.
Transmission of this wave in the far field is the basis of all
modern radio communication. In some systems such as transmission
lines (coaxial cables), the propagation of these waves is
restricted as much as possible via special shielding as they
constitute a power loss. For antennas its the inverse,
propagation is encouraged. When the propagating wave from the
reader collides with a tag antenna in the form of a dipole
(see fig 7)
, part of the energy is absorbed to power the tag and a small
part is reflected back to the reader in a technique known as
back-scatter. Theory shows that for the optimal energy transfer
the length of the dipole must be equal to
l/2 ,which gives a dimension of around 16 cm. In reality the dipole
is made up of two
l/4 lengths. Deviating
from these dimensions can have a serious impact on performance.
Just as for lower
frequency tags using near field inductive coupling, a passive
UHF tag does not have the power to transmit independently.
Communication from tag to reader is achieved by altering the
antenna input impedance in time with the data stream to be
transmitted. This results in the power reflected back to the
reader being changed in time with the data i.e. it is modulated.
From an applications
point of view, using the technique of far field back-scatter
modulation introduces many problems that are not so prevalent in
HF and lower frequency systems. One of the most important of
these is due to the fact that the field emitted by the reader is
not only reflected by the tag antenna, but also by any objects
with dimensions in the order of the wavelength used. These
reflected fields, if superimposed on the main reader field can
lead to damping and even cancellation.
Tag Orientation (polarization)
How tags are placed
with respect to the polarization of the readers field can have a
significant effect on the communication distance for both HF and
UHF tags, resulting in a reduced operating range of up to 50%,
and in the case of the tag being displaced by 90° (see fig 9),
not being able to read the tag . The optimal orientation for HF
tags is for the two antenna coils (reader and tag) to be
parallel to each other as shown below in fig4. UHF tags are even
more sensitive to polarization due to the directional nature of
the dipole fields. The problem of polarization can be overcome
to a large extent by different techniques implemented either at
the reader or tag as shown in table 4 below.
Table 4. Managing the problem of tag orientation
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Reader - Antenna |
Tag |
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UHF- Antenna Circular field polarization |
UHF – Two antennas polarized 90° out of |
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HF |
-Antennas |
physically placed at |
phase –eg Matrics double dipole |
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different |
locations |
with |
in |
different |
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orientations (XYZ) |
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two antennas 90° out of phase with each |
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other |
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3D Tunnel readers |
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Fig 9. HF Tag orientation with different antenna configurations
1-D field,
Tag
Tag
readable
Un-readable
Antenna
Tag standards
A very important aspect of RFID technology are the associated
standards and regulations. They are designed to ensure safe
operation with respect to other electrical and radio equipment,
and guarantee interoperability between different manufacturers
readers and tags. Regulations are mainly concerned with reader
power emissions and allocation of frequency bands, whilst
standards like the ISO (International Standards Organization)
define the Air interface communication between Reader->Tag and
Tag->Reader, and include parameters such as;
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Communication protocol
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Signal Modulation types
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Data coding and frames
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Data Transmission rates
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Anti-collision (detection and sorting of many tags in the Reader
field at the same
time) The history of RFID standards over the last 10 years has
unfortunately been far from ideal ,leading to too many
variations and confusion. The situation for the supply chain and
Item management is no different due to the two air interface
standards currently being proposed by ISO and the BRTglobal (see
below). Some initiatives are under way to try and harmonize the
two into one global standard, which would certainly be the
recommended way to ensure the wide spread adoption, and high
volumes of RFID tags within the supply chain.
Table 5.
ISO
18000 Information Technology AIDC Techniques-RFID for Item
Management - Air Interface
�• 18000 –1 Part 1 – Generic Parameters for Air Interface
Communications for Globally Accepted Frequencies
�• 18000 – 2 Part 2 – Parameters for Air Interface
Communications below 135 KHz
�• 18000 – 3 Part 3 – Parameters for Air Interface
Communications at 13.56 MHz
�• 18000 – 4 Part 4 – Parameters for Air Interface
Communications at 2.45 GHz
�• 18000 – 5 Part 5 – Parameters for Air Interface
Communications at 5.8 GHz
�• 18000 – 6 Part 6 -Parameters for Air Interface
Communications at 860 – 930 MHz
Table 6. BRT standards evolution:
Protocol
Frequency
Description
Class 2 UHF A proposed read-write tag
Source: RFID Journal Jan 2004 issue
Regional Regulations and Frequency Allocation
RFID tags and readers
fall under the category of short range devices (SRD’s), which
although they do not normally require a license, the products
themselves are governed by the laws and regulations which vary
from country to country. Today, the only globally accepted
frequency band is the HF 13.56 MHz. For passive UHF RFID the
problem is much more complicated as frequencies allocated in
some countries are not allowed in others, due to their proximity
to already allocated bands for devices such as mobile phones and
alarms.
This discontinuity has
resulted in the ITU (International Telecommunications Union)
dividing the world into three regulatory regions, these being;
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REGION 1: |
Europe,
Middle East, Africa and the former Soviet Union
including Siberia |
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REGION 2: |
North
and South America and Pacific east of the International
Date Line |
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REGION 3: |
Asia,
Australia and the Pacific Rim West of the International
date line |
The main
regulatory bodies in the different regions
REGION 1:
In the USA, the Federal Communications Commission (FCC).
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For UHF regulations see the FCC-Part 15 (15.249). which can be
found at the
following web site: http://www.access.gpo.gov/nara/waisidx_01/47cfr15_01.html
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Allocated UHF band (902-928 MHz),
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Max Power emission 4W EIRP - Frequency Hopping
REGION 2: In Europe, CEPT (European Conference of Postal and
Telecommunications) has the responsibility of frequency
assignment and output power.
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For UHF regulations see publication ERC REC 70-73 which can be
found at the
following web site http://www.ero.dk/doc98/official/pdf/rec7003e.pdf
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Allocated UHF fixed band ( 869.3 –869.65 MHz)
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Power emissions limited to 500mW EIRP
( expected to be enhanced to 2W in 2004)
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Readers must operate within a 10% duty cycle – No Frequency or
channel hopping
, REGION 3:<