RFID - The Technology
How does it work?
In the last section we mentioned many
things that you are relevant to an RFID system. In this section
we will try to explain all of them.
The diagram below explains the basic schematic of all RFID
The Tag or Transponder can be either
active or passive. It responds to a signal from the
Interrogator (reader/ writer/ antenna) which in turn sends a
signal to the Computer.
Taking each piece in turn:
Tag comes in a variety of shapes. It is made up from a chip
(IC) and an antenna. Depending on your application it may be
embedded in glass, or epoxy, or it may be in a label, or a
card. See below for a selection of shapes.
tag can be passive, battery assisted, or active.
Passive tags get all their power from the signal sent by the
interrogator. As well as using this radio wave to carry the
data, the tag is able to convert it into power. This means
that the tag is only powered when it is in the beam of the
interrogator. The tag then uses a technique called
backscatter to reply to the interrogator. This does not
involve a transmitter on the tag, but is a means of
"reflecting" the carrier wave and putting a signal into that
Battery assisted tags are just like passive tags (they use
backscatter) but they have a battery to provide the power to
the chip. This provides a big advantage, because the tag is
not dependent on the strength of the carrier from the
interrogator to provide the power it needs. Now it can use
all the power from the battery and so is able to work at a
greater distance from the interrogator.
Active tags, have not only a battery, but also some form of
transmitter on the tag. Now we can really talk about long
The disadvantage of having a battery is two fold. One, it
adds cost to the tag, and two they run out of power
eventually. The decision on which one is right for you will
depend on your application.
The tag is made of an IC and an antenna. The IC will include
memory and some form of processing capability. The memory
may be read only or read/write, the type selected will
depend on the application.
The tag talks to the interrogator using what is called the
air-interface. This is a specification for how they talk to
each other and includes the frequency of the carrier, the
bit data rate, the method of encoding and any other
parameters that may be needed. ISO 18000 is the standard for
the air interface for item management.
a part of this air interface is what is commonly called the
anti-collision protocol (if the tag support it). This is a
means of allowing many tags in the field to talk "at the
same time". There are several ways of doing this, and each
manufacturer has developed their own way of implementing it.
Simplistically, consider a first grade teacher talking to
his/her class. She says "Call out your name if you are here
today". What she hears is 20 (or more) kids all shouting at
the same time. So she says, "If your name begins with an A,
shout out your name". Maybe she only hears one name now, or
maybe she hears several. If she hears several, she refines
the command, "If you name begins AA". By telling a child to
keep quiet after she is able to record the name, she is now
able to collect all the names.
Two other terms you may hear are "Reader talks first" (RTF)
and "Tag talks first" (TTF). With a RTF system, the tag just
sits there, until it hears a request from the interrogator.
This means that even though a tag may be illuminated
(receiving power) from the interrogator, it does not talk
until it is asked a question. With TTF the tag talks as soon
as it gets power, or in the case of a battery assisted tag
or active tag, it talks for short periods of time, all the
time. This gives you a much faster indication of a tag
within sight of the interrogator, but it also means that the
airwaves have constant traffic.
The antenna in a tag is the physical interface for the RF to
be received and transmitted. Its construction varies
depending on the tag itself and the frequency it operates
on. Low frequency tags often use coils of wire, whereas high
frequency tags are usually printed with conducting inks.
Another form of tag is often called the smart label. This is
really a paper (or similar material) label with printing,
but also with an RFID tag embedded in it. Examples are shown
below (with the antenna structure shown in the corner).
What about the Frequency?