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
systems.
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:
The 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.
The 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 reflection.
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 range.
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.
Also 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).
