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 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).

What about the Frequency?