Radio frequency identification (RFID) is one of the most useful technologies around and has applications in a wide range of fields. Today, we find it being used everywhere but very few people understand exactly what it is or how it works. In this article, we aim to put that straight and give you a detailed look at a technology that is revolutionising the way many of us work and live.
What is RFID?
RFID technology uses radio waves to send signals between a reader and a tagged object. Readers come in a variety of sizes and shapes, such as hand-held scanners, access control door scanners and mobile phones. Tags are small chips that can be embedded into objects such as smart cards, key fobs, cars, sticky labels and even animals. When you get your pet micro chipped, the vet uses an RFID tag.
The technology works through the following process: the reader sends out radio waves and when a tagged object comes within range, it receives those signals and sends feedback that enables the reader to identify it and receive any other data that is transmitted. As all tags have individual identities, it enables readers to track and communicate with multiple objects at the same time.
An example of use could be an employee using a smart card to gain access to their place of work. On arrival, an access control reader sends out a radio signal to the smart card and when the card responds, the reader is able to identify that the employee has the right to enter the premises. The reader then enables the door to be opened.
Types of tag
There are three different types of tag that are used in an RFID system. These are active, passive and semi-passive tags. Active tags are ones which have their own energy source, such as a battery, which they use to transmit the feedback signal to the reader and for sending further data once the tag has been identified. Passive tags do not have an energy source and rely completely on the radio waves sent from the reader to generate their power. Semi-passive tags use a battery for the initial feedback signal, but then use the reader’s radio waves for further communication.
What is inside an RFID reader?
As we’ve already mentioned, readers can come in various forms, however, they are all composed of three essential components: an RF signal generator, a microcontroller and a signal detector (also known as a receiver).
During operation, the RF signal generator transmits radio waves via the device’s antenna. When a tag responds, the radio waves it sends back are picked up by another antenna on the signal detector. The microcontroller then processes the data contained in those waves. That data can then be used by the device which the reader is built into, for example, to open a door, carry out a payment transaction or to display a reading.
What is inside a tag?
The majority of tags used today are passive. This is because, without the need for their own power supply, they are significantly cheaper to produce and are much more compact in size. A passive tag has four main components: a transponder, a rectifier circuit, a controller and memory.
The transponder is the component that receives radio waves coming from the reader and sends the feedback signal. The rectifier circuit is what provides the energy, the controller processes the sending and receiving of data and the memory is where the data is stored. Although tags, because of their size, do not store significant qualities of data, they can be either read/write or read-only.
The way RFID systems work
RFID tags operate at either low, high or UHF frequencies. Low-frequency tags can send and receive radio waves up to a distance of around 10cm, high-frequency tags can transmit up to one metre and UHF tags have a range of up to 10 – 15 metres.
There are two methods of operation for tags, known as ‘working principles’ and these depend on the frequency at which the tags operate. Low frequency (LF) and high frequency (HF) tags, use a working principle known as ‘Near Field Coupling’ or ‘inductive coupling’; UHF tags, on the other hand, use ‘electromagnetic coupling’.
Near Field Coupling
In Near Field Coupling, the LF and HF radio waves do three things: they induce power in the tag, provide a synchronisation clock for the tag, and act as a carrier for the return data to the reader. To achieve these things, the reader and tag have to be in close proximity.
Power is generated when the radio waves from the reader hit the tag’s rectifier circuit. This induces a voltage, which is used as a power supply for the controller and memory components.
As the reader’s radio waves are what generate the power, the voltage they induce has an identical frequency. As a result, the tag’s synchronisation clock is able to determine the frequency at which to transmit the signal back.
The process by which data is sent is known as load modulation. By turning the current load on and off in a way that corresponds to the data in the memory, it creates a signal that the reader can recognise.
For UHF tags using electromagnetic coupling, the data is transferred not by turning the load on and off at a specific frequency, but by changing the strength of the signal at a specific frequency. This is known as the backscatter modulation.
The strength of the signal depends upon the load inside the tag’s rectifier circuit. By changing the intensity of the load, it is possible to change the intensity of the backscatter signal. Using this method, the load can be changed to a frequency that transmits the data in a way the reader can interpret.
As the UHF tags work over a longer distance, the initial signal from the UHF tag must always be strong to ensure it has been identified by the reader before it varies strength to send the rest of the data.
As you will have been able to see from this article, small does not always mean simple. The technology used in RFID systems uses some quite complicated physics in order to work. Hopefully, from reading this, you will now have a better understanding of the ways in which RFID systems are built and ways that different tags and readers work together.
About the Author
Universal Smart Cards is a leading global supplier of smart card technology and offer advice and consultancy services for organizations looking to implement this technology in their business.