RFID technology, standards and system design

Massimo Incerti, Field Applications Engineer, Future Electronics (Italy)

READ THIS TO FIND OUT ABOUT: RFID principles and system architecture Short-range and long-range RFID Reader and transponder design

 

 

Developed in the early 1940s for military applications, the concept of Radio Frequency Identification (RFID) is now mature. Many modern services such as vending machines, lending libraries, clothes stores or public transport may well be relying on RFID technology, even if most users are not aware of it. Massimo Incerti, Field Applications Engineer, Future Electronics (Italy), describes the technology, design principles, and suitable components.

 

Elements of an RFID system

The physical concept behind RFID is very straightforward: it simply involves applying the basic laws of electro-magnetism.

A generic RFID system is composed of two different devices: a transponder and a reader. The transponder, or tag, (Figure 1) is an electronic device with data storage and the ability to reply to an electro-magnetic stimulus generated by a reader.

The reader is an electronic device able to generate an electro-magnetic field, following a defined protocol that is able to transmit and receive information to and from the transponder.

There are three types of transponders, defined by the type of power supply they use: passive, semi-active and active. Passive transponders do not need any source of power to operate, whereas the semi-active and active devices have a separate source of power independent from the reader’s power supply.

The power supply of a transponder directly affects its reading distance. A system using active transponders can achieve reading distances in the region of a few kilometres, while in systems with passive tags, reading distances are limited to a few metres.

 

Passive RFID

Since passive RFID tags have no internal power supply, the transponder must obtain the required power from the reader’s incoming radio frequency signal in order to transmit a response.

There are essentially two different transponder reply mechanisms: near-field communication is based on an electro-magnetic coupling between the tag and the readers, while far-field communication relies on the back-scattering of the incoming reader signal.

In near-field conditions, the inductive coupling can be understood by reference to a model similar to a transformer. Faraday's law states that a voltage is generated each time a change occurs in the magnetic field; this is the working principle on which all low- and high-frequency systems are based.

In far-field designs, the generally adopted method of communication is the back-scattering effect. Far-field tags reflect, just like a mirror, a fraction of the power transmitted to it by the reader. Such systems work on the same principle as radar systems.

Far-field tags use different wavelengths from near-field tags, their physical dimensions are different and they have longer operating distances.

 

 

Near-field RFID systems: architecture

As stated previously, passive, near-field tags obey the laws of inductive coupling. This, in practice, involves two different antennas working at the same resonant frequency. Reader-to-tag communication uses an amplitude-modulated carrier signal generated by the reader. On the other hand, tag-toreader communication is constrained by the limited power that the tag is able to absorb. It is accomplished by changing the impedance of the tag's antenna according to a modulated signal which comes from data in the tag’s internal memory. The response of a passive RFID tag could be an identification number or may be data from an internal EEPROM. Figure 2 illustrates the internal functions of an RFID transponder chip, showing connections to the antenna.

 

Choosing the right kind of passive tag for your application

In classifying the kinds of tags on the market, the first aspect to be taken into account is the operating frequency. This always lies in one of the ISM bands. Using this type of classification, the transponders fall into two groups: low-frequency or high-frequency.

Low-frequency transponders, operate in the frequency range 125kHz-135kHz. These tags have been on the market since the early 1980s. Their advantages include low cost and high power level together with high reading distance, good rejection of metal and tolerance of noisy environments. Typical applications for these frequencies include animal ID tags meeting ISO11784/5, ISO14223-1 and ISO18000-2.

High-frequency transponders, operate at 13.56MHz. These tags, according to the ISO14443, ISO15693 and ISO18000-3 standards, can be used for the identification of people and goods. ISO14443 defines the directives for so-called proximity cards which are used for financial transactions as well as identification of people. The ISO15693 standard covers vicinity cards, which are typically used for tracking goods.

For both classes of tags the most important system-design challenge is the inductive coupling mechanism between reader and tag.

In particular, decisions made here affect the distance at which a reader can detect a tag.

 

 

The physical principle: Faraday’s law

In a closed loop, an induced electromotive force is generated that is equal to the time rate of change of magnetic flux through the loop itself. Figure 3 highlights the similarity between RFID coupling and transformer operation.

The implication for an RFID system is that the design of the radio critically influences the coupling characteristics of the reader and tag, and hence determines the effective range within the available power budget. In particular, the coupling characteristics are determined by decisions about the reader, such as the power of the radio signal; about the reader’s antenna; and about the tag’s antenna.

Fortunately, the semiconductor vendors resolve many of the difficulties for the designer of short-range systems, with reading distances up to 10cm. Dedicated RFID ICs can handle all the functions associated with the communications protocol and carrier signal generation.

Good examples include the HTRC110 and MFRC5xx from NXP Semiconductors for low- and high-frequency systems respectively. These systems also require a microcontroller to deal with the transactions with the RF ICs, as well as the interface with the external world via a keyboard, display or serial communication. Melexis also provides a range of RFID single-chip devices. Similarly, Tyco subsidiary M/A-COM, offers a wide range of components for building RFID readers. Its RFID readers include amplifiers, modulators, circulators, switches, mixers and modules. M/ACOM has also developed a complete RF front-end solution comprising three modules.

The design of long-range systems is much more difficult. The designer has to implement the RF protocol using a dedicated processor, typically a DSP, together with a power stage usually implemented using an RF wideband transistor. In addition to the software challenges associated with implementing a complete RFID protocol, further difficulties include hardware tasks such as laying out RF power components on the PCB. The designer considering implementing such a system for the first time should seek specialist advice before starting.

Common to the design of all RFID systems, however, is the importance of the antenna. In the design of short-range readers, PCB antennas are widely used, especially in High-Frequency (HF) applications. The typical geometry used for this kind of antenna is a rectangular multi-turn coil. This allows calculations based on semi-empirical formulae, including those governing the inductance of the antenna itself.

Designers can find guides to calculating the inductance of specific geometries freely available on the internet.

Once the inductance is known, it is possible to design the crucial tuning circuit. There is generally no other way to approach the design than via a trialand- error process, in which various resistor and capacitor networks are chosen in order to find the desired resonant frequency.

Once the PCB antenna and the related tuning circuit have been designed, another important step is to choose an adequate impedancematching circuit. Faulty impedance matching can lead to the complete malfunctioning of the system.

The best way to achieve an effective matching circuit is through the use of a network analyser which allows the designer to analyse the behaviour of a system even in the presence of carrier-frequency variation. These instruments are expensive, however. Alternatively, the task may be accomplished less conveniently by use of an ordinary oscilloscope. A good example of tuning and impedance-matching circuits is shown in figure 4, which illustrates how to interface NXP Semiconductors’ MFRC5xx or MFRC4xx devices to external antennas. An optional EMCfiltering stage is also included in this diagram.

Note that in this case the tuning network is simply a resistor network, Rext, and the matching circuit is made only by a capacitor network, Cs and Cp. Tag antenna design is simpler: for instance, there is no need for an impedance-matching circuit because there is no external power supply. While the antenna design is constrained by the shape, size and packaging of the tag itself, most designers find that an ordinary circular multi-turn antenna is effective. The key design issue is to correctly calculate the number of turns required to provide the desired inductance while minimising physical size.

 

 

Conclusion

There is a very great difference in the difficulty of designing short-range and long-range RFID systems. For short-range systems, semiconductor manufacturers such as NXP have built many of the necessary functions into dedicated RFID ICs.

The main challenge for the creator of a short-range system therefore comes merely in the design of the reader and tag antennas. So while widespread adoption of long-range RFID systems might be hampered by the considerable RF-design issues involved in implementing them, the relatively simple challenges involved in short-range RFID systems promise to see them proliferate widely as a convenient means for identifying people and goods in the workplace and in many other commercial environments.