RFID technology is already used for some applications like the smart cards used in access control systems, but many other applications are currently under investigation. Wal-Mart, for example, is working on wireless cards so that the supermarket of the future can have an automatic checkout as customers walk out the door. The European Union has announced plans to embed RFID tags in all Euro notes by 2005 to prevent forgery. As the density and interconnectivity of RFID readers increases, even more bizarre futuristic possibilities such as tracking the location of specific people become possible. An RFID system uses low-power, short-range wireless communications technology. The key components of an RFID system are the RFID tag or transponder, and the RFID reader or interrogator. The RFID tag consists of a microchip attached to an antenna, and operates at a frequency which is selected according to the needs of the system including read range and the environment in which the tag will be read. Tags are either active (integrating a battery) or passive (having no battery). Passive tags derive the power to operate from the field generated by the reader. The RFID reader, which is usually connected to a personal computer, serves the same purpose as a barcode scanner. It can also be battery-powered to allow mobile transactions with RFID tags. The RFID reader handles the communication between the information system and the RFID tag and system. The RFID antenna connected to the reader can be of various types and sizes, depending on the communication distance required for a given system’s performance. The antenna activates the RFID tag and transfers data by emitting wireless pulses. Systems which use a magnetic field as a means of transferring data or power are said to use a electromagnetic coupling, while systems using the inducing of a current in a coil as a means of transferring data or power are said to use inductive coupling. About 90% of current RFID systems are inductively c coupled.
Transmission protocols
RFID systems have to incorporate ‘anti-collision’ capability so that multiple tags can be accessed simultaneously. They also require specialized protocols and transmission checking functions to facilitate reliable communications. Because of the proliferation of applications which take up various parts of the electromagnetic spectrum, strict regulatory regimes (which can vary across countries and regions) apply to the use of RFID systems as seen in table 1. Hence there is a need for testing to ensure conformance to the relevant standards as well as to verify the correct operation of the equipment. The particular nature of the RFID signal presents several challenges in terms of test and measurement. The pulsed signals contain small bursts of modulation, and this modulation can take different forms and serves different purposes. The most common form of modulation is Amplitude Shift Keying (ASK) for the reader and load modulation for the transponder. The other variable is the type of coding used (Manchester, NRZ, RZ, etc). The testing requirements for RFID signals cover not only the traditional RF measurements of frequency, spectral emissions, power and power versus time (because of the time-varying nature of the signal) but also analysis of the short bursts of modulation. The situation is further complicated by the fact that signals are present only for a short time, have vastly differing power levels and could even be frequency-hopping. Moreover, interactions between devices may last for a long time, and interference between signals is common. Traditional, RF measurements on modulated signals have been carried out using a swept spectrum analyser, but the partic-ular constraints of the RFID environment pose limitations for this type of instrument. Signals have to be analysed in the time, frequency and modulation domains, and the signals are pulsed and contain many different ‘sections’ of modulation. The sweeping architecture of the swept spectrum analyser makes it difffficult to capture small RF pulses, and it is not generally possible to combine high measurement speed and good dynamic range at the same time. In addition, these instruments lack the ability to analyse the time-domain characteristics of a complete communication sequence.
Real time spectrum analysis
Fortunately, a solution is now available that suits the needs of the RFID industry: the real-time spectrum analyser (Figure 1). An RTSA captures a full time record of the signals across the entire span within its real-time bandwidth all at once, and so encompasses everything that occurred across the chosen span during the instant the frame was taken. The frame includes all signal information of that moment in time, much like a still photograph includes an image of a particular moment in time. This is combined with powerful triggering capabilities, including a feature known as frequency mask trigger, which allow the instrument to trigger on signals which other spectrum analysers would miss. Seamlessly capturing the complete signal into memory ensures that even the fastest-changing pulses are captured, and enables all aspects of the signal to be analysed. The ability to zoom in and analyse any part of the captured signal means that the user has only to capture the signal once for multiple measurements to be made, with the certainty that all the measurements are correlated. Other benefits of this class of instrument include a large capture memory to ensure that all the signals are captured, plus multi-domain analysis capabilities which allow the complete analysis of time, frequency and modulation characteristics. For example, once the entire transmitted pulse is stored in memory, the user can look at the power/time plot or the AM demodulation plot for even greater resolution. The initial part of an RFID signal exhibits very deep modulation, but the data is encoded as much smaller variations that follow later on in the pulse. Using the RTSA’s on-screen markers, the user can measure the time offsets between various parts of the pulse. In a typical measurement set up, an RTSA is connected to an RFID reader/writer and transponder via a magnetic field probe or loop antenna. Figures 2 and 3 show typical measurements to ensure conformity to RFID industry standards.ISO/IECEC10536: Close-coupling (2mm) IC card: 4.92MHzISO/IEC14443: Proximity (10cm) IC card: 13.56MHzISO/IEC10373-6: Test methodISO/IEC15693: Vicinity (70cm) IC card: 13.56MHzJIS X 6323, ISO/IEC10373-7: Test methodISO/IEC15961 to 3: RFID for managing objectsISO/IEC18000-1 to 7: Air interface(frequency, modulation, rate, encoding)TR18001, 18046, 18047: Performance test, conformance testISO11784, 11785, 14223: Tags for animals: 134kHzISO10374, 18185: Maritime containers: 915MHz, 2.45GHz
