When a passive tag receives a CW signal from a reader, it rectifies the RF energy to create a small amount of power to run the tag. It then changes the absorption characteristics of its antenna to modulate the signal and reflect it back to the reader via backscattering. RFID systems usually employ modulation techniques and coding schemes that are simple to produce. However, most simple modulation schemes tend to be spectrally inefficient, requiring substantial bandwidth for a given data rate. Before modulation, the data must be encoded into a serial information stream. There are many types of bit-encoding schemes available, each with unique advantages in their baseband spectral properties, complexity to encode and decode, and difficulty to clock into memory. Passive tags place unique requirements on the coding schemes used due to the impracticality of precision timing sources on board the tag - challenging bandwidth requirements -, and the need for maximum RF power transport to energise the tag. Finally, some sort of anti-collision protocol is required to enable reading of all tags in the reader's field of view.

Testing overview

Every RFID communication system must pass regulatory requirements and conform to whatever standard is in use. Today, however, it is system optimisation that separates the winners from the losers in this fast-growing industry. RFID technologies present several uncommon engineering measurement challenges such as transient signals, bandwidth-inefficient modulations, and backscattered data. Traditionally, swept tuned spectrum analysers, vector-signal analysers, and oscilloscopes have been used for wireless-data-link development. However, each of these tools has disadvantages when used for RFID testing. Swept tuned spectrum analysers have difficulty capturing and characterising transient RF signals accurately. Vector analysers have virtually no support for spectrally inefficient RFID modulations and their special decoding requirements. Fast oscilloscopes have substantially less measurement dynamic range and lack modulation and decoding capability. Real-time spectrum analysers (RTSAs) are now capable of solving the limitations of these traditional measurement tools thanks to their optimisation for transient signals and their ability to reliably trigger on specific spectral events in complex real-world spectral environments via the Tektronix patented frequency-mask trigger.

Regulatory testing

Every producer of electronic equipment must meet regulatory standards where the equipment will be sold or used. In numerous countries, the regulatory laws are changing to catch up with the unique data-link characteristics of a passive RFID tag. Most regulators prohibit CW transmissions from devices unless it is for a short-term test. Passive tags require the reader to send a CW signal to power the tag and modulate via backscattering. Even though passive tags don't have a typical transmitter, they still produce a modulated signal. However, many regulations do not address non-transmitter-based modulation. A variety of spectral emission tests, which may not be explicitly contained in the RFID standard for the reader, become requirements.Government regulations require that transmitted signals be controlled in power, frequency, and bandwidth. These regulations prevent harmful interference and ensure that each transmitter is a spectrally good neighbour to other users of the band. Power measurements of pulsed signals can be challenging for many spectrum analysers, especially swept analysers. An RTSA can analyse the power characteristics of a complete packet transmission. It can also make direct measurements of the carrier frequency of a frequency-hopping signal, eliminating the need to attempt to place the signal in the centre of the span. An analyser such a this can recognise the modulation of a transient signal and make regulatory measurements of power, frequency and bandwidth at the touch of a button, allowing for the process of pre-compliance testing to be swift and easy.

Standards conformance

Reliable reader/tag interaction requires conformance to industry standards such as the ISO 18000-6 C specifications. This requirement adds many tests beyond those essential to meet government spectral-emissions requirements. Pre-programmed measurements can reduce the set-up time required to make these tests. Some RFID devices use proprietary communications schemes optimised for specific applications. In this case, engineers need an analyser that provides multiple modulation and coding schemes that can be programmatically adjusted for the specific format in use.

Optimisation

Once the basic specifications are met, it is important to optimise the RFID product's features to gain a competitive advantage in a particular market segment. Dimensions of performance include speed of tag reading, ability of a tag to operate in a reader-rich environment, and distance between the tag and the reader. In consumer applications, the speed of the tag-reader communication directly translates into customer acceptance. For instance, RFID-enabled bus passes did not gain wide acceptance until the read time dropped from five seconds to less than half a second. In industrial applications, speed translates into throughput, and this means a more efficient usage of capital and human resources. Since passive tags get the energy they need to operate from the reader, multiple readers can cause a tag to attempt to respond to every reader interrogating it. Improving throughput in a multiple-reader installation requires the use of some sort of anti-collision protocol. Available RF power, path fading, and altered symbol rates can increase the time it takes for the tag to reply to the reader's query. The slower the reply, the longer it will take to read many tags. Quick measurement of turn-around time is essential for optimising the speed of an RFID system.

Conclusion

The RFID industry encompasses a broad array of technologies and applications, many of which differ from the typical communications link. Engineers need tools that make regulatory testing, standards compliance and optimisation measurements quickly and easily. Whether meeting government spectral regulations, ensuring that a tag or reader conforms to a specific communication standard, or debugging a development issue, RTSAs are uniquely suited for analysing RFID signals generated by both readers and tags.