Radio frequency identification (RFID) applications are expanding rapidly. The promise of accurate and cost-effective asset tracking, secure point-of-sale transactions and supply chain management all contribute to this growth along with US Department of Defense mandates. The adoption of RFID technology is also leading to an increase in available RFID systems and the associated need for pre-compliance and compliance testing.
For the designer, pre-compliance testing can greatly reduce the risk of test failure during compliance verification and the resulting time-to-market problems. However, pre-compliance testing has traditionally been a time-consuming manual process. By gaining an understanding of RFID pre-compliance tests and techniques for automating many test procedures, pre-compliance testing can become practical during a rapid product development cycle.
The Case for Pre-Compliance Testing
In order to display the certification trademark that indicates performance compatibility, most industry standards require successful completion of a battery of compliance tests. These certifications are important to prospective customers since they provide independent assurance that the RFID system will function correctly with a variety of readers and tags from different suppliers.
Compliance testing is similar to any impartial, objective test. Come prepared and it can be an exciting and rewarding experience. Come ill-prepared and it can represent a significant setback. In the fast moving RFID industry, a major cost of being poorly prepared for compliance certification is often the opportunity cost of missing market windows. Failing a compliance certification test and having to reschedule another test can mean weeks of lost revenue from a late product introduction or missing the design socket of an important customer.
The only way to prevent these unpleasant scenarios and pass the compliance test first time out is with intense preparation throughout the design cycle and upfront testing. By committing to pre-compliance testing, companies position themselves for a quick pass on the first try and accelerated time to market. It is much less costly to discover a problem before a design leaves the company, then to discover a problem at the compliance certification laboratory. With efficient pre-compliance testing (see Figure 1), a few extra days can save weeks of lost revenue.
Figure 1 Pre-compliance measurements like settling time can help ensure interrogators pass compliance testing the first time.
Certified test laboratories use custom automatic testing systems to conduct compliance testing. These tests are more exhaustive than the typical bench development testing and often simulate real-world conditions that are hard to replicate in the lab. The time required for most engineers to exhaustively test their designs may not make sense if the measurements are arduously slow to make. The right combination of test equipment and software tools make pre-compliance testing a realistic option.
RFID Testing Challenges
Technical challenges for developers of RFID test systems encompass both the RF and protocol characteristic of RFID technology. In RFID communication there are high-level signals (in terms of power) transmitted by the readers and low-level back-scattered signals coming from the tags. One of the big challenges is to trigger and capture both types of signals at the same time within microseconds. Testing all the protocol requirements of RFID devices hinges on the ability to generate different types of modulated signals, protocol commands and responses.
For RFID interrogators, typical RF measurements required are frequency accuracy, RF envelope and power-up/power-down waveforms, while for protocol testing it is necessary to verify link timing parameters. For testing RFID tags, the engineer must simulate the interrogator commands for communicating with a given tag. Examples of RF measurements required here are demodulation capability, duty cycle and operating frequency range. On the protocol side, among other requirements, link timing and tag states and transitions have to be checked.
Swept tuned spectrum analyzers, vector signal analyzers and oscilloscopes have traditionally been used for wireless data link development. For RFID product development, however, inherent limitations of these instruments begin to surface. The spectrum analyzer has been used to characterize the RF spectral output of a transmitter to ensure compliance with regulatory emission restrictions. But the traditional swept tuned spectrum analyzer was developed primarily for the analysis of continuous signals not the intermittent RF pulses associated with modern RFID products. This can lead to a variety of measurement issues, particularly the accurate capture and characterization of pulsed RF signals.
The vector signal analyzer also possesses little ability to capture transient RF signals since it was initially developed for continuous wave (CW) signals. Accurate triggering must be coupled with data acquisition to make sure the desired waveform is captured with the limited memory of most vector signal analyzers. Though most vector signal analyzers have extensive demodulation capability for popular spectrally efficient modulations, current offerings have virtually nothing to support the spectrally inefficient RFID modulations and their special Pulse Code Modulation (PCM) decoding requirements. This makes the current generation of vector signal analyzers of little value to the RFID engineer.
The oscilloscope has long been a valuable tool for analysis of baseband signals. In recent years some oscilloscopes have extended their sampling speed to very high microwave frequencies. They are, however, still sub-optimal tools for UHF or higher frequency measurements on RFID systems. Relative to the modern real-time spectrum analyzer, the fast oscilloscope has less measurement dynamic range and lacks modulation and decoding capability.
Pre-Compliance Testing Solutions
The real-time spectrum analyzer (RSA) solves the limitations of the traditional measurement tools by providing an efficient test and diagnostic tool when coupled with RFID measurement software that allows rapid characterization of the many critical industry specification requirements. This combination provides the speed and repeatability to make pre-compliance testing feasible even under the most pressure-filled design conditions.
Pulsed tag reads and writes require an RF analyzer optimized for transient signals. Modern RSAs excel at characterizing transient signals through the use of a real-time architecture and time-correlated displays. The RSA has the digital processing speed necessary to transform input signal from time domain samples into the frequency domain with a real-time Fast Fourier Transform (FFT) prior to capturing a recording of data. Time-correlated analysis in multiple domains greatly enhances the diagnostic insight and reliability by pinpointing the anomaly responsible for an event showing correlated behavior in the different displays of time, frequency and modulation versus time.
RSAs include a built-in tabular data display that enables the engineer to search for compliance issues quickly. With traditional test equipment the labor required to make many of the measurements is so great that only a few spot checks are logistically possible. The high speed at which the RSA can take measurements allows the engineer to approximate the exhaustive compliance test much more closely.
Most of the important RF measurements are easily set up and accomplished using RFID measurement software, allowing the engineer to quickly check and recheck compliance with specifications. This reduces the possibility of a surprise failure during the actual compliance test. For example, careful pre-compliance measurement under a variety of conditions testing the interrogator’s data burst power-on, power-down and RF envelope ripple can help avoid issues during certification.
If an issue does arise during the compliance test, one approach is to use a transportable RSA to quickly troubleshoot circuits on location. Once a compliance failure condition is known, multi-domain time correlated analysis capability can give the insight needed to trace a failure to its root cause. This can help engineers rescue compliance efforts by rapidly identifying issues.
Spectral Emissions Pre-Compliance Testing
To show how a RSA with RFID analysis software can be used for pre-compliance purposes, consider an example such as the key measurements necessary to characterize spectral emissions for government regulatory compliance. Government regulations require that transmitted signals be controlled in power, frequency and bandwidth. The intention is to minimize interference and ensure each transmitter is a spectrally good neighbor to other users of the band.
Power measurements of pulsed signals can be challenging for many spectrum analyzers. In contrast, the RSA is able to optimize transient signals to identify power in a pulsed RFID packet transmission, and FFT analysis presents a complete spectral frame for any given period of time during the packet transmission. This capability eliminates the need to synchronize tuning sweeps with packet bursts—an issue older swept tuned spectrum analyzers had. Also, traditional spectrum analyzers use correction factors to compensate for Successive Log Video Amplifier (SLVA) peak detection circuits. A modern RSA, on the other hand, uses a true RMS detection approach that accurately reads power for most regulatory measurements.
The carrier frequency of the signal is another important spectral emission measurement. There are two ways this measurement can be expressed: actual absolute carrier frequency or carrier frequency error from a given assigned channel frequency. The RSA first displays carrier frequency error when demodulating a signal while the absolute carrier frequency can be displayed in spectrum analysis mode. It’s worth noting that demodulated carrier frequency measurement doesn’t require the signal to be positioned at the center of the span. This is useful for working with frequency hopping signals.
Other measurements include Occupied Bandwidth (OBW) measurements and Emission Bandwidth (EBW). These can be obtained in two ways. In the demodulation mode the RSA displays the OBW and EBW as well as the carrier frequency and transmission power levels. When these are preprogrammed as automatic measurements, engineers can quickly and accurately determine if their designs meet regulatory requirements (see Figure 2). This eliminates the drudgery of attempting to coax a traditional spectrum analyzer into making measurements on a pulsed RFID signal.
Figure 2 Key regulatory spectral measurements can be quickly made by displaying the spectrum and choosing the OBW/EBW measurements.
Meeting Industry Standards
Reliable interrogator and tag interaction requires that designs conform to industry standards such as the ISO 18000-6 Type C specifications. This adds many tests beyond those needed to meet government spectral emissions requirements. RF conformance tests ensure reliable interoperability among tags and readers.
The combination of the RSA and RFID software makes the task of ensuring interoperability much easier. A complete RFID software package contains measurements needed for a broad range of standards such as ISO 18000-4 Mode 1 and ISO 18000-6 Type A, B and C. Preprogrammed measurements eliminate most of the setup time required to check out these signal formats. A sampling is shown in Table 1.
Table 1 Preset measurements greatly reduce setup time when checking compliance with standards
For example, one important measurement for ISO 18000-6 Type C is the power-on and power-down time. The rise time of carrier energy must be turned on promptly to ensure the tag collects enough energy to function properly. The signal must also settle out to a stable level. At the end of the transmission, the fall time of the signal burst must be quick enough to avoid disrupting other transmissions. Once the RSA is configured for the test, it will automatically measure power on rise time, power off fall time, power settling time, overshoot and undershoot (see Figure 3). By also displaying the waveform characteristics in a measurement window, the instrument gives the engineer a more detailed perspective for further analysis and debugging.
Figure 3 Power-on and power-down measurements for query session can be tested using automated RFID software on the RSA.
Communications between interrogator and tag are accomplished with ASK signal bursts during the power-on period. These signal bursts make up the RF envelope and are important for interoperability. The modulation pulse envelope contains characteristics necessary to assure compatibility between reader and tag. Here, the RFID software can be tapped to automatically measures RF envelope specifications like on width, off width, duty cycle, on ripple, off ripple and the slopes of the RF envelope edges.
Another application is to characterize a variety of modulation envelopes including DSB-ASK, SSB-ASK and PR ASK using the RSA. To simplify keeping track of protocol transmissions, it is useful to have RFID software that first labels individual bursts with an index number and then further subdivides bursts into envelope numbers that show individual symbol parameters.
Once the basic specifications are met and you are sure of obtaining certifications, it is important to optimize some of the RFID product’s features to gain a competitive advantage in a particular market segment. A modern test environment can be extremely valuable in maximizing system performance while at the same time minimizing the engineering commitment necessary to achieve desired goals.
The RFID industry encompasses a broad array of technologies and applications, many of which differ from the typical communications link. For example, the latest international RFID standards call for sophisticated frequency-hopping spread spectrum (FHSS) signals with transient half duplex RF bursts composed of ASK modulations with unusual encoding and robust anti-collision protocols. Given the rapid pace of change, the level of complexity involved and the rigor of certification testing, designers can no longer take the risk of going to certification labs without having first performed pre-compliance testing.
The modern RSA combined with a comprehensive RFID analysis software package that includes international RFID standards support greatly speeds development diagnostics and provides a feasible way of conducting pre-compliance testing. The RSA also helps engineers perform RFID measurements that either cannot be made or would require elaborate, time-consuming test setups on traditional swept spectrum analyzers or vector signal analyzers.
Darren McCarthy earned his BSEE degree from Northwestern University in Evanston, IL. He is currently technical marketing manager for RF Test at Tektronix. He has worked extensively in various test and measurement positions for the last 18 years, including R&D engineer, R&D management, product planning and business development. During his career, he has also represented the US on several IEC technical committees for international EMC standards.