Testing Challenges: Analysis of Testing Techniques

As the price of readers and tags decreases and the global market expands, the application of radio frequency identification (RFID) is increasing day by day. Tags can be powered by readers (passive tags) or by the tag's on-board power supply (semi-active tags and active tags). As the cost of sub-micron passive CMOS tags decreases, inventory and other applications are increasing rapidly. Some estimates show that as the price of passive tags continues to drop, almost every product sold will have an RFID tag inside. Due to the importance of passive RFID tags and their unique engineering challenges, this article will focus on passive tag systems.

　　When receiving a CW signal from a reader, a passive tag rectifies the radio frequency (RF) energy to generate a small amount of energy required to keep the tag working, then changes the absorption characteristics of its antenna to modulate the signal and reflect it to the reader through backscattering. RFID systems usually use simple modulation techniques and coding schemes. However, simple modulation techniques have low spectral efficiency and require more RF bandwidth for a given data rate. Before modulation, the data must be encoded to form a continuous information stream. There are many types of bit coding schemes available, each of which has its unique advantages in baseband spectrum performance, complexity of encoding and decoding, and difficulty in writing data to memory under clock drive. Passive tags have unique requirements for the coding scheme used because the timing source on the tag board is difficult to achieve the actual required accuracy, as well as challenging bandwidth requirements and maximizing RF energy transmission to supply energy to the tag. Finally, some kind of anti-collision protocol is required so that the reader can read all tags within its coverage range.

　　RFID Testing Overview

　　Every RFID communication system must pass regulatory requirements and comply with applicable standards. However, today, system optimization separates the winners from the losers in this fast-growing industry. This article discusses the testing challenges facing designers of RFID communication systems: regulatory testing, standards conformance, and optimization.

　　RFID technology presents several unusual engineering test challenges, such as transient signals, bandwidth-inefficient modulation techniques, and backscatter data. Traditional swept-tuned spectrum analyzers, vector signal analyzers, and oscilloscopes have been used in the development of wireless data links. However, these tools have some shortcomings when used for RFID testing. Swept-tuned spectrum analyzers have difficulty accurately capturing and characterizing transient RF signals. Vector signal analyzers do not actually support RFID modulation techniques with low spectrum efficiency and special decoding requirements. Fast oscilloscopes have a small measurement dynamic range and do not have modulation and decoding capabilities. Real-time spectrum analyzers (RTSAs) overcome the limitations of these traditional test tools, have optimization for transient signals, and can reliably trigger specific spectrum events in complex real-world spectrum environments through Tektronix's patented frequency mask trigger.

　　Regulatory testing

　　Every electronic device manufacturer must comply with regulatory standards wherever the device is sold or used. Many countries are modifying regulations to keep up with the unique data link characteristics of passive RFID tags. Most regulatory agencies prohibit CW transmissions from devices except for short-term testing. Passive tags require the reader to send a CW signal to supply energy to the tag and modulate it through backscattering. Even though passive tags do not have a typical transmitter, they can still send a modulated signal. However, many regulations do not address modulation based on no transmitter. Various spectrum emission tests are not explicitly included in the RFID standards for readers, but are included in regulations.

　　Government regulations control the power, frequency, and bandwidth of transmitted signals. These regulations prevent harmful interference and ensure that each transmitter is a friendly neighbor to other users of the frequency band.

　　Making such measurements is challenging with many spectrum analyzers, especially swept spectrum analyzers that are commonly used for pulsed signal energy measurements.

　　The RTSA can analyze the energy characteristics of a complete packet transmission process and can directly measure the carrier frequency of a frequency-hopping signal without centering the signal in a span. At the push of a button, the analyzer can identify the modulation method of an instantaneous RFID signal and perform regulatory measurements of power, frequency, and bandwidth, making the pre-compliance testing process very flexible and convenient [see Figure 2]. Pre-compliance testing helps ensure that the product passes compliance testing the first time without the need for redesign and retesting.

　　Standards Conformance

　　Reliable interaction between readers and tags requires compliance with industry standards such as ISO18000-6C type specifications. This requirement adds many tests beyond the basic requirements to meet government spectrum emission requirements. RF conformance testing is critical to help ensure reliable interoperability between tags and readers.

　　Pre-programmed measurements can reduce the setup time required to perform these tests. For example, an important measurement for ISO18000-6C type is the startup time and shutdown time. The carrier energy rise time must be fast enough to ensure that the tag collects enough energy to operate normally. The signal must also reach a steady state quickly. At the end of the transmission, the carrier energy fall time must be fast enough to prevent interference with other transmissions.

　　Some RFID devices use proprietary communication schemes that are optimized for specific applications. In these cases, engineers need an analyzer that provides multiple modulation and coding schemes that can be programmed to adjust for the specific format being used.