Passive RFID tag system testing

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 reflects 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 stream of information. There are many types of bit encoding schemes available, each with its own unique advantages in baseband spectrum performance, encoding and decoding complexity, and difficulty in writing data to memory under clock drive. Passive tags have unique requirements for the encoding scheme used due to the difficulty of achieving the actual required accuracy of the timing source on the tag board, 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 require controls on 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 for many spectrum analyzers, especially swept spectrum analyzers that are typically used to measure the energy of pulsed signals. RTSA can analyze the energy characteristics of a complete packet transmission process and can directly measure the carrier frequency of frequency hopping signals without centering the signal in a span. At the touch of a button, the analyzer can identify the modulation method of a transient RFID signal and can perform regulatory measurements of power, frequency, and bandwidth, making the pre-compliance testing process very flexible and convenient. Pre-compliance testing helps ensure that the product passes compliance testing the first time without redesigning 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.

optimization

Once the basic specifications are met, it is important to optimize the performance of RFID products to gain a competitive advantage in a specific market space. Performance indicators include the read speed of the tag, the ability of the tag to operate in a multi-reader environment, and the distance between the tag and the reader. In consumer applications, the communication speed between the tag and the reader directly affects user satisfaction. For example, the public transportation industry using RFID did not gain widespread acceptance until the read time was reduced from 5 seconds to less than half a second. In industrial applications, speed means throughput: the higher the throughput, the more efficient the use of financial and human resources. Since passive tags obtain the energy they need to operate normally from the RFID reader, multiple readers may cause the tag to try to respond to every reader that asks it. In the case of multiple readers, some kind of anti-collision protocol needs to be used to improve the throughput of the system. Finally, to maximize the read range of the tag, the carrier to noise requirement should be minimized, but this may conflict with the need to prevent the tag from running out of energy by minimizing the carrier's inactivity time. These optimization measures pose challenges to engineers and measurement equipment.

Let's look at a specific example - optimizing communication speed, also known as turnaround time TAT (hereafter referred to as TAT). Available RF energy, path fading, and altered symbol rates can increase the time it takes a tag to respond to a reader query. The slower the response, the longer it takes to read multiple tags. Quickly measuring TAT is essential to optimizing the speed of an RFID system.

TAT can be easily measured using an RTSA. First, a frequency mask trigger is installed to capture the entire interrogation between the tag and the reader. The RTSA's power vs. time view allows the user to watch the entire transmission process. It is customary to think of the time between the end of one downlink transmission (from reader to tag) and the beginning of the next downlink transmission as the TAT for a half-duplex system. By placing one marker at the end of the tag interrogation and a second delta marker at the end of the backscatter or the beginning of the next reader data transmission, the TAT time can be accurately measured. Maintaining the shortest TAT under wide-range downlink conditions will help maximize system throughput.

The RTSA can also demodulate the symbols or bits associated with the tag query. The user simply selects the appropriate RFID standard, modulation type, and decoding format. The analyzer automatically detects and displays the link's bit rate. To further increase the engineer's productivity, the recovered data symbols are color-coded based on function. The RTSA automatically identifies the leading symbols and colors those symbols yellow. This makes it easy to identify the actual data payload and compare it to known values.