Analysis of RFID Testing Technology

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 the small amount of energy needed to keep the tag operational, then modulates the signal by changing the absorption characteristics of its antenna and reflects it back to the reader via backscattering [see Figure 1]. RFID systems typically use simple modulation techniques and coding schemes. However, simple modulation techniques are spectrally inefficient 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 coding schemes available, each with its own unique advantages in baseband spectral performance, encoding and decoding complexity, and difficulty in writing data to memory under clocking. Passive tags have unique requirements for the coding scheme used due to the difficulty of achieving practical 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 area.

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 spectrally inefficient RFID modulation techniques 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.

Figure 1: Passive tags rectify and modulate RF energy, then backscatter it to a reader.

Regulatory testing

Every electronic device manufacturer must comply with regulatory standards wherever the device is sold or used. Many countries are in the process of 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 power the tag and modulate it via backscatter. Even though a passive tag does not have a typical transmitter, it can still emit a modulated signal. However, many regulations do not address modulation based on the absence of a transmitter. Various spectrum emission tests are not explicitly included in the RFID standards for readers, but are included in regulations. [page]

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 the small amount of energy needed to keep the tag operational, then modulates the signal by changing the absorption characteristics of its antenna and reflects it back to the reader via backscattering [see Figure 1]. RFID systems typically use simple modulation techniques and coding schemes. However, simple modulation techniques are spectrally inefficient 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 coding schemes available, each with its own unique advantages in baseband spectral performance, encoding and decoding complexity, and difficulty in writing data to memory under clocking. Passive tags have unique requirements for the coding scheme used due to the difficulty of achieving practical 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 area.

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 spectrally inefficient RFID modulation techniques 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.

Figure 1: Passive tags rectify and modulate RF energy, then backscatter it to a reader.

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. Multiple spectrum emission tests are not explicitly included in the RFID standards for readers, but are included in regulations.

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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.

Figure 4: TAT can be easily measured using an RTSA.

TAT can be easily measured using an RTSA [see Figure 4]. 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. The TAT for a half-duplex system is considered to be the time between the end of one downlink transmission (from reader to tag) and the beginning of the next downlink transmission. By placing one delta 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 can be accurately measured. Maintaining the shortest TAT over a wide range of 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.

Conclusion

The RFID industry encompasses a vast array of technologies and applications, many of which differ from typical communication links. Engineers need tools that enable them to quickly and easily perform regulatory testing, standards compliance, and optimization measurements.

The RSA3408A is a tool that meets these needs, supporting multiple RFID international standards, time-correlated multi-domain measurements, customized RFID communication systems, demodulation of multiple RFID signals, and symbol decoding. The instrument greatly improves engineering efficiency while shortening the time to market. Whether it is to meet government spectrum regulations, ensure that tags or readers comply with specific communication standards, or debug a problem encountered during development, the RTSA is a unique tool for analyzing RFID signals emitted by readers and tags.