Using Real Time Spectrum Analyzers to test passive RFID tags

Elaine May, Tektronix Inc.

February 7, 2007

Elaine May, Tektronix Inc.

Radio Frequency Identification (RFID) applications are rapidly growing as both reader and tag prices drop, and global markets expand. As costs for submicron passive CMOS tags drop, inventory and other applications increase rapidly. Some have predicted that as the price for passive tag continues to drop, every product sold will have an RFID tag in it.

When a passive tag receives a continuous wave (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 back-scattering (Figure 1, below).

Figure 1: A passive tag uses the energy from the reader to run itself.

RFID systems usually use simple modulation techniques and coding schemes. However, simple modulation schemes may be spectrally inef- ficient, requiring substantial RF bandwidth for a given data rate. Note that before modulation, the data must be encoded into a serial information stream.

Different kinds of bit encoding schemes are available, each with unique advantages in their baseband spectral properties, complexity to encode and decode, and difficulty to clock into memory.

Passive RFID 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 energize the tag. Finally, an anticollision protocol is required to enable reading of all tags in the reader's field of view.

Every RFID communication system must pass regulatory requirements and must conform to standards. Today, however, system optimization separates the winners from the losers in this fast growing industry. Designers of an RFID communication system are faced with challenges for regulatory testing, standards conformance and optimization.

RFID measurement challenges
RFID technologies present several uncommon engineering measurement challenges such as transient signals, bandwidth inefficient modulations and backscattered data.

Traditionally, swept-tuned spectrum analyzers, vector signal analyzers and oscilloscopes were used for wireless data-link development. However, each tool presents disadvantages when used for RFID testing. Swept-tuned spectrum analyzers cannot accurately capture and characterize transient RF signals.

Vector signal analyzers have virtually no support for spectrally inefficient RFID modulations and their special decoding requirements. Fast oscilloscopes have limited measurement dynamic range, and lack modulation and decoding capability. Real-Time Spectrum Analyzers (RTSAs) solve these limitations of traditional tools due to their optimization for transient signals and their ability to trigger specific spectral events in complex real-world spectral environments.

Producers of electronic equipment must meet regulatory standards where the equipment will be sold or used. Regulatory laws across countries are evolving to cope with the unique data-link characteristics of a passive RFID tag. Most regulations prohibit CW transmissions from devices unless done for a short-term test. Passive tags require the reader to send a CW signal to power the tag and modulate via backscattering.

Even without a transmitter, passive tags can still produce a modulated signal. However, many regulations do not address non-transmitter- based modulation. A slew 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 neighbor to other users of the band. Power measurements of pulsed signals can be challenging for many spectrum analyzers, especially swept spectrum analyzers used for these measurements.

An RTSA can analyze 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 place the signal at the center of the span.

An analyzer that can recognize the modulation of a transient RFID signal and make regulatory measurements of power, frequency and bandwidth at the touch of a button makes the process of pre-compliance testing swift and easy (Figure 2, below).

Pre-compliance testing ensures first-pass success during compliance test, eliminating the need for redesign and retest.

Figure 2: Pre-compliance testing ensures first-pass success during compliance test.

Standards conformance
Reliable reader and tag interaction requires conformance to industry standards such as the ISO 18000-6 Type C specifications. This requirement adds many tests beyond those needed to meet government spectral emissions requirements. RF conformance tests are critical to ensure reliable interoperability among tags and readers.

Pre-programmed measurements can reduce the setup time required to make these tests. One important measurement for ISO18000-6 Type C is the power on and power-down time.

The carrier energy rise time must be fast enough to ensure that the tag collects enough energy to function properly. The signal must also settle out quickly to a stable level. At the end of the transmission (Figure 3, below), the fall time must be quick so that other transmissions are not disrupted.

Figure 3: At the end of the transmission, fall time must be quick so it does not disrupt other transmissions.

Some RFID devices use proprietary communications schemes optimized for specific applications. In this case, engineers need an analyzer that provides multiple modulation and coding schemes that can be programmatically adjusted for the format in use. Once the basic specifications are met, it is important to optimize the RFID product's features to gain competitive advantage in a particular market segment.

Performance considerations include speed of tag reading, ability of a tag to operate in a reader rich environment, and distance between the tag and 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 5 seconds to less than 0.5 seconds.

Speed, throughput
In industrial applications, speed translates into throughput - the higher the throughput, the more efficient the usage of capital and human resources. Since passive tags get the energy they need to operate from the RFID reader, multiple readers causes a tag to attempt to respond to every reader interrogating it. Improving throughput in a multiple reader installation requires the use of an anti-collision protocol.

Finally, to maximize tag read range, the carrier-to-noise requirements should be minimized, but this may conflict with the need to keep the tag from running out of power by minimizing the off time of the carrier. Each of these optimization considerations places challenges on the engineer and on the measurement equipment.

Let's take one example - optimizing communication speed, also called turnaround time (TAT). Available RF power, path fading and altered symbol rates can prolong 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 TAT is key to optimizing the speed of an RFID system (Figure 4, below).

Figure 4: A real-time spectrum analyzer easily measures turnaround time.

TAT is easily measured with an RTSA . First, a frequency mask trigger is set up to capture the entire query between tag and reader. The power vs. time view of the RTSA allows the user to view the entire transmission.

Convention dictates that the period between the end of one downlink transmission (R > T) and the beginning of the next downlink transmission is 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 backscattering or beginning of the next reader data transmission, a precise measurement of TAT can be made. Maintaining the shortest TAT for the widest range of downlink conditions helps maximize the system's throughput.

An RTSA can also demodulate the symbols or bits associated with the tag query. The user merely selects the appropriate RFID standard, modulation type and decoding format. The analyzer can automatically detect and display the link's bit rate.

To further enhance the engineer's productivity, the recovered data symbols are color-coded based on function. The RTSA automatically recognizes the preamble and colors those symbols yellow. This makes the actual data payload easily recognizable for comparison to known values.

Elaine May is director of marketing for the Real-Time Spectrum Analyzer Product Line at Tektronix Inc.