How to use spectrum analyzer for RFID and NFC

RFID systems, especially those with backscatter passive tags, present unique challenges for testing and diagnostics. Timing measurement is a particular concern because it may require the system reader to read ID data from multiple tags very quickly and without error.

Most RFID systems use a transient time division duplex (TDD) scheme, where the interrogator and the tag communicate sequentially on the same channel. To read multiple ID tags in a very short period of time using a serial TDD multiplexing scheme, the standard requires very precise timing. Therefore, timing measurements on data exchanges present unique RFID challenges. Transient RFID signals typically contain spectrally inefficient modulation, which is encoded and decoded using specialized PCM symbols. Debugging homodyne interrogators or tags that receive these anomalous signals requires special signal analyzer capabilities. Traditionally, swept spectrum analyzers, vector signal analyzers, and oscilloscopes have been used for wireless data link development. These limitations make their application in modern RFID product development and production inefficient.

Historically, spectrum analyzers have been the tool of choice for characterizing a transmitter’s RF spectrum output to ensure that radiated emissions regulatory limits are met. Traditional swept spectrum analyzers were primarily developed for analyzing continuous signals, not the intermittent RF transients associated with modern RFID products. This leads to a variety of measurement problems, especially accurately capturing and characterizing transient RF signals. Similarly, vector signal analyzers have very little capability for capturing transient RF signals, having been originally developed for CW signals. While most vector signal analyzers offer comprehensive demodulation capabilities for popular spectrally inefficient modulations, current products offer little support for spectrally inefficient RFID modulations and the specialized PCM decoding requirements. This makes the current generation of vector signal analyzers of little value to RFID engineers.

Oscilloscopes have long been an important tool for analyzing baseband signals. In recent years, some oscilloscopes have extended sampling speeds to very high microwave frequencies. However, they are still suboptimal tools for measurements at UHF or higher frequencies on RFID systems. Fast oscilloscopes have significantly lower measurement dynamic ranges than modern real-time spectrum analyzers and lack modulation and decoding capabilities.

Real-time spectrum analyzers address the limitations of traditional measurement tools, providing RFID engineers with a much more efficient test and diagnostic experience. Pulsed terminal reading and writing require the use of an RF analyzer optimized for transient signals. Due to the unique real-time architecture and time-correlated display capabilities, the Tektronix real-time spectrum analyzer product line is particularly suitable for characterizing transient signals. The RTSA has the necessary digital processing speed to seamlessly transform the input signal from the time domain to the frequency domain using a real-time fast Fourier transform (FFT) and then capture the data record. This enables the RTSA to compare the spectrum amplitude with the frequency mask set by the user in real time. The RTSA can then trigger to capture the spectrum event of interest and then perform detailed offline analysis.

This is an important feature for RFID applications because it allows engineers to begin capturing the entire transient interrogator and tag interaction starting with the initial spectral burst. In addition, Tektronix patented Frequency Mask Trigger (FMT) technology can reliably capture complex interrogator and tag interactions in real-world spectrum environments where the actual amplitude of other signals may be much larger.

The RSA3408A has ample memory and high-precision triggering capabilities to capture the entire exchange between the reader and the terminal. The complete data record can then be used to quickly diagnose system interactions. The real-time spectrum analyzer fully uses time-correlated multi-domain analysis technology, and users can use high-precision time-correlated rulers between different screens to display multiple measurement domains. For example, a ruler can be placed on the wrong sign bit, and the instrument will associate this ruler and instantly correspond to the power change over time screen or spectrum diagram.

Time-correlated multiple analysis domains greatly enhance diagnostic capabilities and reliability, and can effectively determine the abnormal waveform causing the event in different frames.

In addition to being able to effectively trigger and capture transient RFID signals, the RTSA also provides the test and measurement industry's first RFID analysis software package, which allows the RSA3408A to demodulate, decode, and measure the special signals used in many RFID applications.

Compared to traditional test equipment used in RFID applications, the modern RSA3408A can provide a much faster and more efficient diagnostic and characterization experience. To demonstrate the RTSA's complementary tools, let's look at some common RFID measurements...

RFID Measurement

RFID engineers face a variety of design challenges when bringing products to market. First, the product must meet local frequency regulations related to radiating energy into the spectrum. Second, the interrogator and terminal interaction must work together reliably. To do this, the interrogator and terminal must comply with the appropriate industry standards. Finally, to remain competitive, the performance of the RFID system must be optimized to appeal to a certain market segment. This may mean maximizing the number of transactions per second, operating in a dense reader environment, or expanding the reader's ability to communicate over longer distances.

To illustrate how the RTSA and RFID analysis software are integral components of RFID testing, we first describe the key measurements that must be made when characterizing spectral emissions to comply with government regulations.

Comply with government regulations

Government regulations require that transmitted signals must be controlled in power, frequency, and bandwidth. These regulations prevent harmful interference and ensure that each transmitter is a good neighbor to other users of the frequency band. The RSA3408A with RFID software can easily measure the spectrum parameters required by government regulations.

Power measurements on pulsed signals can be challenging for many spectrum analyzers. The RTSA's transient signal optimization feature makes it easy to measure power in pulsed RFID packet transmissions. FFT analysis provides a complete spectrum frame for any given time period during the packet transmission, without the need to synchronize the tuning sweep with the packet burst as with older swept spectrum analyzers. In addition, traditional spectrum analyzers require correction factors to compensate for the successive logarithmic video amplifier (SLVA) peak detection circuit, while the RTSA uses true RMS detection to accurately read power for most regulatory measurements.

Another important spectrum emission measurement is the signal's carrier frequency. This measurement can be expressed in two ways: the actual absolute carrier frequency or the carrier frequency error about a certain assigned channel frequency. When a signal is demodulated, the RTSA will display the carrier frequency error. In spectrum analysis mode, the absolute carrier frequency can be displayed by selecting the measurement button and then using the Carrier Frequency softkey. A significant advantage of the demodulated carrier frequency measurement is that it does not require the signal to be in the center of the span. This is particularly useful for frequency hopping signals.

Similarly, occupied bandwidth (OBW) or radiated bandwidth (EBW) can be obtained in two ways. In demodulation mode, the RTSA displays OBW and EBW along with carrier frequency and transmit power level. In real-time spectrum analyzer mode, bandwidth measurements are also available under the measurement key.

By using these pre-programmed automatic measurements, essential regulatory data can be obtained quickly and accurately. This eliminates the hassle of connecting coaxial cables to traditional spectrum analyzers to measure transient RFID signals. The RTSA identifies the modulation and provides answers at the touch of a button.

Meeting industry standards

Reliable interrogator and tag interaction requires compliance with various industry standards, such as the ISO 18000-6 Class C specification. This adds many tests on top of the basic testing to meet government spectrum emission requirements. RF conformance testing is critical for reliable interoperation between tags and readers.

The RSA3408A's RFID software includes key measurements required for ISO 18000-4 Mode 1 and ISO18000-6 Type A, B, and C. Pre-programmed measurements on the RTSA eliminate much of the setup time required to examine these signal formats.

For example, an important measurement in ISO 18000-6 Class C is the turn-on time and turn-off time. The rise time of the carrier energy must turn on immediately to ensure that the terminal collects enough energy to operate correctly. The signal must also settle to a stable level. At the end of the transmission, the fall time of the signal burst must be fast enough to avoid interfering with other transmissions.

In demodulation mode, select the corresponding RFID standard and type, press the analyzer's soft key, and select Power On/Down. Then RTSA will automatically measure the power-on rise time, power-off fall time, power stabilization time, overshoot and undershoot. To view more detailed information, RTSA also displays waveform characteristics in the measurement window.

Communication between the interrogator and the tag is accomplished through bursts of ASK signals during the power-on cycle. These signal bursts constitute the RF envelope, which is very important for interoperability. The modulated pulse envelope contains the characteristics that are essential to ensure compatibility between the reader and the tag. RTSA's RFID software automatically measures RF envelope indicators such as power-on pulse width, power-off pulse width, duty cycle, power-on ripple, power-off ripple, and RF envelope edge slope.