As the cost for passive tags drops thanks to advances in submicron Complementary Metal Oxide Semiconductors (CMOS), the use of RFID for inventory applications is becoming nearly universal. Many experts believe the 96-bit Electronic Product Code (EPC), as shown in Figure 1, will be the next generation of the Universal Product Code (UPC), the familiar General Trade Identification Number (GTIN) imprinted in the barcode on a majority of products sold today. The varying applications of EPC RFID tags have moved the industry to classify the basic types of RFID devices, ranging from 1 to 5 according to the tag's read/write capability and passive or active power source.

RFID overview and design challenges
The passive class 1 tag in the 900 MHz and 2.45 GHz frequency ranges is ideal for many high-volume applications. The high frequency allows the interrogator to read the tag with a directional antenna for a greater communication range. Passive tags at higher frequencies also work with smaller, less complicated antennas making them more suitable for consumer applications.

Reading passive tags is somewhat different than the traditional full duplex data link. Unlike traditional active data links, the passive tag relies on the RF energy it receives to power the tag. Passive tags modulate some of the energy being transmitted by the interrogator to the tag in a process known as backscattering, as show in Figure 2. By changing the loading of the antenna from absorptive to reflective, a Continuous Wave (CW) signal from the interrogator can be modulated.

Passive tag readers are typically configured as a homodyne or single frequency conversion receiver as shown in Figure 3. A precision frequency source in the interrogator generates both the transmitter signal and the local oscillator for the reader's receiver. The unique homodyne architecture of the Class 1 RFID system presents some unusual challenges for the engineer. The backscattered modulation is far weaker than the CW signal from the reader's transmitter used to power the tag during backscattering. At baseband in the reader's receiver, the CW leakage translates to a large DC offset that can saturate sensitive amplifiers and digitizers.

Another challenge with the passive tag RFID system is the powering of the tag from received RF energy. Even though submicron CMOS requires very little power to operate, at a range of only a few meters very little power (- 10 to -15 dBm) is available. Complicating matters further, regulatory bodies worldwide have different maximum Effective Isotropic Radiated Power (EIRP) limits.

Since the uplink from the Tag (T) to the Reader (R) (denoted T=>R) is modulated from the interrogator's CW signal, it is possible to use spread spectrum techniques such as frequency hopping. Any spreading on the interrogator's signal will automatically be removed in the homodyne down conversion of the receiver since it shares the same Local Oscillator (LO) signal. After down conversion the interrogator's homodyne receiver has separated In phase (I) and Quadrature phase (Q) signals. The down-converted base-band signal is then digitized with Analog to Digital Converters (ADC) and digitally processed to determine the tag's ID.

Modulation and coding
RFID systems usually use simple-to-produce modulation techniques and coding schemes that lead to some design tradeoffs. A typical example is ISO 18000 Type C (also known as EPC Gen2, Class 1) which calls for Double Side Band-Amplitude Shift Keying (DSB-ASK), Single Side Band-ASK (SSB-ASK) and Phase Reversal-ASK (PR-ASK). ASK and PR-ASK Modulation are illustrated in Figure 4.

Amplitude shift keyed digital modulations are spectrally inefficient, requiring substantial RF bandwidth for a given data rate. Bandwidth efficiencies of 0.20 bits per Hertz of RF bandwidth are not uncommon for DSB-ASK. One approach to improving bandwidth efficiency is to use SSB-ASK. This is particularly important in European countries where bandwidth restrictions may preclude DSB-ASK.

The power efficiency of both DSB-ASK and SSB-ASK is dependent on the modulation index. With a modulation index of one or On and Off Keying (OOK) of the carrier, the lowest Carrier to Noise (C/N) required to achieve a given Bit Error Rate (BER) is obtained for DSB-ASK and SSB-ASK. Unfortunately, this also provides the least amount of RF power transport on the downlink to supply the tag with energy. Ideally, the off time of the carrier should be minimized so that the tag doesn't run out of power. The carrier to noise requirements should also be minimized to maximize ID read range. For many modulations these are conflicting goals.

PR-ASK is a modulation that can minimize the carrier to noise requirement in a narrowband while maximizing the power transport to the tag. This modulation has carrier to noise and bandwidth requirements more closely matching PSK than DSB-ASK, making it attractive for narrowband and longer-range applications. DSB-ASK is the least bandwidth efficient modulation, but the easiest to produce by On and Off Keying (OOK) of the carrier signal.

Data encoding considerations
Before modulation, the data must be encoded into a serial information stream. There are many types of bit encoding schemes available as shown in Figure 5, each with different strengths. Data encoding is critical for RFID applications due to such factors as the lack of precision timing sources on board the passive tag, challenging bandwidth requirements and the need for maximum RF power transport to energize the tag.

Manchester-L (Bi-Phase-L) and Pulse Interval Encoding (PIE) are popular for interrogator to tag (R=>T) communications. These coding schemes are based on transitions and are self-clocking, greatly reducing the complexity of the synchronization circuitry required in the power-starved tag.

PIE encoding is based on a given minimum pulse duration or interval such as 20 s. This period, called a Tari, is named after the ISO 18000-6 Type A Reference Interval. One and zero bits as well as special symbols like Start Of Frame (SOF) and End Of Frame (EOF) are composed of differing numbers of Tari periods. This makes the transmission length for a given number of bits variable, but since PIE encoding is self-clocking the variable length has little effect.

The Tari length is also the minimum pulse width for the modulated signal, an important factor in determining the bandwidth of the transmitted signal. The shorter the Tari length, the greater the bandwidth requirement for the signal. More recent standards such as the ISO 18000-6, Type C allow for several Tari lengths (6.25, 12 and 25 s) to accommodate differing worldwide regulatory spectral emission requirements.

Another important property for RFID Pulse Code Modulation (PCM) coding schemes is the DC spectral component. Backscattering tags modulate a carrier signal. The carrier signal is then filtered out as a baseband DC level back in the tag reader, leaving only the much weaker uplink modulation from the tag. Coding schemes in the tag require the uplink to the reader to have little or no DC energy to conflict with the carrier signal.

Miller and FM0 encoding share this property of little or no DC energy in their spectrums. ISO 18000-6 Type C further enhances the Miller encoding by offering different sub-carrier rates. One, two, four and eight times the sub-carrier frequency enable adjustment of the modulation encoding to optimize read range, speed or bandwidth.

Amplitude-based modulations used in many RFID systems are susceptible to rapid signal fading conditions. Pallets of tags traveling on forklifts past readers located between metal trucks and warehouses can undergo devastating multi-path conditions. Rapid Rayleigh fading or shadowing can be indistinguishable from amplitude modulation, leading to bit errors.

Another RFID consideration is some form of anti-collision protocol to enable reading all the tags in the interrogator's field of view. There are two basic types of anti-collision protocols, deterministic and probabilistic. Popular RFID protocols are the deterministic binary tree and the probabilistic ALOHA and slotted ALOHA approaches.

The binary tree method searches for tag IDs that fit a specific binary number while the probabilistic ALOHA protocol allows the tag to send its message and if the message doesn't get through, it simply tries again later until it does. The slotted ALOHA approach uses synchronization between all the tags, so communications packets are not interrupted mid-stream in the transmission. Additional efficiency gains are possible by using Listen Before Talk (LBT) schemes.

RFID testing overview
RFID systems, particularly those with backscattering passive tags, present a number of challenges for test and diagnostics. Timing measurements are of particular concern, as system readers can be required to read the ID data from many tags very quickly without error.

Most RFID systems use transient Time Division Duplexing (TDD) schemes, where the interrogator and tags take turns communicating on the same channel. To read many ID tags within a very short period of time with a serial TDD multiplexing scheme, standards call for precise timing on the data interchange, thus creating one of the more important RFID test challenges.

Transient RFID signals often contain spectrally inefficient modulations using special PCM symbol encoding and decoding. Troubleshooting the homodyne interrogators or tags that receive these unusual signals requires special signal analyzer capabilities.

Traditionally, swept tuned spectrum analyzers, vector signal analyzers and oscilloscopes have been used for wireless data link development. The limitations of these instruments make their application to modern RFID product development and production inefficient.

The spectrum analyzer has historically been used to characterize the RF spectral output of a transmitter to ensure compliance with regulatory emission restrictions. The traditional swept tuned spectrum analyzer was developed primarily for the analysis of continuous signals not the intermittent RF transients associated with RFID.

Similarly, vector signal analyzers struggle to capture transient RF signals. Though most vector signal analyzers have extensive demodulation ability for popular spectrally efficient modulations, current offerings are limited in their support of spectrally inefficient RFID modulations and their special PCM decoding requirements.

The oscilloscope has long been a valuable tool for analyzing base-band signals. In recent years some oscilloscopes have become available with faster sampling speed to handle high microwave frequencies. They are, however, not well suited for UHF or higher frequency measurements. Relative to the modern real-time spectrum analyzer (RSA) the fast oscilloscope has less measurement sensitivity, memory depth, and it lacks modulation and decoding capability.

The RSA solves the limitations of the traditional measurement tools to provide a more efficient test and diagnostic experience for the RFID engineer. The modern RSA has the digital processing speed necessary to transform the input signal from time domain samples into the frequency domain with a real-time Fast Fourier Transform (FFT) prior to capturing a recording of data. This enables the RSA to compare spectral amplitudes to a frequency mask set by the user in real-time. The RSA can then trigger a capture on a spectral event of interest for subsequent detailed off-line analysis.

This is an important capability for RFID applications as it allows the engineer to begin a capture of the entire transient interrogator and tag interaction starting with the initial spectral burst. As shown in Figure 7, features such as frequency mask triggers enable reliable capture of interrogator and tag interactions in complex real-world spectral environments where other signals might actually be larger in amplitude.

Testing to government regulations
Government regulations require that transmitted signals be controlled in power, frequency and bandwidth. These regulations prevent harmful interference and ensure 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. In comparison, an RSA can optimize transient signals to simplify measurement of the power in a pulsed RFID packet transmission. The lets the engineer examine a complete spectral frame for any given period of time during the packet transmission. In turn, this eliminates the need to synchronize tuning sweeps with packet bursts as required with older swept tuned spectrum analyzers.

Another important spectral emission measurement is the carrier frequency of the signal. There are two ways this measurement can be expressed: actual absolute carrier frequency or carrier frequency error from a given assigned channel frequency. Using an RSA, the display carrier frequency error is displayed when the instrument is demodulating a signal, and the absolute carrier frequency can be displayed in spectrum analysis mode. One notable advantage of the demodulated carrier frequency measurement is it doesn't require the signal to be positioned at the center of the span. This can be very useful for measuring the frequency error of frequency hopping signals.

Similarly, the Occupied Bandwidth (OBW) measurement or the Emission Bandwidth (EBW) can be obtained in two ways. In the demodulation mode the RSA displays the OBW and EBW as well as the carrier frequency and transmission power levels. The bandwidth measurements are also available in real-time spectrum analyzer mode.

Meeting industry standards
Reliable interrogator and tag interaction requires conformance to industry standards such as the ISO 18000-6 Type C specifications. This adds many tests beyond those for government spectral emissions.

The combination of the RSA and RFID software makes the task of ensuring interoperability much easier. A complete RFID software package contains measurements needed for a broad range of standards such as ISO 18000-4 Mode 1 and ISO 18000-6 Type A, B and C. Preprogrammed measurements are helpful to eliminate most of the setup time required to check out these signal formats.

An example, as shown in Figure 8, is power on and power down time as specified by a number of standards. The carrier energy rise time must be turned on promptly to ensure the tag collects enough energy to function properly. The signal must also settle out to a stable level. At the end of the transmission the fall time of the signal burst must be quick enough to avoid disrupting other transmissions.

Communications between interrogator and tag are accomplished with ASK signal bursts during the power on period. These signal bursts make up the RF envelope and are important for interoperability. The modulation pulse envelope contains characteristics necessary to assure compatibility between reader and tag. Here's where RFID software can be employed to measure RF envelope specifications like on width, off width, duty cycle, on ripple, off ripple and the slopes of the RF envelope edges.

Testing proprietary communication schemes
Many RFID and NFC devices use proprietary communications schemes that are optimized for specific market applications. These require manually configured measurements including modulation type, decoding format and data rate. The frequency can be set to test systems including the Low Frequency (LF) band (125 kHz to 135 kHz), High Frequency (HF) band (13.56 MHz), Ultra High Frequency (UHF) band (868 MHz to 928 MHz) and in some cases S-band microwave (2.45 GHz).

For example, a user can manually set an RSA to test compliance of the NFC devices that adhere to ISO 18092, as well as testing interoperability with devices conforming to ISO 14443, Type A and Type B. In such a scenario, the engineer would set the RSA's frequency to 13.56 MHz, modulation type to ASK or BPSK (Type B card/target) decoding format to Modified Miller, Manchester or NRZ and the data rate to 106, 212 or 424 Kb/s.

Gaining a competitive edge
Once the basic specifications are met, it is important to optimize some of the RFID product's features to gain a competitive advantage in a particular market segment. An RSA can be used to maximize system performance while at the same time minimizing the engineering commitment necessary to achieve the desired goal.

One such example is optimizing the number of tag reads possible in a given amount of time. An important element in maximizing capacity is minimizing the Turn Around Time (TAT) for each tag reply. Available RF power, path fading and altered symbol rates can lengthen the time it takes for the tag to reply to the interrogator's query. The slower the reply, the longer it will take to read many tags.

The ability to quickly measure the turnaround time for a half duplex system is essential to optimizing performance. Using an RSA, the entire query between the interrogator and a tag is first captured into the instrument. In the demodulation mode with symbol table chosen, under the view define window, the user sets the RSA to a power versus time display in the sub window. Next, the sub window is zoomed into the portion of the waveform where the tag is backscattering.

Convention dictates that the period between the end of one downlink transmission (R=>T) to the beginning of the next downlink transmission is the turnaround time or TAT for a half duplex system. Thus by placing a marker at the end of the tag interrogation and a second delta marker at the end of the backscattering or beginning of the next interrogator data transmission, a precise measurement of turnaround time can be made. An example is shown in Figure 9. Maintaining the shortest TAT for the widest range of downlink conditions helps maximize the system's throughput.

The RSA can also demodulate the symbols or bits associated with a 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.

Optimizing communications often requires extensive diagnostics. Advanced frequency mask triggering capability to reliably capture important spectrums, comprehensive ASK demodulation and specialized RFID symbol decoding help engineers more efficiently troubleshoot complex RFID systems. Traditional signal analyzers fail to provide this level of insight, which in turn reduces productivity.

The real-time spectrum analyzer's time-correlated multi-domain displays mean that multiple displays can be viewed at once with time correlation between markers in each display. Time-correlated multi-domain displays are particularly useful for troubleshooting and diagnostic work.

A marker placed on an anomaly in a spectrogram will correlate to a marker on the exact symbol that corresponds to the event. Time-correlated displays take the guesswork out of diagnostic analysis and greatly improve the reliability of problem insight. The engineer doesn't have to assume a power versus time glitch is causing a data error because the time-correlated markers verify the two events occurred simultaneously.

Conclusion
The RFID industry encompasses a broad array of technologies and applications, many of which differ from the typical communications link. These include sophisticated FHSS signals with transient half duplex RF bursts composed of ASK modulations with unusual encoding and robust anticollision protocols.

An RSA together with RFID software helps the engineer mitigate the need for elaborate or time consuming test setups as required for traditional swept spectrum analyzers or vector signal analyzers. Further, the RSA provides detailed diagnostic insight for reliable troubleshooting assessments can help ensure that designs conform to government and industry standards.

About the Author
Darren McCarthy is Technical Marketing Manager for RF Test at Tektronix. He has worked extensively in various test and measurement positions for over 20 years including R&D engineer, R&D management, product planning and business development. During his career, he has also represented the U.S. on several IEC technical committees for international EMC standards. He holds a BSEE from Northwestern University in Evanston, Illinois.