Rfid Using Spectral Analyzer

8/3/2019 Rfid Using Spectral Analyzer

1/4

32 www.rfdesign.com April 2006

Test & Measurement

Evaluating and optimizing RFID and NFCsystems using real-time spectrum analysis

Developing systems based on increasingly complex RFID technology, as well asachieving compliance with RFID regulations, requires advanced instrumentation.One instrument, the real-time spectrum analyzer, is particularly useful.

By Darren McCarthy

The evolution of radio frequency identification (RFID) technologiesis transpiring at a frenzied pace. RFID is also associated with the

term near field communications (NFC) on mobile appliances such ascell phones, new appliances and utilities in a range of industries thatcontinue to emerge. These include commercial applications, such asautomated inventory control and ticketless entry; agricultural appli-

cations, with electronic tracking of livestock and food; and transportapplications, such as automated toll collection, airline baggage tracking,tire pressure monitoring systems (TPMS) and vehicle managementsystems like real-time locations systems (RTLS).

The latest RFID technologies are yielding larger memory capacitiesfor extended article identification, wider frequency reading ranges toadapt to dynamic RF environments and faster processing to improvesystem throughput. Over time, tags are becoming lighter, smaller,cheaper and able to hold more informationin many cases, personalinformation. Instead of a simple one or zero or an ON or OFF signalfor theft-deterrent systems like electronic article surveillance (EAS),tags will send user name, account number and billing information forsecure banking applications.

This ability to transmit personal data wirelessly is bringing to theforefront the most important issues RFID manufacturers will face:information privacy, security and communication assurance. Asthe future unfolds, RFID manufacturers will be tasked with rigidadherence to regulations and standards to meet interoperability,privacy and security mandates. In fact, key technical issues are alreadyimpacting the deployment of RFID systems, namely interoperability,radiated emissions conformance and assurance of operations in acomplex RF environment.

To assure the operation of the installed system, the physics involvedwith setting up and maintaining the tag and reader system in a com-plex RF operating environment must be considered. Considerationof interference signals, tag read times, speed of the tagged articlethrough the reading system, the number of tags read at a time, theRF frequency and propagation characteristics of the material and

environment are all important factors in determining the appropriatetag and reader system.To confront these challenges, RFID engineers must have the ability

to characterize and troubleshoot their increasingly complex, transientRF systems in accordance with RF standards and regulations. Of course,they must also meet the tight time-to-market deadlines required by aprogressively demanding and competitive market. Real-time spectralanalysis can be a valuable aid for efficiently evaluating an RFIDsystems compliance to ISO/IEC standards, as well as governmentregulations regarding intentional transmitters.

While an oscilloscope is a useful bench tool in RFID applications,a real-time spectrum analyzer (RTSA) is optimized for the types ofRF measurements required for conformance testing and optimizationof RFID systems. For example, an RTSA offers the advantage of asignificant increase in dynamic range and record length of tag/readerinteraction when compared to the use of an oscilloscope. This isimportant in RFID testing because the response from the tag is oftenat the same frequency as the reader but at a significantly lower powerlevel (on the order of 50 dB to 80 dB depending on tag type, distanceand RF propagation characteristics).

The RTSA allows a user to see a complete time record of readerand tag interaction using an over-the-air trigger, memory captureand demodulation of analog and digital signals. It is able to capture,measure and display RFID system interactions in multiple domains,including frequency vs. time (spectrogram), time (pulse response)and demodulation. Previously, the ability to view and analyze a signalin these various domains required the use of an oscilloscope, sweptspectrum analyzer and vector signal analyzer.

The RSA3408A from Tektronix can instantaneously capture 36MHz of spectrum. This will enable the entire hopping range of thetag/reader system to be analyzed for proper operation. The RTSA re-cords more than one second of data (increased as analysis bandwidthis reduced) to capture the entire tag/reader interactions to be recordedespecially during a multitag environment where collisions require mul-tiple transmit/receive events. Detailed analysis of timing events witha 20 ns timing resolution enables precise system performancecharacteristics to be validated.

Once captured in memory, the data can be analyzed in multipledomains including frequency, frequency over time for hopping signals,power over time for pulsed signals, modulation and more (Figure 1).These capabilities render the RTSA an optimal instrument for measur-ing and analyzing collision, modulation rate and quality, frequencydeviation, occupied bandwidth, transmission spurious emissions and

field strength of RFID systems.Figure 1. RTSA shows a multidomain view of RFID system interaction.


2/4


Collision

The RTSA offers the ability to store signals to internal memoryfor post-capture analysis. This can be useful for measuring interac-tions between multiple tags to verify collision management. ISO/IEC18000-3 section 6.2.7.9 calls for reading 500 tags within 390 ms. Italso calls for reading 50 words of data within 930 ms from static tagsand 944 ms from active tags. If only using a PC to time stamp theinteractions and test for compliance, there is no way to know why theinteraction takes so long, which particular tag is being problematicor at what point a collision is occurring. However, by using a RTSA

Subcarrier 212 kHz

Data stream baseband coded ASK- Modulation 1

ASK- Modulation 2= Load modulationModulated subcarrier

Carrier signal 13.56 MHz

Load modulated signal with subcarrier

Figure 2. Load modulation diagram.

Figure 3. Demodulation of BPSK subcarrier signal that shows power vs.time (bottom), frequency vs. time (upper left), demodulated symbol table(upper right).

and monitoring the over-the-air interface during polling, it is possibleto troubleshoot when a collision occurs and determine the cause. Thebasis of a collision may be interference, faulty tag or hopping patternerror, to name a few.

The RTSA can be valuable in maximizing system performance ina competitive market. Turn around time (TAT) is often a benchmarkfor system throughput and capacity. Overall capacity can be increasedmaking it appealing for high-volume applications. For these complexenvironments using multiple tags, the RTSA allows a user to set upa record length of seconds, minutes or even hours, depending onthe frequency span. In instances where frequency hopping is used,the RTSA can seamlessly capture and record signals in a 36 MHzbandwidth, allowing the RFID hopping sequence to be viewed and


3/4


analyzed. The RTSA then displays the tag/reader interactions. If desired, no trigger eventneed occur; the RTSA will gather data withoutany time gaps in a free run mode for thelength of the time record set by the user.

Modulation rate and qualityThe RTSA permits measurement and analy-

sis of interrogator and transponder modula-tion rate and quality. The modulation used inRFID (as defined by ISO/IEC 18000-3) is ASK(either 10% or 100%) for the downlink (reader

to tag) and load modulation for the uplinkwith a rate defined as a division of the carrier.As illustrated in Figure 2, the load modulationproduces subcarriers, which use binary phaseshift keying (BPSK) modulation.

Future development of RFID devices willinclude more complex modulation to enablemore information to be transmitted. The RTSAoffers the ability to demodulate complexdigital signals such as BPSK, QPSK, GMSKand QAM. It is possible to simultaneouslyview three domains (constellation, frequency,

time or spectogram), as well as display signalquality measurements, such as EVM,frequency error or symbol data (Figure 3).

Frequency deviation and occupied

bandwidthBending tags or placing them in closeproximity to conductive objects and other tagscan detune the tag antenna, prevent them fromgoing into resonance and render them inop-erative or significantly reduce their operatingrange. RFID systems must also operate withina specified bandwidth to prevent interfer-ence with other signals. For these reasons,frequency deviation and occupied bandwidthmeasurements are critical to ensuring compli-ance with RFID and transmitter standards.

The RTSA can make extremely accuratefrequency and bandwidth measurements witha resolution of 1 Hz. For example, frequencyaccuracy for a 13.56 MHz RFID interrogatoris typically specified at7 kHz. By enablingthe carrier frequency feature found on theRTSA, a user can scroll through the capturedreader/tag interaction and measure to anaccuracy of 4 Hz. Figure 4 shows theoccupied bandwidth capability of the RTSA.

Interrogator transmission spuriousemissions and field strength

RFID readers and tags are intentional trans-mitters that fall under regional regulations,such as FCC 47 Part 15 in the United States,EN 300 330 in Europe and ARIB STDT60/

T-82 in Japan. As RFID heads toward globalacceptance, the most stringent of these regula-tions will apply. Also under close scrutiny isthe effect of human exposure to RFID elec-tromagnetic fields as spelled out in documentssuch as IEEE C.95-1 and EN50364.

Various regulations will define limitsin different units. The power flux densityS [mW/cm2], electric field strength E [V/m]and magnetic field strength H [A/m] are inter-changeable according to the equation below.

S = E2 / (3700) = (37.7) H2

By use of a directional antenna (and, insome cases, a pre-amplifier), effective isotro-

pic radiated power (EIRP) and field-strengthmeasurements can be made with a RTSA.A frequency mask trigger found in newerRTSAs allows a user to create a pass/fail mask by using the mouse to draw the allowableEIRP parameters (Figure 5).

The RTSA suits RFIDThe RFID and NFC industries encompass

a broad array of technologies and applica-tions. From low frequency to high frequency,to ultrahigh frequency and up to microwavefrequencies, the tag and reader systems areselected for environmental considerations,

system capacity and function. These often must


4/4


share the same frequency allocations of other communications systemsand interoperate with other tag/reader systems in the same frequency.

Whether developing a tag and reader system to optimize performancein the lab, performing testing during manufacturing or diagnosing thesystem installation and operating in situ, the RTSA delivers advan-tages to designers working with RFID signals of all kinds. With thefirst comprehensive RFID analysis software package, the real-timespectrum analyzer supports a variety of international RFID and NFCstandards. Only the RTSA offers the over-the-air triggering, memorystorage and analysis features to help designers understand the range ofRFID interrogator and transponder behavior. Perhaps most important,the RTSA is the solution that can and will evolve with emerging trends

in RFID design. RFD

Figure 4. A reader signal being measured for compliance with ISO/IEC

18000-6, which allows 423.75 kHz40 kHz for a single subcarriermodulated signal.

Figure 5. When violated (as shown here), a RTSAs frequency mask triggercan capture and store the spectral data.

ABOUT THE AUTHOR

Darren McCarthy is a market development manager for RF Test atTektronix. He has worked in test and measurement positions for thelast 17 years, including R&D engineer, R&D management, productplanning and business development. Much of his career has beenfocused on the development and applications of signal surveillance,monitoring and analysis for commercial and defense electronicsapplications. He has represented the United States on IEC technicalcommittees for international EMC standards. He holds a BSEEfrom Northwestern University in Evanston, Ill.

Documents

Rfid Using Spectral Analyzer