Using Real Time Spectrum Analyzers to test passive RFID tags

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

When a passive tag receives a continuous wave (CW) signal froma reader, it rectifies the RF energy to create a small amount of powerto run the tag. It then changes the absorption characteristics of itsantenna to modulate the signal and reflect it back to the reader viaback-scattering (Figure 1, below ).

Figure1: A passive tag uses the energy from the reader to run itself.

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

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

Passive RFID tags place unique requirements on the coding schemesused due to the impracticality of precision timing sources on board thetag, challenging bandwidth requirements, and the need for maximum RFpower transport to energize the tag. Finally, an anticollision protocolis required to enable reading of all tags in the reader's field ofview.

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

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

Traditionally, swept-tuned spectrum analyzers, vector signal analyzers and oscilloscopes were used forwireless data-link development. However, each tool presentsdisadvantages when used for RFID testing. Swept-tuned spectrumanalyzers cannot accurately capture and characterize transient RFsignals.

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

Producers of electronic equipment must meet regulatory standardswhere the equipment will be sold or used. Regulatory laws acrosscountries are evolving to cope with the unique data-linkcharacteristics of a passive RFID tag. Most regulations prohibit CWtransmissions from devices unless done for a short-term test. Passivetags require the reader to send a CW signal to power the tag andmodulate via backscattering.

Even without a transmitter, passive tags can still produce amodulated signal. However, many regulations do not addressnon-transmitter- based modulation. A slew of spectral emission tests,which may not be explicitly contained in the RFID standard for thereader, become requirements.

Government regulations require that transmitted signals becontrolled in power, frequency and bandwidth. These regulations preventharmful interference and ensure that each transmitter is a spectrallygood neighbor to other users of the band. Power measurements of pulsedsignals can be challenging for many spectrum analyzers, especiallyswept spectrum analyzers used for these measurements.

An RTSA can analyze the power characteristics of a complete packettransmission. It can also make direct measurements of the carrierfrequency of a frequency hopping signal, eliminating the need to placethe signal at the center of the span.

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

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

Figure2: Pre-compliance testing ensures first-pass success during compliancetest.

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

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

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

Figure3: At the end of the transmission, fall time must be quick so it doesnot disrupt other transmissions.

Some RFID devices use proprietary communications schemes optimizedfor specific applications. In this case, engineers need an analyzerthat provides multiple modulation and coding schemes that can beprogrammatically adjusted for the format in use. Once the basicspecifications are met, it is important to optimize the RFID product'sfeatures to gain competitive advantage in a particular market segment.

Performance considerations include speed of tag reading, ability ofa tag to operate in a reader rich environment, and distance between thetag and reader. In consumer applications, the speed of the tag-readercommunication directly translates into customer acceptance. Forinstance, RFID enabled bus passes did not gain wide acceptance untilthe read time dropped from 5 seconds to less than 0.5 seconds.

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

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

Let's take one example – optimizing communication speed, also calledturnaround time (TAT). Available RF power, path fading and alteredsymbol rates can prolong the time it takes for the tag to reply to thereader's query. The slower the reply, the longer it will take to readmany tags. Quick measurement of TAT is key to optimizing the speed ofan RFID system (Figure 4, below ).

Figure4: A real-time spectrum analyzer easily measures turnaround time.

TAT is easily measured with an RTSA . First, a frequency masktrigger 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 entiretransmission.

Convention dictates that the period between the end of one downlinktransmission (R > T) and the beginning of the next downlinktransmission is the TAT for a half-duplex system.

By placing one marker at the end of the tag interrogation and asecond delta marker at the end of the backscattering or beginning ofthe next reader data transmission, a precise measurement of TAT can bemade. Maintaining the shortest TAT for the widest range of downlinkconditions helps maximize the system's throughput.

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

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

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

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