Using Real-Time Spectrum Analyzers to Test Passive RFID tags

By Elaine May

February 12, 2007

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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).

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.

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