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Product How To: Integrate and debug RFID

Alan Wolke- October 23, 2012

Once the next big thing in tech, RFID (radio frequency identification) technology has gone through the hype cycle of over-inflated expectations followed by disillusionment. Over the last few years, however, RFID has started making a solid comeback with nearly 3 billion tags shipped in 2011 and analysts calling for steady growth of about 20 percent for the next five years.

With that kind of growth, even if you’ve forgotten about RFID since 2003 when Walmart said it would require suppliers to attach RFID tags to every pallet by 2006, chances are good that you will be asked to integrate and debug RFID systems at some point in the future. With that in mind, this article discusses ways to overcome the test challenges presented by integrating RFID technology into various products

These challenges include not only the need to characterize, debug, and test the RF interface, but also to ensure that the RFID systems interact nicely with the rest of the electronics in the device. Interference and interaction from RF sources in the environment, as well as other electrical signals in the system, need to be characterized and understood in order to successfully integrate RFID technology into a product.  

RFID Basics
RFID systems consist of a reader and a tag. The reader interrogates the tag via an RF signal, and the tag responds with its stored data. Tags can be classified as passive or active. Active tags contain a battery to aid in sending the response to the reader.  Automotive toll transponders are a good example of an active tag system. Passive tags contain no power source. They use the RF energy from the reader to power themselves up and transmit the response. The passive tag’s response is often communicated to the reader by altering the reader’s own RF signal by absorbing, reflecting or creating a backscatter response. The reader must then detect and demodulate these tag-induced changes on its transmitted signal in order to read the tag’s data. The majority of RFID systems in use today employ passive tags. These include inventory control tags and most card-entry systems.

Reader-Tag RF Signals
RFID communications or transactions are bursty or intermittent in nature. Many readers transmit their interrogation signal periodically, or in some cases only when a tag is in close proximity. Passive tags, and most battery-assisted tags, will only transmit their data when interrogated by a reader. Certainly, the passive tags can only transmit when they are interrogated because their power source is the reader’s RF signal.  The tag’s signal is often orders of magnitude weaker than the reader’s signal. Therefore, the test equipment used to capture and analyze the RFID transaction must have good sensitivity, good dynamic range, as well as the ability to detect, trigger and record the transaction over time. These are typical properties of a spectrum analyzer that includes time domain analysis (vector signal analysis) capabilities.

The spectrum analyzer in a mixed domain oscilloscope, such as the Tektronix MDO series, has the ability to trigger on the reader’s RF transmission, and then capture and record the RF activity during the reader-tag interrogation (see Figure). The RF vs. time traces available in an MDO can be used to show the magnitude of the RF envelope vs. time (amplitude vs. time), as well as the frequency or phase vs. time.  These traces can be thought of as basic AM, FM or PM demodulation.  Most RFID systems employ relatively simple AM, ASK, PR-ASK or FSK modulations that can easily be viewed and characterized with the RF vs. time traces in an MDO.



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