Articles
Passive RFID — A Primer For New RF Regulations
May 13, 2005
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Passive RFID refers to an RFID tag. Tags that are passive in nature obtain operational power from an active RF field emitted by a corresponding RFID interrogator (reader). This is most unlike typical devices that only communicate via RF and are actively powered by any number of methods including batteries, utility power, solar, etc. Whilea good amount of RF energy could be in an RF field – the realistic applications involve human interaction, and for safety reasons, limit the amount of energy a reader can emit. Although these limits vary worldwide and at different frequencies, current passive RFID systems are functional anywhere from about 1-10 meters. Tag operational power requirements can go as low as 200 microwatts absorbed from the RF field.
Since there is not enough field power to operate a traditional RF transmission, passive RFID tags communicate back to the reader with a technique called ‘backscatter'. In effect, what the tag does is alter the impedance of the antenna match so that more or less energy is reflected. The reader's job then is to detect this change in reflected signal. Several schemes are used to encode digital information from the tag to the reader (baseband signaling is the most common), but all are based upon the backscatter technique.
