Installation guide

89
Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
RFID Tag Considerations
The passive RFID tag is available commercially packaged wide variety of designs, from mounting on a
simple substrate to creating a classic “hard” tag sandwiched between adhesive and paper (commonly
referred to as an RFID “smart” label). The form factor used depends primarily on the application
intended for the passive RFID tag and can represent the bulk of the passive RFID tag cost. Passive RFID
tags typically operate at low frequencies (125–135 kHz), high frequencies (13.56 mHz), and ultra high
frequencies (868, 915 mHz).
Semi-Passive RFID Tags
Semi-passive RFID tags overcome two key disadvantages of pure passive RFID tag design:
• The lack of a continuous source of power for onboard telemetry and sensor asset monitoring circuits
• Short range
Semi-passive tags differ from passive tags in that they use an onboard battery to provide power to
communication and ancillary support circuits such as temperature and shock monitoring. It is interesting
to note that although they employ an onboard power source, semi-passive RFID tags do not use it to
directly generate RF electromagnetic energy. Rather, these tags typically make use of backscatter
modulation and reflect electromagnetic energy from the RFID reader to generate a tag response similar
to that of standard passive tags (see Figure 60). The onboard battery is used only to provide power for
the backscatter enabling circuits on the tag and not to generate the RF energy directly.
Figure 60 Backscatter Modulation in Semi-Passive RFID Tags
Semi-passive RFID tags operating in the ISM band (shown in Figure 61) can have a range of up to
30 meters with onboard lithium cell batteries lasting several years. Range is vastly improved over
conventional passive RFID tags primarily because of the use of a backscatter-optimized antenna in the
semi-passive design. Unlike a conventional backscatter-modulated passive RFID tag, the antenna
contained in a semi-passive tag is dedicated to backscatter modulation and is not relied on to provide
power for tag operation. Therefore, this antenna can be optimized to make most efficient use of the
backscatter technique and provide far better performance.
190593
Tag
Reader
Reader detects changes
in reflected power
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