Installation guide

87
Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
RFID Tag Considerations
Figure 56 Passive RFID Interrogators
Passive RFID tags (shown in Figure 57) consist of a coil and a microcircuit that includes basic
modulation circuitry, an antenna, and non-volatile memory.
Figure 57 Passive RFID Tags
Passive RFID tags can vary in how they communicate data to RFID readers and how they receive power
from the RFID readers inductive or electromagnetic field. This is currently performed via two basic
methods:
• Load modulation and inductive coupling—In this approach (shown in Figure 58), the RFID reader
provides a short-range alternating current magnetic field that the passive RFID tag uses for both
power and as a communication medium. Via inductive coupling, this field induces a voltage in the
antenna coil of the RFID tag, which powers the tag. The tag transmits its information to the RFID
reader by taking advantage of the fact that each time the tag antenna draws energy from the RFID
readers magnetic field, the RFID reader can detect a voltage drop in its antenna. The tag can
communicate binary information to the reader by switching on and off a load resistor to perform
load modulation. The RFID reader detects this as amplitude modulation of the signal voltage at the
reader antenna. Load modulation and inductive coupling can be found among passive RFID tags
operating in the 125–135 KHz and 13.56 MHz frequencies.