Understanding Smart Sensors - Nomads.usp

Transceivers, Transponders, and Telemetry 1918.3.7 RF-IDRF-ID tags are being used to track inventory in warehouses; work in process(WIP) in manufacturing plants; and animals in laboratories, on farms, or in thewild; as well as for automatic toll collection for vehicles. The RF tag is a transponderthat is read and decoded by an RF reader. For inventory and WIP inharsh environments or non-LOS operations, RF-ID is an alternative to a barcodesystem [24]. The tags can be active or passive. Active tags have onboardbatteries and larger data capacities approaching 1 Mb. Decision making abilityis included in some varieties with the addition of a microcontroller. Passive tagsobtain their operating power from RF energy transmitted from an antenna andas a result have limited data capacities, typically 1 to 128 bits.RF-ID systems operate in both low- and high-frequency ranges. Lowfrequencysystems typically operate in the frequency range of 100 kHz to1.5 MHz and also have lower data transfer rates. They are cost effective inaccess control and asset tracking applications. High-frequency systems operatein the spread spectrum range of 900 MHz to 2.4 GHz and have higher datatransfer rates. High-frequency systems cost more than low-frequency systems,but they can operate at distances up to 100 ft (30m).The main blocks of a passive RF-ID tag are shown in Figure 8.11 [25].An RF burst is received, rectified, and used to charge a capacitor that serves asthe power supply for the tag circuitry. A voltage regulator keeps the voltageacross the capacitor at a constant 2V. Logic circuitry interprets the commandfrom the RF burst. An interrogation request is answered with transmission of anumber from the EEPROM. Information in the EEPROM can be changed by15 VVoltageregulatorVdd2 VDataextractorLogicTransmitterChargepumpEEPROMFigure 8.11 RF ID block diagram.