Passive RFID tag technology

I read up recently on RFID tag technologies. RFID tags can be active or passive. From the Auto-ID Center's website:

Active RFID tags have a battery, which is used to run the microchip's circuitry and to broadcast a signal to a reader (the way a cell phone transmits signals to a base station). Passive tags have no battery. Instead, they draw power from the reader, which sends out electromagnetic waves that induce a current in the tag's antenna.

Active tags are useful when tags need to be tracked at longer ranges but are expensive (a dollar or more) which makes them infeasible for applications such as tagging low-ticket-price items. On the other hand, passive tags are much cheaper (pennies vs dollars) but are useful only when the ranges required are smaller. The Auto-ID Center is primarily working on passive tags. There are two major approaches for passive tags: inductive coupling and backscatter coupling. See here for an overview.

(Click below to continue reading)

Inductive coupling exploits a near-field effect (operative when the separation of the tag and the reader is much less than a wavelength). The radiating antenna on the reader creates an alternating magnetic field which penetrates the antenna coil of the tag. By Lenz's Law, this induces a voltage in the tag. A capacitor is connected in parallel to the antenna coil in the tag to create an L-C circuit whose resonant frequency matches the frequency of the radiator. In this situation, the tag draws energy from the magnetic field created by the radiator. By altering the load in the tag's circuit, it is possible to vary the amount of energy drawn from the field and these variations can be detected by the reader. It is thus possible to transfer data from the tag to the reader by using the data to modulate the switching on or off of a load in the tag's circuit.

Backscatter coupling uses a technique similar to radar. The tag uses an antenna with a high reflection cross-section for incident electromagnetic waves. Thus a portion of the power radiated by the reader antenna is reflected off the tag's antenna and is then received by the reader. By switching on or off a load connected in parallel to the reader's antenna, it is possible to vary the amount of reflected power. By modulating the switching with data, information can be transferred from the tag to the reader. The reflected power is received at the reader, passed through a directional coupler (to decouple it from the transmitted signal) and the data is decoded.