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    RFID: A Tutorial

1. Operation of RFID Systems

   1. RFID Building blocks

   2. RFID Tag block schematic, and operation

   3. Backscatter Modulation

a. RFID Building Blocks:

Tags: A tag is the data carrier and normally contains the ID number, and unique EPC code programmed into the Tag
Tag Antenna: The tag antenna is connected to the chip in tag. It could be wire or printed using conductive ink.
Reader Antenna: It is a coil included in plastic or similar case, and normally measures 12 -18 inches square
Reader: A reader captures the data provided by the tag within the detectable area of the Reader. There can be one or more tags within the capture area. A reader is typically capable of reading multiple tags simultaneously.
Savant: This is normally a middleware that interacts with the readers, and communicate with External databases

b. RFID Block Schematic:

A simplified block schematic of an RFID tag (also called transponder) is shown in the diagram below. Various components of the tag are as shown. Normally, the antenna is external to the tag chip, and large in size.

The operation of the RFID tag is described below:

Handshaking with the Reader (interrogator):

1. The reader continuously emits RF carrier signals, and keeps observing the received RF signals for data.
2. The presence of a tag (for our discussion, we consider only passive tag) modulates the rf field, and the same is detected by the reader.
3. The passive tag absorbs a small portion of the energy emitted by the reader, and starts sending modulated information when sufficient energy is acquired from the rf field generated by the reader. Note that the data modulation (modulation for 0s and 1s) is accomplished by either direct modulation or FSK or Phase modulation.
4. The reader demodulates the signals received from the tag antenna, and decodes the same for further processing.

c. Backscatter Modulation:

  Backscatter is one of the most widely used modulation schemes for modulating data on to rf carrier. In this method of modulation, the tag coil (load) is shunted depending on the bit sequence received. This in turn modulates the rf carrier amplitude as shown in the diagram below. The reader detects the changes in the modulated carrier and recovers the data.

The above diagram provides a simplified modulated carrier signals from the RFID tag. As seen in the diagram, the encoded binary digits modulate rf carrier. A 1 is represented with high carrier level, and a 0 is represented by a low carrier level (tag coil shunted). The reader demodulates the signals to recover the data, and note that this data is still encoded. The reader decodes the data using suitable decoder, and forwards it for further processing to a computer (or any backend server).

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