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Radio Frequency Identification (RFID) systems use radio frequency to identify, locate and track people, assets and animals. Three components compose passive RFID systems – a reader (interrogator), passive tag and host computer. The tag is composed of an antenna coil and a silicon chip that includes basic modulation circuitry and nonvolatile memory. A time-varying electromagnetic radio frequency (RF) wave that is transmitted by the reader energizes the tag. This RF signal is called a carrier signal. AC voltage is generated across the coil when the RF field passes through an antenna coil. This voltage is rectified to result in DC voltage for the device operation. When the DC voltage reaches a certain level, the device becomes functional. Reflecting, or loading, the reader's carrier transfers the information stored in the device to the reader. This is often called backscattering. By detecting the backscattering signal, the information stored in the device can be fully identified.

Various RF remote sensing applications, specifically in access control and animal tracking applications, use the passive RFID system because of its simplicity for use.

Application demands increased dramatically in recent years. In most cases, unique packaging form factor, communication protocol, frequency, etc., are used in each application. The passive tag deals with very small power (~ µw) because it is remotely powered by the reader’s RF signal. Thus, the read range (communication distance between reader and tag) is typically limited within a proximity distance. Design parameters, such as frequency, RF power level, reader’s receiving sensitivity, size of antenna, data rate, communication protocol, current consumptions of the silicon device, etc., cause the read range to vary.

Because of the availability of silicon devices, low frequency bands (125 kHz to 400 kHz) were traditionally used in RFID applications. Typical carrier frequency (reader’s transmitting frequency) in today’s applications range from 125 kHz to 2.4 GHz.

In recent years with the advent of new silicon devices, the applications with high frequency (4 to 20 MHz) and microwave (2.45 GHz) bands have risen. Each frequency band has advantages and disadvantages. Advantages of both low (125 kHz) frequency and microwave (2.4 GHz) bands are offered by the 4 to 20 MHz frequency bands. Therefore, this frequency band becomes the most dominant frequency band in passive RFID applications.

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