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RF ID or radio frequency identification tags represent a relatively new market for RF products. RFID is based on the concept that a simple radio receiver can provide a highly mobile non-contact method of keeping track of an object. An RF transmitter - receiver unit (interrogator) is used to query a transponder or tag located at a distance. The tag detects the interrogator signal and transmits a response signal back to the interrogator's receiver. The response signal is encoded with data which may be as simple as 1-bit (tag is present or absent) to a serial stream of data. Applications are as varied as locating manufacturing goods in a factory, tracking livestock, remote toll paying on the highways, and anti-theft devices for automobiles.
There are two basic kinds of tags. The simplest are devices that respond to an external interrogator with a very short, pre-set message. These are referred to as read-only tags. The second, more complex tag type adds the ability to have information "written" to it by the interrogator, and typically can store longer messages. These are called "read-write" tags.
Tags may be either active (having a transmitter) or passive (using modulated backscatter). Passive tags use Schottky diodes to detect an interrogating signal and re-transmit the fundamental or a harmonic directly back to the interrogator. Active tags with batteries make use of high sensitivity Schottky diodes to detect and demodulate the interrogating signal and to activate transponder electronics from a power-conserving standby mode. Active tags may also be implemented without a battery by using Schottky diodes to rectify a portion of the received RF signal to supply direct current bias to power transponder electronics. Schottky diodes with high rectification efficiency are essential for this function.
PIN diodes may be used to prevent high level signals from damaging the transponder's detector.
Most systems operate in the 900 - 6 GHz range.
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