A Designer‘s Guide to RFID

　　二十一世紀電子產品最有影響力的趨勢之一是通過無線方式進行通信。平板電腦、智能手機以及他們的同類技術都有魅力，但是大量的大規模的無線射頻識別設備（RFID）正在起到巨大的推動作用。

　　RFID是一種非常短距離的通信方式，在低數據速率下非常便宜。它被用來跟蹤高速公路上收費的軟件包，用于庫存管理，等等。被跟蹤的項目被稱為“標簽”，幾乎總是不需要電池或其他電源，這將RFID與所有其他無線通信方案區別開來。

　　RFID提供了一種非接觸的方式來收集關于產品或設備的信息。一般來說，轉發器（或“標簽”）花費很少——通常是一美元以下，因此可以或多或少地一次性使用。圖書館使用標簽跟蹤個人圖書。沃爾瑪試圖將幾乎所有銷售的商品都貼上標簽，以便更好地進行庫存管理。標簽可以在惡劣和骯臟的環境中使用，不利于條形碼或其他技術。

　　RFID is difficult to pin down as the term encompasses a wide range of device types and frequencies. The E-ZPass? automated toll collection system used in the USA‘s Northeast， for instance， uses RFID transponders operating at 915 MHz. Other bands in the GHz range are available as well.

　　But nearly all RFID work is done in two lower-frequency bands： 13.56 MHz and （approximately） 125 KHz. Unlike the shorter-wavelength bands， 13.56 and 125 KHz are authorized for these applications in just about every country.

　　RFID is further confused by the types of units. Active transponders have a battery and work very much like a traditional radio. Battery-assisted units， which are not particularly common outside of key fobs， may do some energy harvesting but require some source of power.

　　Passive units are the most common; a passive unit has no battery and does not use mains power. A passive tag harvests energy from the applied electromagnetic field created by a reader （see Figure 1）。

　　Passive tags are further subdivided into read-only and read-write varieties. The former have pre-programmed IDs which are used mostly for tracking items and the latter can store some information， somewhat like a web-browsing cookie.