A Designer‘s Guide to RFID

　　A Designer‘s Guide to RFID

　　二十一世纪电子产品最有影响力的趋势之一是通过无线方式进行通信。平板电脑、智能手机以及他们的同类技术都有魅力，但是大量的大规模的无线射频识别设备（RFID）正在起到巨大的推动作用。

　　RFID是一种非常短距离的通信方式，在低数据速率下非常便宜。它被用来跟踪高速公路上收费的软件包，用于库存管理，等等。被跟踪的项目被称为“标签”，几乎总是不需要电池或其他电源，这将RFID与所有其他无线通信方案区别开来。

　　RFID提供了一种非接触的方式来收集关于产品或设备的信息。一般来说，转发器（或“标签”）花费很少——通常是一美元以下，因此可以或多或少地一次性使用。图书馆使用标签跟踪个人图书。沃尔玛试图将几乎所有销售的商品都贴上标签，以便更好地进行库存管理。标签可以在恶劣和肮脏的环境中使用，不利于条形码或其他技术。

　　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.