for the function of tracking large numbers of items many of which use the basic elements shown in fig1 typically the rfid tag 11 is a passive device attached to the item being tracked the reader 12 is an active rfid device that communicates with large numbers of passive rfid tags and typically either stores data in the reader and or relays data to a central database 13 the central database keeps data for all items in the system in many applications for example large retail outlets the rfid readers are mobile devices that are moved around the vicinity of the rfid tags to record the rfid tag data mobility in this application is necessary since the transmission distance between the rfid tags and the rfid readers is very limited for example tens of meters maximum and typically less than 10 meters the rfid readers are typically powered which extends the range of transmission between the rfid readers and a remotely located receiver that allows the option of using a rfid reader to simply relay rfid tags to a central database more typically the reader reads the passive rfid and stores the information locally this data may be downloaded to the central store periodically by placing the reader in a docking device that is connected by wireless or hardwired link to the central database in the latter case a wireless link between the rfid reader and a remote receiver may or may not be used a passive rfid tag is shown at 21 in fig2 rfid tags are miniaturized as much as practical to allow for the essential elements of a semiconductor ic chip 22 typically a cmos.