Pasive RFIDs are tags that do not require a power source to operate. Instead, the reader for the RFID is operated on a voltage source. With this voltage source, the reader is able to construct a magnetic field when it senses a tag is near. From this, a current is induced through the magnetic field, which allows the tag to send its signal to the reader.

Most passive tags signal by backscattering the carrier wave from the reader. This means that the antenna has to be designed both to collect power from the incoming signal and also to transmit the outbound backscatter signal. The response of a passive RFID tag is not necessarily just an ID number: The tag chip can contain non-volatile data.

The reading and writing depend on the chosen radio frequency and the antenna design/size. Passive tags have a variety of ranges depending on the antenna incorporated into the tag. Some have a read distance ranging from about 11 cm (with near-field) and up to approximately 10 meters (with far-field)! There are even some tags that can reach up to 183 meters when combined with a phased array. Due to the simplicity in design they are also suitable for manufacture with a printing process for the antennas.

The lack of an onboard power supply means that the device can be quite small. Commercially available products exist that can be embedded in a sticker, or under the skin in the case of low frequency (LowFID) RFID tags. This information can be even retreived from over a couple hundred meters. Because of it's low-profile and simplicity, there are several applications for passive tags, ranging from inventory to paying for products remotely. More about its applications and setbacks will be found in upcoming posts.

(via Wikipedia, image credit: Sandia National Lab)