TABLE OF CONTENTS Tags identify objects. When the objects are very expensive, the cost of the tags is of little consequence, but their endurance is of great import, since expensive objects, and our interest in them, are usually also long lived. When the objects are cheap, the tags must be cheaper. These are the fundamental dynamics of tag design.
As a consequence, tags that are intended to label long-lived, expensive objects (typically viewed as assets on someone's books) are usually active tags, with their own radio transmitter and receiver, powered by a local battery. Since battery technology has progressed very slowly relative to semiconductor technology, the key issues in designing a active tag are to minimize the duty cycle the proportion of time during which the tag is doing something other than sleeping and to minimize the power required both to support the active state and the sleep or idle state. While these are not trivial design challenges, the technology used to fabricate an active tag is substantially similar to that used in other radios including the reader radio: discrete components and ICs are soldered to a printed circuit board, with the whole attached to a compact antenna and placed within a plastic housing.
Passive tags are mostly meant to identify inexpensive objects, and must thus, submit to an economic asceticism that eschews such luxuries. Conventional batteries are far too bulky and expensive to be considered. A conventional radio transmitter or receiver, with the complex and expensive oscillators, mixers, and...
