Identification system

Claim 1

A transponder for use in an identification system of the class comprising an interrogator adapted to transmit a continuous RF interrogation signal of a high frequency to said transponder and to receive and process an identification signal that is transmitted from said transponder, said transponder comprising; means for detecting said RF interrogation signal, means coupled to said detecting means for deriving a synchronous clock signal from said RF interrogation signal by division thereof in integer division means, means coupled to said detecting means for deriving first and second transmission frequencies from said RF interrogation signal by division thereof in integer division means, said first and second frequencies being of individually different frequencies that are low relative to the frequency of said RF interrogation signal and that are high relative to the frequency of said derived synchronous clock signal, means for storing an identification code comprising a sequence of digital bits in solid state memory means, means responsive to said derived synchronous clock signal for retrieving said identification code from said storing means, and means coupled to said identification code retrieving means and to said means for deriving said first and said transmission frequencies for transmitting a frequency-shift-keyed identification signal varying between said first and second transmission frequencies, said identification signal comprising a number of data bits transmitted in bit cells synchronized to said derived clock signal and therefore also to said RF interrogation signal, the data in each of the bit cells of said identification signal varying in accordance with said stored identification code.