Transponder arrangement

A transponder system comprising:

• an interrogation unit for communicating with cooperating responder units comprising;
• a microprocessor,
• a transmitter for transmission of at least one RF interrogation signal of a first frequency having a duration predetermined by the microprocessor, and
• a receiver for receiving signal information at the termination of the interrogation signal;
• and
• a responder unit located in spaced relation with respect to said interrogation unit for receiving on a resonant circuit said at least one interrogation signal transmitted form the transmitter of said interrogation unit and returning data as signal information on said resonant circuit to the receiver of said interrogation unit in response to the reception of said at least one interrogation signal, said responder unit comprising;
• an energy accumulator for storing the energy contained in said at least one interrogation signal as received by said responder unit,
• a carrier wave generator operable for providing a FSK modulated carrier wave having at least two frequencies, one of said two frequencies being said first frequency contained in said at least one interrogation signal and a second frequency selectively shifted from said first frequency,
• circuitry operably connected to the output of said carrier wave generator for producing control signals for maintaining and modulating the carrier wave,
• circuitry for transmitting the FSK modulated carrier wave and data from said responder unit back to the receiver of said interrogation unit as signal information, and
• circuitry for initiating operation of said carrier wave generator in response to the detected power level of the RF interrogation signal decreasing and the presence of a predetermined energy amount stored in said energy accumulator.