Method and apparatus for modulating and detecting a subcarrier signal

Abstract

A transponder system including a transponder and an interrogator having a transmission coil for producing a high-intensity electromagnetic field for inductively powering the transponder and two receiver coils for receiving a low-intensity electromagnetic field reradiated by the transponder. The transmission coil includes one or more conductive windings circumscribing a substantially polygonal volume of space. The receiver coils are diametrically disposed opposite one another within the volume space of the transmitter and are electrically connected to each other in a differential circuit relationship, the output of which is at a minimum when the two receiver coils receive approximately equal quantities of energy and is at a maximum when one of the receiver coils receives more electromagnetic energy from the transponder than the other receiver coil. The transponder uses a coil to derive both a power source and a clock pulse from the high-intensity field for powering and driving an identification circuit. The frequency of the clock pulse is then divided by 8 to drive the memory array and logic network of the identification circuit so that it clocks out a new data bit every 16 cycles of the carrier frequency. The output of the memory array is then used to phase modulate a subcarrier, which is at 1/2 the frequency of the carrier frequency. The subcarrier frequency of 1/2 the carrier frequency is chosen to maximize the amplitude of the transmitted subcarrier. The transponder also includes a double pulse lockout circuit which prevents rapid consecutive short-outs of the transponder's coil.