Signal-powered frequency-dividing transponder

Abstract

A batteryless, portable frequency divider, such as used in presence detection systems for article surveillance or as used for article-location determination, includes a series LC resonant circuit connected directed across a parallel LC resonant circuit. One circuit is resonant at a first frequency and the other circuit is resonant at a second frequency that is a plural-integer-divided quotient of the first frequency. In one class of embodiments, either or both of the series and parallel resonant circuits includes a variable capacitance element, such as a varactor, in which the capacitance varies in accordance with the voltage across the variable capacitance element. The variation of the capacitance of the variable capacitance element in response to variations in energy in the higher-frequency resonant circuit resulting from receipt electromagnetic radiation at the first frequency causes the lower-frequency resonant circuit to transmit electromagnetic radiation at the second frequency. In another class of embodiments, the parallel circuit is resonant at the higher first frequency and the series circuit is resonant at the frequency-divided second frequency; the frequency divider includes a three-terminal semiconductor switching device having a control terminal, a reference terminal, and a controlled terminal, which is connected directly across both resonant circuits and between the inductance and the capacitance of the series resonant circuit and which switches on and off in response to variations in energy in the parallel resonant circuit resulting from the parallel resonant circuit receiving electromagnetic radiation at the first frequency to cause the series resonant circuit to transmit electromagnetic radiation at the second frequency.