Semipassive responder utilizing a low voltage, low power drain reflective varactor phase modulator

Claim 1

A semipassive responder for generating, in response to incident interrogation signals, a reply signal, said reply signal being shifted in phase in accordance with a sequence of code bits, said responder comprising: an antenna having a first predetermined impedance; a transmission line, connected to said antenna, said transmission line having a second predetermined impedance and a predetermined electrical length; an inductor, coupled to said transmission line, said inductor having a predetermined inductance; a voltage-controlled variable-capacitance device, responsive to bias signals applied thereto, connected in series with said inductor and said transmission line; and biasing means, coupled to said variable-capacitance device, for generating said bias signals, said bias signals being at a selected one of a plurality of voltage levels in accordance with said code bits, the impedance measured across said transmission line looking toward said variable-capacitance device being respectively a third predetermined impedance and a fourth predetermined impedance in response to a respective first and second selected ones of said voltage levels; said first and second predetermined impedances, said predetermined electrical length and said inductance being chosen such that twice the difference between the arctangent of the ratio of said third predetermined impedance to said first predetermined impedance and the arctangent of the ratio of said fourth predetermined impedance to said first predetermined impedance is equal to a predetermined differential phase shift.