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This paper presents a compact backscatter modulator tag operating at 1.76 GHz. The modulator is composed of a series combination of a variable resistor and a varactor. These components modulate the backscatter signal by adjusting the input impedance or input reflection coefficient. A gallium nitride (GaN) on silicon carbide (SiC) high electron mobility transistor (HEMT) is adopted to implement the variable resistor. The modulator can support a variety of modulation schemes without increasing the size and design complexity. The tag can be continuously powered through ambient RF energy in the 2.45 GHz frequency band. A differential rectifier harvests RF energy to power a microcontroller eliminating the need for a battery. Experimental results show that the modulator can support 16-QAM with an error vector magnitude (EVM) of 1.73%. The measured RF to DC power conversion efficiency (PCE) of the energy harvester is 57.1% at a received power of 1.7 dBm.

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