A 2.4 GHz Frequency Selective Backscatter Modulator for Bluetooth and Wi-Fi Backscatter

This paper presents a frequency selective backscat-ter modulator for the 2.4 GHz industrial, scientific, and medical (ISM) band. Unlike conventional wideband backscatter modulators, which modulate every incoming signal that is incident on an antenna, a frequency selective modulator mitigates undesirable spectral pollution by preferentially backscattering signals over a desired range of frequencies and minimizing backscatter elsewhere. The modulator architecture presented here is designed for Wi-Fi (IEEE 802.11) and Bluetooth backscatter devices operating in the 2400–2483 MHz ISM band. Three different band-selection filters are presented, based on microstrip and low-temperature co-fired ceramic (LTCC) implementations. Additionally, three different modulators based on varactor diodes, PIN diodes, and pHEMT FETs are presented. The wideband differential reflection coefficient is simulated and measured for each combination, demonstrating the frequency selective backscatter properties of the presented modulator architecture. For example, the pHEMT based frequency selective modulator exhibits a 5856 MHz 3 dB modulation bandwidth without filtering, while the addition of an LTCC filter reduces the 3 dB modulation bandwidth to only 328 MHz. This modulator has an energy-per-bit figure of merit of only 1.9 pJ/bit,