A Low-Power, High-Resolution Successive Approximation RF Reflectometer

This article presents a monolithically integrated radio frequency (RF) scalar reflectometer based on a successive approximation register (SAR) approach with boosted measurement resolution. The limiting factors of the SAR-based architecture are investigated and the corresponding solutions are brought up accordingly for the improvement. The directional coupler used in the reflectometer adopts a reconfigurable design with which the directivity can be optimized for various frequency bands by loading the tuning ports with impedance tuning network. The device directly converts the return loss of the load (|T_L|) into a digital output code by controlling the attenuator with the SAR codes. By modulating the attenuator with a ΣΔ -modulator (ΣΔ M ), the step size of the attenuation can be reduced to as low as 0.25 dB, reducing the quantization error and boosting the resolution of the SAR-based reflectometer. The device is designed and implemented in a 90-nm RF SOI CMOS switch technology, occupying a core area of 700×1000 μm . The implemented reflectometer demonstrates a measurement error of 0.061 in terms of (|T_L|) for VSWR ranging from 1.4 and 20 while consuming a supply current of 86 μA in conversion mode and 12.4 μA in idle mode from a dc supply of 1.8 V.