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 (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {\lvert {\Gamma }_{L} \rvert }$ </tex-math></inline-formula>) into a digital output code by controlling the attenuator with the SAR codes. By modulating the attenuator with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Sigma \Delta $ </tex-math></inline-formula>-modulator (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Sigma \Delta $ </tex-math></inline-formula>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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$700\times 1000~{\mu}{\mathrm {m}}$ </tex-math></inline-formula>. The implemented reflectometer demonstrates a measurement error of 0.061 in terms of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {\lvert {\Gamma }_{L} \rvert }$ </tex-math></inline-formula> for VSWR ranging from 1.4 and 20 while consuming a supply current of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$86~{\mu }\text {A}$ </tex-math></inline-formula> in conversion mode and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$12.4~{\mu }\text {A}$ </tex-math></inline-formula> in idle mode from a dc supply of 1.8 V.