Single-Unit 3-Bit and 4-Bit Reflection-Type Phase Shifters With Flat Phase Responses

Digital phase shifters (DPSs) are essential for phased-array systems, but the conventional cascaded architecture for multibit implementation suffers from a large footprint, high insertion loss (IL), and accumulated phase errors. This article presents single-unit multibit reflection-type phase shifters (RTPSs) based on the star-shaped switchable reactance block topology, which enable multiple phase-shifting bits within a single circuit. Each star-shaped structure consists of a connecting transmission line with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> parallel open-circuited stubs selectively connected via PIN diodes. The core innovation is leveraging a strong linear relationship between input susceptance and phase shift. This linearity allows the use of the superposition principle, enabling all 2<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">^N</i> phase states to be generated through simple switch combinations. Crucially, to simultaneously achieve accurate linearity at the center frequency and excellent phase flatness across the operating band, an adaptive gradient descent (AGD) algorithm is developed for parameter extraction. For validation, 3-bit (11.25°/22.5°/45°) and 4-bit (5.625°/11.25°/22.5°/45°) prototypes are fabricated and measured. A 4-bit design with an extended range (11.25°/22.5°/45°/90°) is also simulated, which further validates the capability of the star-shaped structure to achieve a large phase shift range. The measured results show excellent agreement with simulations, achieving maximum in-band phase errors below ± 5.5°, the figure of merit (FOM) better than 0.017 dB/°, return loss (RL) greater than 14 dB, and Root-Mean-Square (RMS) phase errors under 3.35° over 10% fractional bandwidth. Owing to the single-unit structure and the use of significantly fewer elements and switches, the proposed single-unit multibit RTPSs exhibit highly compact dimensions and lower IL.