A 28-GHz Phased-Array Transmitter Achieving 24% Peak Efficiency, 0.26-mm2/ Element Area Efficiency with Completely Orthogonal Phase and Gain Control

This paper presents a Ka-band 4-element phased-array transmitter (TX) featuring an intermediate frequency local-oscillator phase shifting (IFLO PS) architecture, demonstrating superior power and area efficiency, highly-linear gain-invariant PS, and phase-invariant gain control. The TX adopts sliding-IF architecture, integrating an on-chip phase-locked loop (PLL) with a divider to generate a $22.4-\mathrm{GHz}$ radio-frequency LO (RFLO) and a $5.6-\mathrm{GHz}$ IFLO. The phase shift and variable gain control functions are executed at IFLO and IF signals, respectively, with precise and area-efficient implementations. By utilizing a multiplex-and-interpolation-based IFLO phase shifter, the TX attains an outstanding RMS phase error of $0.63^{\circ} \sim 0.85^{\circ}$, with a minimal RMS PS-induced gain error of $0.02 \sim 0.05 \mathrm{~dB}$. A proposed transadmittance-transimpedance IF VGA offers a 13-dB gain control range with a $0.3^{\circ} \sim 0.6^{\circ} \mathrm{RMS}$ gain control-induced phase error. The TX attains completely orthogonal PS and gain control. The sliding-IF scheme enables a lowered RFLO frequency centering at 22.4 GHz, allowing for inductor-less RFLO buffers. This is in contrast to the direct-conversion architecture, where an inductively tuned RFLO buffer is required. The proposed architecture significantly simplifies the mmWave path, incorporating only an RF mixer and a power amplifier. As a result, the phased-array TX achieves a state-of-the-art $0.26-\mathrm{mm}^{2} /$ element area efficiency and 24.1% peak power efficiency.