AM-AM and AM-PM Distortion in D-Band BiCMOS Vector-Interpolation Phase Shifters

Sub-THz phased-array transceivers need phase shifters for a coherent combination of the transmitted and received signals. Vector-interpolation phase shifters demonstrate wide bandwidth, low loss, and compact footprint, but may suffer from poor linearity, restrained by the need of high dynamic range variable gain amplifiers (VGAs). This article investigates the role of VGA impairments in a vector-interpolation phase shifter, with primary focus on the AM-AM and AM-PM distortions. Then, three common VGA structures are analyzed and compared, revealing a best candidate to optimize linearity. A single-ended input/output phase shifter is finally implemented in 55 nm SiGe BiCMOS technology. The test chip allows experimental validation of the theoretical results by using the VGAs in different operating modes. It is also shown that, in a single-ended phase shifter, the common-mode current generated by the VGAs may compromise gain and linearity. Solutions to the problem are introduced in the design, including a novel current-mode balun that provides large (4:1) impedance transformation and high common-mode rejection with equal primary and secondary inductors. Experimental results prove wideband operation, from 130 to 175 GHz, with 3.5 dB gain. The phase shift is programmable in 10° steps with rms phase and amplitude errors across frequency < 5° and < 0.8 dB, respectively. With the optimal VGA operation mode, the output power at 1 dB gain compression (OP1dB) is above 1.8 dBm with < 10° AM-PM distortion. Experiments compare favorably against previous works in the same band and in similar technologies.