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In this paper, a 4-GHz, four-channel, analog-beamforming direct-conversion down-converter in 90-nm CMOS is presented. Down-converting vector modulators (VMs) in each channel multiply the inputs with complex beamforming weights before summation between the different channels. The VMs are based on a phase-oversampling technique that allows the synthesis of inherently linear, high-resolution complex gains without complex variable gain amplifiers. A bank of simple passive mixers driven by a multiphase local oscillator (LO) in each VM performs accurate phase shifting with minimal signal distortion, and a pair of transimpedance amplifiers (TIAs) combines the mixer outputs to perform beamforming weighting and combining. Each individual channel achieves 360° phase shift and gain-setting programmability with 8-bit digital control, a complex gain constellation with a mean error-vector magnitude (EVM) of <;2%, and a measured phase error of <; 5.5° at a back-off of 4 dB from the maximum gain setting. The beamformer demonstrates >24-dB blocker rejection for blockers impinging from different directions and 17-dB signal EVM improvement in the presence of an in-channel blocker.
Date of Publication: Nov. 2010