Frequency-Stitching-Based Ultrawideband Signal Generation for 6G Component/System Testing: Achieving 12-GHz Instantaneous Bandwidth and 96-Gbps Data Rate at D Band

This article introduces a novel method for generating ultrawideband (UWB) modulated signals at millimeter-wave (mmW) and sub-THz frequency bands using readily available high-resolution digital-to-analog converters (DACs) with sampling rates lower than twice the target bandwidth. The method exploits the periodicity of test signals to divide them into frequency subbands, with each subband generated by a dedicated channel consisting of an IQ-DAC followed by an IQ mixer. Phase coherent local oscillators (LOs) drive the IQ mixers, and the final UWB signal is synthesized by combining the outputs of all channels. To address inherent nonidealities in the proposed UWB signal generation method, a novel calibration technique is introduced. This technique uses nonuniformly interleaved tones to correct IQ imbalances, phase and magnitude offsets across channels, and linear distortions in each RF chain. The calibration formulation ensures continuity in the phase and magnitude frequency responses across different channels. For experimental validation, the proposed method was used to generate a 256-QAM orthogonal frequency-division multiplexing (OFDM) signal at D band (149 GHz) with an instantaneous bandwidth of up to 12 GHz, achieving a peak data rate of 96 Gbps. The calibration technique effectively compensates for the nonidealities in the proposed signal generator, improving the measured error vector magnitude (EVM) and normalized mean square error (NMSE) from 82.6% and 23.8% to less than 2% and 1%, respectively, when tested with a 12-GHz bandwidth 256-QAM OFDM UWB signal. Furthermore, the method was applied to linearize a D band power amplifier driven by a 256-QAM OFDM signal with a 4-GHz modulation bandwidth. The adjacent channel power ratio (ACPR) and EVM improved from - 27.8/ - 26 dBc and 8.5% before linearization to - 42.8/ - 43.1 dBc and 1.2% after linearization, ensuring a linearization bandwidth over 12 GHz. These results underscore the suitability of the proposed method for generating high-quality UWB modulated signals for component testing at mmW and sub-THz frequency bands.