Browsing by Author "Zhang, Feifei"

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    An Edge-Combining Frequency-Multiplying Class-D Power Amplifier
    Nguyen, Hieu Minh; Zhang, Feifei; O'Connell, Ivan; Staszewski, R. Bogdan; Walling, Jeffrey S. (IEEE, 2022-01)
    The class-D power amplifier (PA) is commonly implemented in CMOS, but its operating frequency is often limited due to the power loss of parasitic capacitances and the lower transition frequency of the the PMOS transistor. In this brief we demonstrate edge-combining frequency-multiplication embedded directly in the output-stage, allowing higher-frequency operation of the class-D PA, while maintaining similar performance to a lower-frequency PA. A 65 nm CMOS prototype achieves output power and system efficiency of 22.3 dBm and 30.2%, respectively. The prototype is tested with a D-BPSK signal and achieves an EVM of 2%-rms. Although the prototype was not embedded with amplitude modulation capability, it can be readily adapted for such operation using switched-capacitor PA techniques.
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    Extensive marine anoxia during the terminal Ediacaran Period
    Zhang, Feifei; Xiao, Shuhai; Kendall, Brian; Romaniello, Stephen J.; Cui, Huan; Meyer, Mike; Gilleaudeau, Geoffrey J.; Kaufman, Alan J.; Anbar, Ariel D. (AAAS, 2018-06-20)
    The terminal Ediacaran Period witnessed the decline of the Ediacara biota (which may have included many stem-group animals). To test whether oceanic anoxia might have played a role in this evolutionary event, we measured U isotope compositions (δ238U) in sedimentary carbonates from the Dengying Formation of South China to obtain new constraints on the extent of global redox change during the terminal Ediacaran. We found the most negative carbonate δ238U values yet reported (−0.95 per mil), which were reproduced in two widely spaced coeval sections spanning the terminal Ediacaran Period (551 to 541 million years ago). Mass balance modeling indicates an episode of extensive oceanic anoxia, during which anoxia covered >21% of the seafloor and most U entering the oceans was removed into sediments below anoxic waters. The results suggest that an expansion of oceanic anoxia and temporal-spatial redox heterogeneity, independent of other environmental and ecological factors, may have contributed to the decline of the Ediacara biota and may have also stimulated animal motility.
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    Global marine redox changes drove the rise and fall of the Ediacara biota
    Zhang, Feifei; Xiao, Shuhai; Romaniello, Stephen J.; Hardisty, Dalton; Li, Chao; Melezhik, Victor; Pokrovsky, Boris; Cheng, Meng; Shi, Wei; Lenton, Timothy M.; Anbar, Ariel D. (Wiley, 2019-07-28)
    The role of O2 in the evolution of early animals, as represented by some members of the Ediacara biota, has been heavily debated because current geochemical evidence paints a conflicting picture regarding global marine O2 levels during key intervals of the rise and fall of the Ediacara biota. Fossil evidence indicates that the diversification the Ediacara biota occurred during or shortly after the Ediacaran Shuram negative C-isotope Excursion (SE), which is often interpreted to reflect ocean oxygenation. However, there is conflicting evidence regarding ocean oxygen levels during the SE and the middle Ediacaran Period. To help resolve this debate, we examined U isotope variations (δ238U) in three carbonate sections from South China, Siberia, and USA that record the SE. The δ238U data from all three sections are in excellent agreement and reveal the largest positive shift in δ238U ever reported in the geologic record (from ~ −0.74‰ to ~ −0.26‰). Quantitative modeling of these data suggests that the global ocean switched from a largely anoxic state (26%–100% of the seafloor overlain by anoxic waters) to near-modern levels of ocean oxygenation during the SE. This episode of ocean oxygenation is broadly coincident with the rise of the Ediacara biota. Following this initial radiation, the Ediacara biota persisted until the terminal Ediacaran period, when recently published U isotope data indicate a return to more widespread ocean anoxia. Taken together, it appears that global marine redox changes drove the rise and fall of the Ediacara biota.
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    Nitrate limitation in early Neoproterozoic oceans delayed the ecological rise of eukaryotes
    Kang, Junyao; Gill, Benjamin C.; Reid, Rachel E. B.; Zhang, Feifei; Xiao, Shuhai (American Association for the Advancement of Science, 2023-03-23)
    The early Neoproterozoic Era witnessed the initial ecological rise of eukaryotes at ca. 800 Ma. To assess whether nitrate availability played an important role in this evolutionary event, we measured nitrogen isotope compositions (δ15N) of marine carbonates from the early Tonian (ca. 1000 Ma to ca. 800 Ma) Huaibei Group in North China. The data reported here fill a critical gap in the δ15N record and indicate nitrate limitation in early Neoproterozoic oceans. A compilation of Proterozoic sedimentary δ15N data reveals a stepwise increase in δ15N values at ~800 Ma. Box model simulations indicate that this stepwise increase likely represents a ~50% increase in marine nitrate availability. Limited nitrate availability in early Neoproterozoic oceans may have delayed the ecological rise of eukaryotes until ~800 Ma when increased nitrate supply, together with other environmental and ecological factors, may have contributed to the transition from prokaryote-dominant to eukaryote-dominant marine ecosystems.