A CMOS Fractional Frequency Synthesizer for a Fully Integrated S-Band Extravehicular Activity (EVA) Radio Transceiver

Extravehicular activity (EVA) is an important aspect of space explorations. It enables astronauts carry out tasks outside the protective environment of the spacecraft cabin. The crew requires EVA radio transceivers to transmit and receive information among themselves and with equipment in space. Communication is done through the S frequency band (2GHz to 4GHz). Since the EVA radio transceiver is part of the space suits the astronauts wear for EVA, it is important that lightweight, low power consumption and miniaturized systems are utilized in their design and implementation. This thesis presents the design and implementation of a fully integrated frequency synthesizer for carrier signal generation in the EVA radio transceiver. The transceiver consists of a dual up-conversion transmitter (TX) and a direct conversion receiver (RX) at 2.4GHz. It supports 10 channels spaced at 6MHz for both video and voice communications, covering the frequency band from 2.4GHz to 2.454GHz. Therefore in the TX mode, the frequencies required are 0.8GHz to 0.818GHz (quadrature) and 1.6GHz to 1.636GHz (differential) for dual up-conversion to prevent the pulling problem between the power amplifier (PA) and voltage controlled oscillator (VCO) of the synthesizer. In RX mode, the frequencies from 4.8GHz to 4.908GHz are synthesized with a divide-by-two circuit to generate quadrature signals of 2.4GHz to 2.454GHz. In order to cover the frequency ranges in both TX and RX modes with a small area and low power consumption, a dual-band VCO fractional-N PLL is implemented. The dual-path loop filter topology is utilized to further reduce chip area. The fractional synthesizer is fabricated in 0.18μm CMOS technology and has a loop bandwidth of around 40kHz. It occupies a relatively small area of 1.54mm^(2) and consumes a low power of 22.68mW with a 1 V supply for the VCO and 1.8V supply for the rest of the blocks. The synthesizer achieves a reference spur performance of less than –62.34dBc for the lower band (LB) and less than –68.36dBc for the higher band (HB). The phase noise at 1MHz for the LB ranges from -125.38 to -130.39 dBc/Hz and for the HB -113.12 to -120.16 dBc/Hz. Thus the synthesizer achieves low power consumption with good spectral purity while occupying a small chip area making it suitable for EVA radio applications

Consistent patterns of common species across tropical tree communities

Trees structure the Earth’s most biodiverse ecosystem, tropical forests. The vast number of tree species presents a formidable challenge to understanding these forests, including their response to environmental change, as very little is known about most tropical tree species. A focus on the common species may circumvent this challenge. Here we investigate abundance patterns of common tree species using inventory data on 1,003,805 trees with trunk diameters of at least 10 cm across 1,568 locations1,2,3,4,5,6 in closed-canopy, structurally intact old-growth tropical forests in Africa, Amazonia and Southeast Asia. We estimate that 2.2%, 2.2% and 2.3% of species comprise 50% of the tropical trees in these regions, respectively. Extrapolating across all closed-canopy tropical forests, we estimate that just 1,053 species comprise half of Earth’s 800 billion tropical trees with trunk diameters of at least 10 cm. Despite differing biogeographic, climatic and anthropogenic histories7, we find notably consistent patterns of common species and species abundance distributions across the continents. This suggests that fundamental mechanisms of tree community assembly may apply to all tropical forests. Resampling analyses show that the most common species are likely to belong to a manageable list of known species, enabling targeted efforts to understand their ecology. Although they do not detract from the importance of rare species, our results open new opportunities to understand the world’s most diverse forests, including modelling their response to environmental change, by focusing on the common species that constitute the majority of their trees.Publisher PDFPeer reviewe

Consistent patterns of common species across tropical tree communities

International audienceAbstract Trees structure the Earth’s most biodiverse ecosystem, tropical forests. The vast number of tree species presents a formidable challenge to understanding these forests, including their response to environmental change, as very little is known about most tropical tree species. A focus on the common species may circumvent this challenge. Here we investigate abundance patterns of common tree species using inventory data on 1,003,805 trees with trunk diameters of at least 10 cm across 1,568 locations 1–6 in closed-canopy, structurally intact old-growth tropical forests in Africa, Amazonia and Southeast Asia. We estimate that 2.2%, 2.2% and 2.3% of species comprise 50% of the tropical trees in these regions, respectively. Extrapolating across all closed-canopy tropical forests, we estimate that just 1,053 species comprise half of Earth’s 800 billion tropical trees with trunk diameters of at least 10 cm. Despite differing biogeographic, climatic and anthropogenic histories 7 , we find notably consistent patterns of common species and species abundance distributions across the continents. This suggests that fundamental mechanisms of tree community assembly may apply to all tropical forests. Resampling analyses show that the most common species are likely to belong to a manageable list of known species, enabling targeted efforts to understand their ecology. Although they do not detract from the importance of rare species, our results open new opportunities to understand the world’s most diverse forests, including modelling their response to environmental change, by focusing on the common species that constitute the majority of their trees