| Abstract |
During the last years, telephone companies around the world are making decisions to include existing twisted-pair loops in broadband access networks. The desirable high bit-rates are not attainable by the existing copper wire, which comes up short when asked to carry voice telephony, interactive video, and high speed data communications at the same time. Fiber all the way to the home (FTTH) is still prohibitively expensive in a marketplace driven by competition rather than costs. An attractive alternative, soon to be commercially practical, is a combination of fiber cables feeding neighborhood Optical Network Units (ONUs) and last leg premises connections by existing copper. This topology, which can be called Fiber to the Neighborhood (FTTN), connects a regional broadband network to a local Central Office (CO) with fiber, while the rest of the infrastructure to the end users remains intact. One of the enabling technologies for FTTN is Very high rate Digital Subscriber Line, or VDSL. In simple terms, VDSL transmits high speed data over short reaches of twisted-pair copper telephone lines, with a range of speeds depending upon actual line length. The maximum downstream rate under consideration is between 51 and 55 Mbps over lines up to 1000 ft (300 meters) in length. Downstream speeds as low as 13 Mbps over lengths beyond 4000 ft (1500 meters) are also in the picture. Upstream rates in early models will be asymmetric, just like ADSL, at speeds from 1.6 to 2.3 Mbps. VDSL chipsets implementation demands digital and analog hardware as well as software necessary for Digital Signal Processing and interfaces. This work deals with the design of a circuit included in the Analog Front End, which is the analog part of the transmitter hardware. The circuit is called a Line Driver and is the last of the units that constitute the Analog Front End. It is capable of transmitting an analog signal that can reach 30MHz of bandwidth, since this is the upper limit of frequencies that are transferred on copper twisted pairs. The signal swing reaches the supply rails and retains it for a wide range of DC offset input values. The amplified signal to be transmitted on the line is matched to a 100-Ohm impedance. The benefits of signal amplification, wide DC input range and impedance matching are combined with its spectral purity, since the line driver produces an output signal with low Total Harmonic Distortion (THD). In this way, the higher-order harmonics of the fundamental frequency that carries the useful data, cause insignificant interference.
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Σχεδίαση ενός οδηγού τεχνολογίας CMOS 0.18μm για το αναλογικό μέρος μετάδοσης σε γραμμή VDSL
