Since the xDSL specifications impose stringent linearity requirements to ensure the integrity of the data transferred, current line drivers use linear amplifiers. The downside of these solutions is their power consumption, as the xDSL modulation has a high peak to average ratio, forcing the amplifier to operate in its least efficient region. An alternative is the use of switching amplifiers, which theoretically have an efficiency of 100%. However, the nonlinear nature of these amplifiers complicates the design. To meet the linearity requirements, the switching frequency should be sufficiently high, but as this is limited by the technology used, additional techniques must be adopted. Therefore, an asynchronous oscillating topology has been chosen and a mathematical model of the loop formulated to predict its behaviour. The model is then validated by numerical simulations on macro-blocks, yielding the values required for an implementation on silicon.

Distortion calculation of an asynchronous switching xDSL line-driver

To satisfy the high linearity requirements of ADSL line-drivers, most solutions to date use linear amplifiers, such as class AB. However their efficiency remains limited because of the modulation scheme used. As such alternative classes of amplifiers are necessary to lower dissipation, increasing the capacity of central offices. In addition the transformer turns ratio and the resulting supply voltage of the amplifier require a high-voltage supply. In this paper a switching differential amplifier is described. The circuit is processed in a 100V extension of a standard 0.7/spl mu/m CMOS technology, using a supply voltage of 50V. A full downstream signal maintains a minimum MTPR of 40dB with an efficiency of 13.1%, while the maximum efficiency into a matched load is more than 40%.

K.-P. Ho

Papers

Broadcast digital subscriber lines using discrete multitone for broadband access