Introduction

In the last few years the demand for high-speed Internet access, local area network, and short-distance data communications has been growing exponentially, and more growth is expected by the year 2005. Telephone lines such as twisted pairs have been used in LANs to transmit data between data terminals and computers, and video on demand (VoD) applications. Techniques similar to these may be used by video designers who want to trade the cost of a coaxial cable for the more cost-effective twisted pair. These applications require very fast speed and need more highly-integrated technologies.

Modem technology has blossomed into a family of digital subscriber loop (DSL) types, collectively referred to as XDSL. In these, analog technology allows a hundred-fold rate increase of data over phone lines. The ever-present problem with digital transmission is that it consumes bandwidth far beyond data rates.

Digital Subscriber Line (DSL) is one of the new technologies and the deployment rate has increased very fast among the other delivery models: cable, fiber optics, and satellite access. This column will focus mainly on ADSL central office (CO) driver design.

We will first discuss the ADSL DMT CO line driver requirements. We will show a typical line driver circuit; from the transformer turns ratio and termination resistor value, we will calculate the driver output current and voltage swing. We will examine the driver linearity and power dissipation -- and derive the resistance required. We will present a thermal resistance testing method and results of LPP, SOL-20, and SO-16 packages. Lastly we will present the power requirements of different ADSL systems and HDSL II system.

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