December 5, 2008 -- The appropriate protective-circuit design for DSL (digital-subscriber-line) loops depends on the type of loop: Loops vary in voltage conditions and in susceptibility to attenuation and degradation in signal integrity. Therefore, protective circuitry that works well in one application may be completely inappropriate in another. A "normal" signal ranges from 2.5V in an HDSL (high-bit-rate DSL) to 260V in a DSL system. The circuit design and choice of circuit-protection devices must take these differences into account. You should also consider the design with respect to effects of varying capacitance.

By Phillip Havens. (Havens is telecom-sector technical manager at Littelfuse.)