Transmission Line (TL) Effects

Abstract

Transmission line (TL) effects are one of the most common causes of noise problems in high-speed DSP systems. When do traces become TLs and how do TLs affect the system performance? A rule-of-thumb is that traces become TLs when the signals on those traces have a rise-time (Tr) less than twice the propagation delay (Tp). For example, if a delay from the source to the load is 2nS, then any of the signals with a rise-time less than 4nS becomes a TL. In this case, termination is required to guarantee minimum overshoots and undershoots caused by reflections. Excessive TL reflections can cause electromagnetic interference and random logic or DSP false-triggering. As a result of these effects, the design may fail to get the FCC certification or to fully function under all operating conditions such as at high temperatures or over-voltage conditions.