I remember someone telling me long ago that if a voltage step is sent down a transmission line, the step will become smeared as it travels down the line, and the rise time will become degraded. I'm not referring to degradation caused by signal reflections, but rather some other limit on \$dv/dt\$ imposed by the transmission line that increases with length, regardless of the termination.
Is there such an effect? What's it called, and what causes it in practical transmission lines?
Best Answer
Yes, there is such a effect. A ideal transmission line is modeled with lots of little series inductors and parallel capacitors. See this answer by Phil.
In such a ideal transmission line, frequencies above a certain amount are removed and the remainder of the step propagates forever unchanged. This approximation is usually good enough for "short" transmission lines.
Real "long" transmission lines differ in that the series resistance matters, which is ignored in the ideal model presented above. This series resistance effectively adds some low pass filtering. Since the resistance accumulates with length, the resulting filter becomes ever lower in frequency. The more low pass filtered edge at the end of a long transmission line therefore looks more spread out since ever more high frequencies are removed over the length of the transmission line.