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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?

Phil Frost
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  • Are you thinking of slew rate? – dext0rb Jul 09 '13 at 15:19
  • @dext0rb I think \$dv/dt\$ is slew rate, yes? – Phil Frost Jul 09 '13 at 15:19
  • Well, *max* dv/dt is slew rate, so when you said "some limit on dv/dt" that made me think slew rate. But I get what you are saying now, you are looking for a property of the T-line that influences the slew rate? I'm guessing increasing capacitance along the T-line will impact this. Then again, I haven't had my morning coffee yet so...perhaps ignore me :) – dext0rb Jul 09 '13 at 15:24
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    What sort of transmission line are you talking about? Ideal transmissions line can carry arbitrary waveforms perfectly, but can only be fabricated from unobtanium. Practical transmission lines differ from ideal ones in ways that depend upon their materials and construction. – supercat Jul 09 '13 at 15:26
  • @supercat practical transmission lines. What is it about them that causes this deviation from ideal, if indeed this is a thing that's significant? – Phil Frost Jul 09 '13 at 15:31
  • @supercat you have reached the answer he is needing, just adding the why. – Kortuk Jul 09 '13 at 15:37
  • @PhilFrost: An ideal transmission line behaves like a chain of infinitely many, infinitesimally small matched series inductors and matched parallel capacitors (it's the limiting case as the number of links goes up and their size goes down, so the line is uniform and continuous). In reality, nothing is perfectly uniform, and there are parasitic series and parallel resistances, capacitances, and inductances everywhere. Different techniques of cable construction yield different kinds of deviation from the ideal. – supercat Jul 09 '13 at 15:40
  • @supercat sounds like most of an answer! – Kortuk Jul 09 '13 at 15:42

1 Answers1

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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.

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Olin Lathrop
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    My paradox detector lit up. Referring Phil to Phil! :) – JYelton Jul 09 '13 at 15:48
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    @JYelton it's more common than I'd expect. Most of my questions are inspired by holes in my understanding discovered by answering other questions. Sometimes the best way to learn is to teach. – Phil Frost Jul 09 '13 at 15:49
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    @Phil I completely agree. I've learned more by trying to help others than I would on my own. – JYelton Jul 09 '13 at 15:50
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    @JYelton: LOL, I didn't notice Phil was the OP until you pointed it out. – Olin Lathrop Jul 09 '13 at 15:58