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As the question says, I want to know why do longer traces/longer lengths of non-terminated signals of the PCB cause reflections and not the short length ones? I am ignoring the negligible amount of reflection a short trace may cause. I am not new to transmissions lines and impedance matching, but somehow I am not able to recall why exactly is reflection is more prominent on long non-terminated transmission lengths?

Is it because for longer trace there is higher chances of encountering discontinuities on the signal path, thereby causing reflection from that particular discontinuity, and even with the termination in place at the source, the signal gets reflected before it reaches back to the termination where it should ideally die off?

I would really appreciate your answer. Even if you link me to some document that answers this question well, it would be great.

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    \$\begingroup\$ All traces cause reflections - but the reflections on a short trace come so soon after the actual signal edge that they're indistinguishable from them. Reflections on longer transmission lines are a problem because they're delayed enough to show up as secondary, spurious signals. \$\endgroup\$ – Nick Johnson Jun 12 '15 at 9:44
  • \$\begingroup\$ Thanks a lot. That's what I really wanted to know. But I am happy I posted this question, as I got to read some really good information from all of you. \$\endgroup\$ – LoveEnigma Jun 16 '15 at 11:46
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All traces cause reflections. As the trace length approaches significant fractions of a given signals electrical length, the reflection can start to interfere with the signal (constructive or destructive interference).

You could also keep the trace length the same and increase the frequency of the signal, the effect will be the same. As mentioned in the comments, when considering the frequency of the signal it is important to note the frequency components. That is, the frequency of the fourier series components of the signal.

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    \$\begingroup\$ Note that it is not about the frequency of the signal but about the frequency components in the signal. A 1 HZ square wave can show serious reflection effects in a situation where a 2 Hz sine shows no such effect. \$\endgroup\$ – Wouter van Ooijen Jun 12 '15 at 6:55
  • \$\begingroup\$ @WoutervanOoijen Yes, an important note for me to add. \$\endgroup\$ – Samuel Jun 12 '15 at 6:57
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    \$\begingroup\$ @LoveEnigma Impedance mismatch at the end of any length trace causes a reflection. It's a change in the medium, like sound echoing off the end of a pipe. \$\endgroup\$ – Samuel Jun 12 '15 at 8:15
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    \$\begingroup\$ I tried to give a very basic understanding of reflections here: electronics.stackexchange.com/questions/171557/… \$\endgroup\$ – sweber Jun 12 '15 at 10:22
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    \$\begingroup\$ @LoveEnigma it can be instructive to take a bath. Make a wave at one end of the abth towards the other end. You can see the water at the far end rise far above the level of the wave itself. Now try to do this in a short 'bath' and the effect will be much less. \$\endgroup\$ – Wouter van Ooijen Jun 12 '15 at 13:55

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