Electronic – How fair is comparing a pair of long wires with a capacitor

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Long ago I was reading an explanation why twisted pair is batter than just a pair of wires. That text was discussing long distances – miles or dozens of miles. It said that two wires in parallel acted like a capacitor, so whenever the sender wanted to send a pulse and raised the voltage that long "capacitor" would take some time to charge and so the receiver would see slowly rising voltage instead of a pulse and when the pulse ended at the sender side by quickly lowering voltage the "capacitor" would continue discharging and so the receiver would see slow decrease in voltage. So transmitting short pulses was just impossible – pulses had so be impractically long.

How fair is such explanation and comparing a long pair of wires with a capacitor?

Best Answer

Summarised Solution:

  • Your text has some correctness to what it says but is misleading.

  • Both a spaced pair of wires and a twisted pair will round pulses. It is likely that a spaced pair of parallel wires will allow faster pulse transmission easier BUT the reason for twisting pairs is largely unrelated to data speed. Wires are twisted to reduce interference. In a twisted pair the distance to a radiating noise source is equal for both wires - there is no net coupling area to couple to the noise source and any induced noise signals cancel. [Simplified answer].

But

  • A pair of wires looks like a capacitor.

  • The effect of the capacitance (and other factors) is to limit the "sharpness" of the pulse which can be transmitted

Because (at least)

  • A capacitor CANNOT have its voltage instantaneously changed.

Plus

  • A pair of wires has inductance and resistance as well.

But

  • In many practical cases, while the shape of the pulse is affected by the wire characteristics, you can still get some very fast very square looking pulses transmitted using adequate technical magic. 1 gigabit per second LANs are a good existence proof of fast pulse transmission. A circuit that severely rounds a 1 GB/s pulse stream will pass a squarish looking 100 mbps pulse stream and a VERY square looking 10 mbps pulse stream.

A pair of wire acts as a capacitor (capacitance C), plus has an inductance L and a resistance R: thus it is an RCL circuit.

These properties (C, R, L) are all "distributed".
Effectively a pair of wires may be modelled as an infinite series of "sections" with each section comprising a series inductor and a series resistor in each "leg" plus a capacitor between the two "legs". When a pair of wires is used to transmit signals in an environment where the pair's RCL characteristics have a significant effect o the signal it is known as a "transmission line".

If you just "lump" all the inductance and all the resistance and all the capacitance together the results differs from when they are distributed.

A transmission line will tend to have increasing attenuation with frequency.
This means that pulse edges tend to get rounded as the squareness of the edges depends on high frequency components.

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