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Old 13th March 2006, 01:32 PM   #1
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Default How does Twisted pair work?

I can understand balanced line: two wires carrying a signal. One has reversed phase. Any noise affects both equaily and when that signal wire is unreversed the noise is also flipped. the positive noise cancelles out the negative noise.

How does twisted pair work?
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Old 13th March 2006, 02:33 PM   #2
moamps is offline moamps  Croatia
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twisted pair = balanced line
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Old 13th March 2006, 02:51 PM   #3
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I wish it was that simple. I have balanced line cables that aren't twisted.
I think balanced is electrical noise cancellation whereas twisted is electromagnetic, but I don't understand the magnetic part.
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Old 13th March 2006, 02:53 PM   #4
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A pair of parallel wire will make a single loop and works as an antenna if exposed to magnetic field and an electric signal proportional to flux change will be present in loop circuit.

A pair of twisted wire will make many loops each of them working as an antenna but electric signal from any loop will cancel signal from next loop.

This is why on a balance system each pair of wire must de twisted, have a look on LAN wire you will see how it is done 8 wires twisted to make 4 pairs.
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Old 13th March 2006, 02:55 PM   #5
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Twisting a balanced pair reduces the amount of electromagnetic induction, as the helix reduces the amount of conductor normal to the field, and so reduces the amount of noise induced. In balanced operation, this is 'balanced' out by the negative conductor, but the benefits of twisted pairs isnt limited to balanced cabling.

Ok?


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Old 13th March 2006, 03:07 PM   #6
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Quote:
helix reduces the amount of conductor normal to the field
............... that's the bit I don't get.
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Old 13th March 2006, 03:24 PM   #7
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Quote:
Originally posted by Pbassred
............... that's the bit I don't get.
While the helix does indeed lower the total magnetic intercept cross section on a loop by loop basis, that is not why the twist works. For reaonable twist pitches, that reduction runs anywhere from 5% to 20% reduction.

As mentioned before, Faraday's law of induction applies. If you take a twisted pair, and project it's metal's "shadow" onto a surface, you will see the wire forms football (american, not the rest of the world) shaped openings. Note that the "positive conductor" in that pair swaps side to side, from one loop to the next.

When a changing magnetic field is going through those loops, each loop will produce a voltage, depending on the rate of change of the magnetic field, and the total area of each loop. The polarity of the voltage generated within each loop will depend on which side the "positive wire" is on. When it swaps side, the voltage polarity also swaps.

This causes the voltage that was generated loop to loop to be opposite sign, so they cancel.

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Old 13th March 2006, 03:26 PM   #8
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Hey John,

That means we should have an even number of twists, no ?

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Old 13th March 2006, 03:30 PM   #9
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Magnetic field will induce voltage in a loop of wire, the voltage is proprtional to the flux change in the loop ( farady,s law ) so for a flux change the voltage is proportional to the loop surface normal to the flux. Using twisted pair reduce surface and also give several surfaces opposite one to the other so several voltages oppotie one to the other this is why you get cancellation of the noise ( induced voltage )

Owen is right but you must use calculus to see how it is going when you read : the helix reduces the amount of conductor normal to the field
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Old 13th March 2006, 03:33 PM   #10
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A even number of loop is ideal but if you have an odd number noise will be limited to the noise of the the last loop if the setup is ideal , in practice it will not make any difference
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