| Re: Hum from phone wires running next to mains? |
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Group: sci.electronics.equipment · Group Profile
Author: FoxtrotFoxtrot Date: Mar 4, 2008 15:44
On Tue 04 Mar 2008 21:22:30, ipal.net> wrote:
> In alt.engineering.electrical gfretwell@ aol.com wrote:
>| On Tue, 4 Mar 2008 19:13:07 -0000, "Graham." privacy.com>
>| wrote:
>
>|>It is quite difficult to induce hum into telephone wiring.
>|>Use twisted pair cabling rather than the flat ready-made
>|>extension cables.
>|
>| Exactly!
>| The phone company has millions of miles of cable running right
>| below power lines and hundreds literally touching each other in
>| the jacket of the cable. That little twist they put in the pairs
>| is excellent in isolating them from crosstalk.
>
> That twist is a great little means to ensure induced signals,
> whatever they may be, are induced in equal amount on both wires, so
> they do not contribute to the actual intended signal that is a
> differential between those two wires.
>
> However, a risk exists when two different pairs are present next to
> each other and each pair is twisted at the same pitch. The signal
> carried by one can end up being induced differentially on the
> other. So don't twist those power lines, or if you do, twist them
> at a pitch with a ratio to the phone line twist that is not a whole
> number.
>
> CAT5 cable is an example. It has 4 different pairs twisting along.
> Each of the pairs has a different twist pitch by design (unless
> you get some cheap cable not manufactured correctly).
>
I do not have any technical knowledge of this area.
I would like to ask about a cable which has two or more twisted pairs
in it.
Is there is a greaterlikelihood of hum if I connect a "2 wire" phone
extension by using one wire from a twisted pair and taking the second
wire from a different twisted pair?
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