HardwareBanter - View Single Post - Hum from phone wires running next to mains?

**George[_5_]**

On Tue, 04 Mar 2008 21:22:30 +0000, phil-news-nospam wrote:

In alt.engineering.electrical wrote:
| On Tue, 4 Mar 2008 19:13:07 -0000, "Graham." 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).

Very interesting.

Please can you advise on how these twisted pairs compare with

1. shielded audio cable
and
2. rf coax.

In case 1 both the wanted signal and the noise are in the audio frequency
range.

In case 2 the electricity supply noise contains harmonics of similar
frequency to the wanted rf signal.