[wplug] Wiring Question

dingimarson at quantapoint.com dingimarson at quantapoint.com
Mon Sep 26 00:43:45 EDT 2005


Kevin,

Short answer:

I don't think you will have a problem with your data circuit. You may or
may not hear some noise on the phone lines, but that depends on what
equipment is running on the 120VAC line that is run close to the phone
line. The coaxial line is essentially "self shielding" not much gets in or
out, at least as far as anything you will care about.

Be careful of the 120VAC. Use electricians tape on any connections -- a
stray conductor shorting to the hot wire can really piss off your data
equipment.




Pedantic answer (because I can't sleep and I'm tired of hacking code ;)

Generally, noise can be coupled into a circuit through the following
mechanisms:

1. Electric Field - this occurs when two conductors (aggressor and victim)
run in parallel (like the plates of a capacitor). Stuff has to be fairly
close to couple significantly. Unless you have run all these inside a
conduit (not recommended) it should be fine. This and the next mechanism
is  also why audiophiles never tie their signal wires into nice, neat
bundles.

2. Magnetic field - this is the most common. Mag field from the aggressor
induces current in the victim. The cure here is to watch the current loop.
The differential pairs on your data cable (I assume you are using
something decent like cat5 or 5e) are twisted, forming little loops --
this ensures that any mag field the cable intersects induces current in
one direction in one loop, and in the opposite direction in the next loop,
effectively canceling out the induced noise (well, almost anyway, nothing
is perfect). Plus the cat5 feeds a differential signaling system which
will take care of the remaining common-mode noise.

Your cat3 phone line doesn't really have the twists (I don't think) and is
certainly NOT differential which is why I say you _may_ hear some
interference depending on what is on that 120VAC circuit you have run next
to it..

Inductive loads or large switching loads will inject switching current or
rapid current changed onto the 120VAC which will couple to the phone
lines. Not large, but you might hear some noise. On the other hand you
might hear nothing if your phone equipment has decent filtering.

The coaxial cable has its loop area entirely enclosed in the body of the
cable -- once again, nothing much gets in or out.

3. Conductively coupled noise. If you create a "ground loop" through
mis-wiring something this can be a problem. Keep all signal returns and
shields separate (power with power, signal with signal, don't connect
signal shields to the 120VAC ground) and all should be well. Your
equipment should be properly isolated inside.


Have fun, be safe...

   -darin





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