Floyd L. Davidson said:
Ahem. That is absolutely false. Telecom engineering
necessarily goes to an extreme effort to reduce what is called
"power line influence". The reasons should be obvious:
telephone and power cables are often run side by side, on the
same poles, and in the same crawl spaces, sometimes for miles at
a stretch. It is not uncommon to see as much as 40 to 50 volts
of power line AC on a telecom cable. That requires an
astounding amount of noise immunity to allow a circuit to work.
Yes they do, in fact we've got a broadcast transmitter site which is fed
by a bit of BT, (British Telecom, the national Telco), overhead wire
for some miles and no hum at all!. And that is on the same pole set as
240 volt mains wiring and I've actually seen 11 kV lines with phone
lines near them. Not that advisable owing to the safety factor!.
Yes of course you can get leakage via induction and capacitance into the
telecom lines but this does not matter as it will inevitably be induced
in both conductors and cancelled out by common mode rejection. Doesn't
matter providing the insulation in the line and transformers will stand
it to have some kilovolts actually on the line as such...
Consider that the test tone level at a customer premise telephone
set is nominally targeted at -9 dBm, and the worst case acceptable
Signal-to-Noise ratio is 24 dB, which means that all noise should be
at least at -33 dBm, which is about 0.0000005 watts. But a 40 volts
hum across a 600 ohm impedance is 2.7 watts, and there is roughly
67 dB difference!
Can you explain how your measuring or have that configured please?..
Do you have any idea how many telephone lines actually have a 67
dB SNR?
So? "Very locally" can mean more than a *mile*...
Often less than in ntl or telewest installations but longer in BT ones.
Ntl care the cableco in the UK but that name is to disappear and their
to be called Virgin!...
What do you mean by "BT system the copper is longer but doesn't
humm.."? They have hum resistance copper??? ;-)
Nope;!, just a way of putting that, see above,...
Virtually *all* "multicore" telecom cable is shielded. (Some
customer premise cable is not. But you won't find anything
within a telephone central office that isn't, and you won't find
any outside plant distribution cable that isn't.)
In a central office most all of it here is twisted pair. I think some
terminology things betwixt the UK and USA are showing up here. All the
cable co Telco multicores I've seen, though not all, are unshielded.
What do you define shielding as, just a wrap of aluminium foil with a
drain wire or a fully woven copper mesh?..
Where are you coming up with these ideas? Have you ever even
seen the specs for any of this?
Heh heh, lets see you try picking any holes in it!
OK then, part 2 "On the other hand cable shields which are bonded at one
end etc". Read that thorough carefully, doesn't make sense. Then take a
lump of Andrews 4-50 Heliax and see what a good radiator that is even
greater number of wavelengths . They didn't even state if it were open
circuit or terminated on a load...
Actually we've had a lot of EMC experience over the years in radio,
audio and automotive environments and what's made by far and away the
biggest effect is bypassing of transistor junctions at RF
frequencies....
You didn't read it, did you? It *does* affect the signal pairs.
It reduces the noise on them, significantly.
Were is this noise coming from then?...
In practice, what they showed was that it improves noise
immunity.
"Floating" makes no difference at all.
Well think about that, Say we have a cable the inner pairs are wrapped
around one of the power lines that you describe, and there are a LOT of
volts induced on that wiring. OK now into a transformer there will be
galvanic isolation i.e. the ends or centre tap of that transformer isn't
connected to anything. Now take a electronically balanced input. At some
point that will be connected to say an input IC which will have supply
rails etc, and that IC will be coupled through to the output of that
line receiving amplifier now don't you think that if there were some
matter of kilovolts on said line, then that will break down the
transistor junctions ?..
Longitudinal balance is
the most significant factor. Magnetic shielding is ineffective
below about 10 kHz, and reverse induction via the shield (by
grounding it at both ends) is much more significant for power
line frequencies and their harmonics (which commonly exist up to
2 or 3 kHz).
I think you have that wrong. Provided that the rejection is what it
should be then whatever is induced on the pairs will cancel out.
We could all agree that common mode rejection is not always
sufficient, and that reverse induction is virtually *always*
applied to outside plant communications cables because of that.
Exactly what you mean by "balanced working", I'm not sure.
What we've been discussing. Take a signal source and connect a
transformer thereto and connect that to a pair of wires twisted together
and then connect that to another transformer and the out put winding of
that to a load. That do?..
It is rarely used for critical circuits where induction
interference from power lines would be important. (For obvious
reasons...)
Yes..
Nothing you have said suggests it could possibly hum, given that
you have not mentioned the presence of any power line related
equipment at all. If this thing is located out in the ocean, on
a floating barge that has no AC electric power, it won't hum.
On the other hand, if you place a fluorescent light fixture close
to it, it might well hum!
Why?.
Regardless, that is one of the worst possible ways to wire 10
meters of cable to a microphone.
Yes agreed and you wouldn't do that, well not in a pro environment
anyway.
Now if say you ground that to the local mains earth at one end, and say
10 meters away at the microphone case end earth that to a driven rod
earth, will it or wont it hummmmmmmmmmm?.....