NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/dcom/modems-part1/701

Internet Message Format | 1996-08-05 | 3.7 KB

Path: polarnet.com!floyd
From: [e-mail] (Floyd Davidson)
Newsgroups: misc.consumers.house,sci.electronics.misc,comp.dcom.modems
Subject: Re: 60Hz buzz on phone line & modem problems
Date: 7 Jan 1996 22:26:06 GMT
Organization: __________
Message-ID: <4cph9u$[e-mail]>
References: <4cf1le$[e-mail]> <[e-mail]> <[e-mail]>
Reply-To: [e-mail]
NNTP-Posting-Host: tanana.imagi.net

In article <[e-mail]>,
A. E. Siegman <[e-mail]> wrote:
>     Besides disconnecting phone lines or phone units one by one to
>see what happens, you could try opening your _circuit breakers_ one
>by one, to see if any particular electrical circuit is the culprit.
>
>     (You may have lots of clocks and other stuff to reset after
>doing this, however.)

That is one good reason not to bother, but a better one is that it
is not likely to be of any benefit, because the culprit will not
be just any particular electrical circuit.

The nature of a twisted pair balanced telephone line is that
"noise" induced into the pair is equally applied to both sides of
the pair.  The desired signal is applied differentially, or
unequally to the two wires.  Hence the receiving end will see the
induced "noise" as equal on both wires, and ignore it (called
common mode rejection) while seeing the signal as the difference
in voltages between the two wires.

Hence, to the degree that the twisted pair is balanced, it is
immune to 60 Hz hum from power circuits.  Even though that
immunity is relatively great, it is still a good idea to not run
long lengths of telephone cable immediately next to power wiring.
But "long" means many yards, and "next to" means within an inch,
and doesn't normally exist as an opportunity in most homes.

The 60 Hz hum on a telephone line is an indirect way of measuring
the imbalance of the telephone circuit.  The amount of 60 Hz
energy in various electical fields through which the cable passes
is _great_, and no one circuit in any given house is likely to
be contributing even a significant percentage.  Hence you can
cut the main power source to not only your house, but to the whole
block where you live, and the 60 Hz hum from an unbalanced line
will not go away!

There are two likely causes of 60 Hz hum on a telephone line.
One is a relative short to ground on either tip or ring wire of
the pair.  Commonly this results in not only 60 Hz hum but a
frying sound very much like hot grease on a griddle.  Detecting
such a condition is easy because it can be measured with a typical
inexpensive ohm meter.

But 60 Hz hum with no crackling sounds is more likely not a short
to ground as such.  It is an imbalance which does NOT cause any
current to flow from either wire of the twisted pair to ground.
Examples would be poor insulation that causes a connection to a
large surface that is not grounded, such as a wet surface or to
things like air duct metal.  Even something as simple as stapling
a cable to the wall and hitting one wire with the staple will
unbalance the cable enough to induce noise.

The most common cause of unbalanced cable hum is using poor cable!
That nice flat satin cord with modular connectors on it is NOT
well balanced (it isn't twisted), and the more of it used the more
60 Hz hum should be expected.

For testing, I go along with the proceedure that others have
mentioned, of getting a known good telephone set, and trying it at
the demarc point.  If it is bad there, call the phone company.  If
it is good there, start disconnecting various parts of the
in-house telephone cabling until the bad cable or piece of
equipment is isolated.

  Floyd
--
Floyd L. Davidson          Salcha, Alaska         [e-mail]