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The phone line
A telephone is usually connected to the telephone exchange by about three
miles (4.83 km) of a twisted pair of No.22 (AWG) or 0.5 mm copper wires, known
by your phone company as "the loop". Although copper is a good conductor,
it does have resistance. The resistance of No.22 AWG wire is 16.46 Ohms per thousand
feet at 77 degrees F (25 degrees C). In the United States, wire resistance is
measured in Ohms per thousand feet; telephone companies describe loop length
in kilo feet (thousands of feet). In other parts of the world, wire resistance
is usually expressed as Ohms per kilometer.
Because telephone apparatus is generally considered to be current driven,
all phone measurements refer to current consumption, not voltage. The length
of the wire connecting the subscriber to the telephone exchange affects
the total amount of current that can be drawn by anything attached at the subscriber's
end of the line.
In the United States, the voltage applied to the line to drive the telephone
is 48 VDC; some countries use 50 VDC. Note that telephones are peculiar in that
the signal line is also the power supply line. The voltage is supplied by lead
acid cells, thus assuring a hum-free supply and complete independence from the
electric company, which may be especially useful during power outages.
At the telephone exchange the DC voltage and audio signal are separated
by directing the audio signal through 2 uF capacitors and blocking the audio
from the power supply with a 5-Henry choke in each line. Usually these two chokes
are the coil windings of a relay that switches your phone line at the exchange;
in the United States, this relay is known as the "A" relay (see
fig.1). The resistance of each of these chokes is 200 Ohms.
We can find out how well a phone line is operating by using Ohm's law and
an ammeter. The DC resistance of any device attached to the phone line is
often quoted in telephone company specifications as 200 Ohms; this will vary
in practice from between 150 to 1,000 Ohms. You can measure the DC resistance
of your phone with an Ohmmeter. Note this is DC resistance, not impedance.
Using these figures you can estimate the distance between your telephone
and the telephone exchange. In the United States, the telephone company guarantees
you no lower current than 20 mA - or what is known to your phone company as a "long
loop." A "short loop" will draw 50 to 70 mA, and an average loop,
about 35 mA. Some countries will consider their maximum loop as low as 12 mA.
In practice, United States telephones are usually capable of working at currents
as low as 14 mA. Some exchanges will consider your phone in use and feed dial
tone down the line with currents as low as 8 mA, even though the telephone may
not be able to operate.
Although the telephone company has supplied plenty of nice clean DC direct
to your home, don't assume you have a free battery for your own circuits. The
telephone company wants the DC resistance of your line to be about 10 megOhms
when there's no apparatus in use ("on hook," in telephone company jargon);
you can draw no more than 5 microamperes while the phone is in that state. When
the phone is in use, or "off hook," you can draw current, but you will
need that current to power your phone, any current you might draw for other purposes
would tend to lower the signal level.
The phone line has an impedance composed of distributed resistance, capacitance,
and inductance. The impedance will vary according to the length of the loop,
the type of insulation of the wire, and whether the wire is aerial cable,
buried cable, or bare parallel wires strung on telephone poles. For calculation and
specification purposes, the impedance is normally assumed to be 600 to 900 Ohms.
If the instrument attached to the phone line should be of the wrong impedance,
you would get a mismatch, or what telephone company personnel refer to as "return
loss." (Radio Amateurs will recognize return loss as SWR.) A mismatch on
telephone lines results in echo and whistling, which the phone company calls "singing" and
owners of very cheap telephones may have come to expect. A mismatched device
can, by the way, be matched to the phone line by placing resistors in parallel
or series with the line to bring the impedance of the device to within the desired
limits. This will cause some signal loss, of course, but will make the device
usable.
A phone line is balanced feed, with each side equally balanced to ground.
Any imbalance will introduce hum and noise to the phone line and increase susceptibility
to RFI.
The balance of the phone line is known to your telephone company as "longitudinal
balance." If both impedance match and balance to ground are kept in mind,
any device attached to the phone line will perform well, just as the correct
matching of transmission lines and devices will ensure good performance
in radio practice.
If you live in the United States, the two phone wires connected to your
telephone should be red and green. (In other parts of the world they may
be different colors.) The red wire is negative and the green wire is positive.
Your telephone company calls the green wire "Tip" and the red
wire "Ring". (In other parts of the world, these wires may be
called "A" and "B".) Most installations have another
pair of wires, yellow and black. These wires can be used for many different purposes,
if they are used at all. Some party lines use the yellow wire as a ground; sometimes
there's 6.8 VAC on this pair to light the dials of Princess type phones. If you
have two separate phone lines (not extensions) in your home, you will find the
yellow and black pair carrying a second telephone line. In this case, black is "Tip" and
yellow is "Ring."
The above description applies to a standard line with a DC connection between
your end of the line and the telephone exchange. Most phone lines in the
world are of this type, known as a "metallic line." In a metallic line,
there may or may not be inductance devices placed in the line to alter the
frequency response of the line; the devices used to do this are called "loading
coils." (Note: if they impair the operation of your modem, your telephone
company can remove them.) Other types of lines are party lines, which may be
metallic lines but require special telephones to allow the telephone company
to differentiate between subscribers. Very long lines may have amplifiers,
sometimes called "loop extenders" on them. Some telephone companies
use a system called "subscriber carrier," which is basically an
RF system in which your telephone signal is heterodyned up to around 100 KHz
and then sent along another subscriber's "twisted pair."
If you have questions about your telephone line, you can call your telephone
company; depending on the company and who you can reach, you may be able to obtain
a wealth of information.
Click on one of the items below to learn
more.
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