The article below is not for publication, but for archiving in the
tutorial section.
UNDERSTANDING TELEPHONES
by
Julian Macassey, N6ARE
First Published
in
Ham Radio Magazine
September 1985
Everybody has one, but what makes it work?
Although telephones and telephone company practices may vary
dramatically from one locality to another, the basic principles
underlying the way they work remain unchanged.
Every telephone consists of three separate subassemblies,
each capable of independent operation. These assemblies are the
speech network, the dialing mechanism, and the ringer or bell.
Together, these parts - as well as any additional devices such as
modems, dialers, and answering machines - are attached to the
phone line.
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 kilofeet (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 call
call your telephone company; depending on the company and who you
can reach, you may be able to obtain a wealth of information.
The Speech Network
The speech network - also known as the "hybrid" or the "two
wire/four wire network" - take the incoming signal and feeds it
to the earpiece and takes the microphone output and feeds it down
the line. The standard network used all over the world is an LC
device with a carbon microphone; some newer phones use discrete
transistors or ICs.
One of the advantages of an LC network is that it has no
semiconductors, is not voltageensitive, and will work
continuously as the voltage across the line is reduced. Many
transistorized phones stop working as the voltage approaches 3 to
4 Volts.
When a telephone is taken off the hook, the line voltage
drops from 48 Volts to between 9 and 3 Volts, depending on the
length of the loop. If another telephone in parallel is taken
off the hook, the current consumption of the line will remain the
same and the voltage across the terminals of both telephones will
drop. Bell Telephone specifications state that three telephones
should work in parallel on a 20 mA loop; transistorized phones
tend not to pass this test, although some manufacturers use ICs
that will pass. Although some European telephone companies claim
that phones working in parallel is "technically impossible," and
discourage attempts to make them work that way, some of their
telephones will work in parallel.
While low levels of audio may be difficult to hear, overly
loud audio can be painful. Consequently, a well designed
telephone will automatically adjust its transmit and receive
levels to allow for the attenuation - or lack of it - caused by
the length of the loop. This adjustment is called "loop
compensation." In the United States, telephone manufacturers
achieve this compensation with silicon carbide varistors that
consume any excess current from a short loop (see fig. 2).
Although some telephones using ICs have built-in loop
compensation, many do not; the latter have been designed to
provide adequate volume on the average loop, which means that
they provide low volume on long loops, and are too loud on short
loops. Various countries have different specifications for
transmit and receive levels; some European countries require a
higher transmit level than is standard in the United States so a
domestically-manufactured telephone may suffer from low transmit
level if used on European lines without modification.
Because a telephone is a duplex device, both transmitting
and receiving on the same pair of wires, the speech network must
ensure that not too much of the caller's voice is fed back into
his or her receiver. This function, called "sidetone," is
achieved by phasing the signal so that some cancellation occurs
in the speech network before the signal is fed to the receiver.
Callers faced with no sidetone at all will consider the phone
"dead." Too little sidetone will convince callers that they're
not being heard and cause them to shout, "I can hear you. Can
you hear ME?" Too much sidetone causes callers to lower their
voices and not be heard well at the other end of the line.
A telephone on a short loop with no loop compensation will
appear to have too much sidetone, and callers will lower their
voices. In this case, the percentage of sidetone is the same,
but as the overall level is higher the sidetone level will also
be higher. overall level
The Dial
There are two types of dials in use around the world. The
most common one is called pulse, loop disconnect, or rotary; the
oldest form of dialing, it's been with us since the 1920's. The
other dialing method, more modern and much loved by Radio
Amateurs is called Touch-tone, Dual Tone Multi-Frequency (DTMF)
or Multi-Frequency (MF) in Europe. In the U.S. MF means single
tones used for system control.
Pulse dialing is traditionally accomplished with a rotary
dial, which is a speed governed wheel with a cam that opens and
closes a switch in series with your phone and the line. It works
by actually disconnecting or "hanging up" the telephone at
specific intervals. The United States standard is one disconnect
per digit, so if you dial a "1," your telephone is
"disconnected" once. Dial a seven and you'll be "disconnected"
seven times; dial a zero, and you'll "hang up " ten times. Some
countries invert the system so "1" causes ten "disconnects" and
0, one disconnect. Some add a digit so that dialing a 5 would
cause six disconnects and 0, eleven disconnects. There are even
some systems in which dialing 0 results in one disconnect, and
all other digits are plus one, making a 5 cause six disconnects
and 9, ten disconnects.
Although most exchanges are quite happy with rates of 6 to
15 Pulses Per Second (PPS), the phone company accepted standard
is 8 to 10 PPS. Some modern digital exchanges, free of the
mechanical inertia problems of older systems, will accept a PPS
rate as high as 20.
Besides the PPS rate, the dialing pulses have a make/break
ratio, usually described as a percentage, but sometimes as a
straight ratio. The North American standard is 60/40 percent;
most of Europe accepts a standard of 63/37 percent. This is the
pulse measured at the telephone, not at the exchange, where it's
somewhat different, having traveled through the phone line with
its distributed resistance, capacitance, and inductance. In
practice, the make/break ratio does not seem to affect the
performance of the dial when attached to a normal loop. Bear in
mind that each pulse is a switch connect and disconnect across a
complex impedance, so the switching transient often reaches 300
Volts. Try not to have your fingers across the line when
dialing.
Most pulse dialing phones produced today use a CMOS IC and a
keyboard. Instead of pushing your finger round in circles, then
removing your finger and waiting for the dial to return before
dialing the next digit, you punch the button as fast as you want.
The IC stores the number and pulses it out at the correct rate
with the correct make/break ratio and the switching is done with
a high-voltage switching transistor. Because the IC has already
stored the dialed number in order to pulse it out at the correct
rate, it's a simple matter for telephone designers to keep the
memory "alive" and allow the telephone to store, recall, and
redial the Last Number Dialed (LND). This feature enables you to
redial by picking up the handset and pushing just one button.
Because pulse dialing entails rapid connection and disconnection
of the phone line, you can "dial" a telephone that has lost its
dial, by hitting the hook-switch rapidly. It requires some
practice to do this with consistent success, but it can be done.
A more sophisticated approach is to place a Morse key in series
with the line, wire it as normally closed and send strings of
dots corresponding to the digits you wish to dial.
Touch tone, the most modern form of dialing, is fast and
less prone to error than pulse dialing. Compared to pulse, its
major advantage is that its audio band signals can travel down
phone lines further than pulse, which can travel only as far as
your local exchange. Touch-tone can therefore send signals
around the world via the telephone lines, and can be used to
control phone answering machines and computers. Pulse dialing is
to touch-tone as FSK or AFSK RTTY is to Switched Carrier RTTY,
where mark and space are sent by the presence or absence of DC or
unmodulated RF carrier. Most Radio Amateurs are familiar with
DTMF for controlling repeaters and for accessing remote and auto
phone patches.
Bell Labs developed DTMF in order to have a dialing system
that could travel across microwave links and work rapidly with
computer controlled exchanges. Each transmitted digit consists
of two separate audio tones that are mixed together (see fig.3).
The four vertical columns on the keypad are known as the high
group and the four horizontal rows as the low group; the digit 8
is composed of 1336 Hz and 852 Hz. The level of each tone is
within 3 dB of the other, (the telephone company calls this
"Twist"). A complete touch-tone pad has 16 digits, as opposed to
ten on a pulse dial. Besides the numerals 0 to 9, a DTMF "dial"
has *, #, A, B, C, and D. Although the letters are not normally
found on consumer telephones, the IC in the phone is capable of
generating them.
The * sign is usually called "star" or "asterisk." The #
sign, often referred to as the "pound sign." is actually called
an octothorpe. Although many phone users have never used these
digits - they are not, after all, ordinarily used in dialing
phone numbers - they are used for control purposes, phone
answering machines, bringing up remote bases, electronic banking,
and repeater control. The one use of the octothorpe that may be
familiar occurs in dialing international calls from phones in the
United States. After dialing the complete number, dialing the
octothorpe lets the exchange know you've finished dialing. It
can now begin routing your call; without the octothorpe, it would
wait and "time out" before switching your call.
When DTMF dials first came out they had complicated cams and
switches for selecting the digits and used a transistor
oscillator with an LC tuning network to generate the tones.
Modern dials use a matrix switch and a CMOS IC that synthesizes
the tones from a 3.57MHz (TV color burst) crystal. This
oscillator runs only during dialing, so it doesn't normally
produce QRM.
Standard DTMF dials will produce a tone as long as a key is
depressed. No matter how long you press, the tone will be
decoded as the appropriate digit. The shortest duration in which
a digit can be sent and decoded is about 100 milliseconds (ms).
It's pretty difficult to dial by hand at such a speed, but
automatic dialers can do it. A twelve-digit long distance number
can be dialed by an automatic dialer in a little more than a
second - about as long as it takes a pulse dial to send a single
0 digit.
The output level of DTMF tones from your telephone should be
between 0 and -12 dBm. In telephones, 0 dB is 1 miliwatt over
600 Ohms. So 0 dB is 0.775 Volts. Because your telephone is
considered a 600 Ohm load, placing a voltmeter across the line
will enable you to measure the level of your tones.
The Ringer
Simply speaking this is a device that alerts you to an
incoming call. It may be a bell, light, or warbling tone. The
telephone company sends a ringing signal which is an AC waveform.
Although the common frequency used in the United States is 20 HZ,
it can be any frequency between 15 and 68 Hz. Most of the world
uses frequencies between 20 and 40 Hz. The voltage at the
subscribers end depends upon loop length and number of ringers
attached to the line; it could be between 40 and 150 Volts. Note
that ringing voltage can be hazardous; when you're working on a
phone line, be sure at least one telephone on the line is off the
hook (in use); if any are not, take high voltage precautions.
The telephone company may or may not remove the 48 VDC during
ringing; as far as you're concerned, this is not important.
Don't take chances.
The ringing cadence - the timing of ringing to pause -
varies from company to company. In the United States the cadence
is normally 2 seconds of ringing to 4 seconds of pause. An
unanswered phone in the United States will keep ringing until the
caller hangs up. But in some countries, the ringing will "time
out" if the call is not answered.
The most common ringing device is the gong ringer, a
solenoid coil with a clapper that strikes either a single or
double bell. A gong ringer is the loudest signaling device that
is solely phone-line powered.
Modern telephones tend to use warbling ringers, which are
usually ICs powered by the rectified ringing signal. The audio
transducer is either a piezoceramic disk or a small loudspeaker
via a transformer.
Ringers are isolated from the DC of the phone line by a
capacitor. Gong ringers in the United States use a 0.47 uF
capacitor. Warbling ringers in the United States generally use a
1.0 uF capacitor. Telephone companies in other parts of the
world use capacitors between 0.2 and 2.0 uF. The paper
capacitors of the past have been replaced almost exclusively with
capacitors made of Mylar film. Their voltage rating is always
250 Volts.
The capacitor and ringer coil, or Zeners in a warbling
ringer, constitute a resonant circuit. When your phone is hung
up ("on hook") the ringer is across the line; if you have turned
off the ringer you have merely silenced the transducer, not
removed the circuit from the line.
When the telephone company uses the ringer to test the line,
it sends a low-voltage, low frequency signal down the line
usually 2 Volts at 10 Hz) to test for continuity. The company
keeps records of the expected signals on your line. This is how
it can tell you have added equipment to your line. If your
telephone has had its ringer disconnected, the telephone company
cannot detect its presence on the line.
Because there is only a certain amount of current available
to drive ringers, if you keep adding ringers to your phone line
you will reach a point at which either all ringers will cease to
ring, some will cease to ring, or some ringers will ring weakly.
In the United States the phone company will guarantee to ring
five normal ringers. A normal ringer is defined as a standard
gong ringer as supplied in a phone company standard desk
telephone. Value given to this ringer is Ringer Equivalence
Number (REN) 1. If you look at the FCC registration label of
your telephone, modem, or other device to be connected to the
phone line, you'll see the REN number. It can be as high as 3.2,
which means that device consumes the equivalent power of 3.2
standard ringers, or 0.0, which means it consumes no current when
subjected to a ringing signal. If you have problems with
ringing, total up your RENs; if the total is greater than 5,
disconnect ringers until your REN is at 5 or below.
Other countries have various ways of expressing REN, and
some systems will handle no more than three of their standard
ringers. But whatever the system, if you add extra equipment and
the phones stop ringing, or the phone answering machine won't
pick up calls, the solution is disconnect ringers until the
problem is resolved. Warbling ringers tend to draw less current
than gong ringers, so changing from gong ringers to warbling
ringers may help you spread the sound better.
Frequency response is the second criterion by which a ringer
is described. In the United States most gong ringers are
electromechanically resonant. They are usually resonant at 20
and 30 Hz (+&- 30 Hz). The FCC refers to this as A so a normal
gong ringer is described as REN 1.0A. The other common frequency
response is known as type B. Type B ringers will respond to
signals between 15.3 and 68.0 Hz. Warbling ringers are all type
B and some United States gong ringers are type B. Outside the
United States, gong ringers appear to be non-frequency selective,
or type B.
Because a ringer is supposed to respond to AC waveforms, it
will tend to respond to transients (such as switching transients)
when the phone is hung up, or when the rotary dial is used on an
extension phone. This is called "bell tap" in the United States;
in other countries, it's often called "bell tinkle." While
European and Asian phones tend to bell tap, or tinkle, United
States ringers that bell tap are considered defective. The bell
tap is designed out of gong ringers and fine tuned with bias
springs. Warbling ringers for use in the United States are
designed not to respond to short transients; this is usually
accomplished by rectifying the AC and filtering it before it
powers the IC, then not switching on the output stage unless the
voltage lasts long enough to charge a second capacitor.
Conclusion
This brief primer describing the working parts of a
telephone is intended to provide a better understanding of phone
equipment. Note that most telephone regulatory agencies,
including the FCC, forbid modification of anything that has been
previously approved or attached to phone lines.
