627 lines
31 KiB
Plaintext
627 lines
31 KiB
Plaintext
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The article below is not for publication, but for archiving in the tutorial section.
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UNDERSTANDING TELEPHONES
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by
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Julian Macassey, N6ARE
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First Published
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in
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Ham Radio Magazine
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September 1985
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Everybody has one, but what makes it work?
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Although telephones and telephone company practices may vary
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dramatically from one locality to another, the basic principles
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underlying the way they work remain unchanged.
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Every telephone consists of three separate subassemblies,
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each capable of independent operation. These assemblies are the
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speech network, the dialing mechanism, and the ringer or bell.
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Together, these parts - as well as any additional devices such as
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modems, dialers, and answering machines - are attached to the
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phone line.
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The phone line
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A telephone is usually connected to the telephone exchange
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by about three miles (4.83 km) of a twisted pair of No.22 (AWG)
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or 0.5 mm copper wires, known by your phone company as "the
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loop". Although copper is a good conductor, it does have
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resistance. The resistance of No.22 AWG wire is 16.46 Ohms per
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thousand feet at 77 degrees F (25 degrees C). In the United
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States, wire resistance is measured in Ohms per thousand feet;
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telephone companies describe loop length in kilofeet (thousands
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of feet). In other parts of the world, wire resistance is
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usually expressed as Ohms per kilometer.
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Because telephone apparatus is generally considered to be
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current driven, all phone measurements refer to current
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consumption, not voltage. The length of the wire connecting the
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subscriber to the telephone exchange affects the total amount of
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current that can be drawn by anything attached at the
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subscriber's end of the line.
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In the United States, the voltage applied to the line to
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drive the telephone is 48 VDC; some countries use 50 VDC. Note
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that telephones are peculiar in that the signal line is also the
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power supply line. The voltage is supplied by lead acid cells,
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thus assuring a hum-free supply and complete independence from
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the electric company, which may be especially useful during power
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outages.
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At the telephone exchange the DC voltage and audio signal
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are separated by directing the audio signal through 2 uF
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capacitors and blocking the audio from the power supply with a 5-
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Henry choke in each line. Usually these two chokes are the coil
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windings of a relay that switches your phone line at the
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exchange; in the United States, this relay is known as the "A"
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relay (see fig.1). The resistance of each of these chokes is 200
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Ohms.
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We can find out how well a phone line is operating by using
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Ohm's law and an ammeter. The DC resistance of any device
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attached to the phone line is often quoted in telephone company
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specifications as 200 Ohms; this will vary in practice from
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between 150 to 1,000 Ohms. You can measure the DC resistance of
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your phone with an Ohmmeter. Note this is DC resistance, not
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impedance.
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Using these figures you can estimate the distance between
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your telephone and the telephone exchange. In the United States,
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the telephone company guarantees you no lower current than 20 mA
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- or what is known to your phone company as a "long loop." A
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"short loop" will draw 50 to 70 mA, and an average loop, about 35
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mA. Some countries will consider their maximum loop as low as 12
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mA. In practice, United States telephones are usually capable of
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working at currents as low as 14 mA. Some exchanges will
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consider your phone in use and feed dial tone down the line with
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currents as low as 8 mA, even though the telephone may not be
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able to operate.
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Although the telephone company has supplied plenty of nice
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clean DC direct to your home, don't assume you have a free
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battery for your own circuits. The telephone company wants the
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DC resistance of your line to be about 10 megOhms when there's no
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apparatus in use ("on hook," in telephone company jargon); you
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can draw no more than 5 microamperes while the phone is in that
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state. When the phone is in use, or "off hook," you can draw
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current, but you will need that current to power your phone, any
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current you might draw for other purposes would tend to lower the
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signal level.
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The phone line has an impedance composed of distributed
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resistance, capacitance, and inductance. The impedance will vary
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according to the length of the loop, the type of insulation of
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the wire, and whether the wire is aerial cable, buried cable, or
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bare parallel wires strung on telephone poles. For calculation
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and specification purposes, the impedance is normally assumed to
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be 600 to 900 Ohms. If the instrument attached to the phone line
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should be of the wrong impedance, you would get a mismatch, or
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what telephone company personnel refer to as "return loss."
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(Radio Amateurs will recognize return loss as SWR.) A mismatch
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on telephone lines results in echo and whistling, which the phone
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company calls "singing" and owners of very cheap telephones may
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have come to expect. A mismatched device can, by the way, be
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matched to the phone line by placing resistors in parallel or
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series with the line to bring the impedance of the device to
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within the desired limits. This will cause some signal loss, of
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course, but will make the device usable.
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A phone line is balanced feed, with each side equally
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balanced to ground. Any imbalance will introduce hum and noise
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to the phone line and increase susceptibility to RFI.
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The balance of the phone line is known to your telephone
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company as "longitudinal balance." If both impedance match and
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balance to ground are kept in mind, any device attached to the
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phone line will perform well, just as the correct matching of
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transmission lines and devices will ensure good performance in
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radio practice.
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If you live in the United States, the two phone wires
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connected to your telephone should be red and green. (In other
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parts of the world they may be different colors.) The red wire
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is negative and the green wire is positive. Your telephone
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company calls the green wire "Tip" and the red wire "Ring". (In
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other parts of the world, these wires may be called "A" and "B".)
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Most installations have another pair of wires, yellow and black.
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These wires can be used for many different purposes, if they are
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used at all. Some party lines use the yellow wire as a ground;
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sometimes there's 6.8 VAC on this pair to light the dials of
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Princess type phones. If you have two separate phone lines (not
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extensions) in your home, you will find the yellow and black pair
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carrying a second telephone line. In this case, black is "Tip"
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and yellow is "Ring."
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The above description applies to a standard line with a DC
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connection between your end of the line and the telephone
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exchange. Most phone lines in the world are of this type, known
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as a "metallic line." In a metallic line, there may or may not
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be inductance devices placed in the line to alter the frequency
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response of the line; the devices used to do this are called
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"loading coils." (Note: if they impair the operation of your
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modem, your telephone company can remove them.) Other types of
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lines are party lines, which may be metallic lines but require
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special telephones to allow the telephone company to
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differentiate between subscribers. Very long lines may have
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amplifiers, sometimes called "loop extenders" on them. Some
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telephone companies use a system called "subscriber carrier,"
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which is basically an RF system in which your telephone signal is
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heterodyned up to around 100 Khz and then sent along another
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subscriber's "twisted pair."
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If you have questions about your telephone line, you call
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call your telephone company; depending on the company and who you
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can reach, you may be able to obtain a wealth of information.
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The Speech Network
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The speech network - also known as the "hybrid" or the "two
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wire/four wire network" - take the incoming signal and feeds it
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to the earpiece and takes the microphone output and feeds it down
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the line. The standard network used all over the world is an LC
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device with a carbon microphone; some newer phones use discrete
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transistors or ICs.
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One of the advantages of an LC network is that it has no semiconductors, is not voltageensitive, and will work
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continuously as the voltage across the line is reduced. Many
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transistorized phones stop working as the voltage approaches 3 to
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4 Volts.
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When a telephone is taken off the hook, the line voltage
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drops from 48 Volts to between 9 and 3 Volts, depending on the
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length of the loop. If another telephone in parallel is taken
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off the hook, the current consumption of the line will remain the
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same and the voltage across the terminals of both telephones will
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drop. Bell Telephone specifications state that three telephones
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should work in parallel on a 20 mA loop; transistorized phones
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tend not to pass this test, although some manufacturers use ICs
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that will pass. Although some European telephone companies claim
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that phones working in parallel is "technically impossible," and
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discourage attempts to make them work that way, some of their
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telephones will work in parallel.
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While low levels of audio may be difficult to hear, overly
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loud audio can be painful. Consequently, a well designed
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telephone will automatically adjust its transmit and receive
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levels to allow for the attenuation - or lack of it - caused by
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the length of the loop. This adjustment is called "loop
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compensation." In the United States, telephone manufacturers
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achieve this compensation with silicon carbide varistors that
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consume any excess current from a short loop (see fig. 2).
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Although some telephones using ICs have built-in loop
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compensation, many do not; the latter have been designed to
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provide adequate volume on the average loop, which means that
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they provide low volume on long loops, and are too loud on short
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loops. Various countries have different specifications for
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transmit and receive levels; some European countries require a
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higher transmit level than is standard in the United States so a
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domestically-manufactured telephone may suffer from low transmit
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level if used on European lines without modification.
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Because a telephone is a duplex device, both transmitting
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and receiving on the same pair of wires, the speech network must
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ensure that not too much of the caller's voice is fed back into
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his or her receiver. This function, called "sidetone," is
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achieved by phasing the signal so that some cancellation occurs
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in the speech network before the signal is fed to the receiver.
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Callers faced with no sidetone at all will consider the phone
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"dead." Too little sidetone will convince callers that they're
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not being heard and cause them to shout, "I can hear you. Can
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you hear ME?" Too much sidetone causes callers to lower their
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voices and not be heard well at the other end of the line.
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A telephone on a short loop with no loop compensation will
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appear to have too much sidetone, and callers will lower their
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voices. In this case, the percentage of sidetone is the same,
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but as the overall level is higher the sidetone level will also
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be higher. overall level
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The Dial
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There are two types of dials in use around the world. The
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most common one is called pulse, loop disconnect, or rotary; the
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oldest form of dialing, it's been with us since the 1920's. The
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other dialing method, more modern and much loved by Radio
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Amateurs is called Touch-tone, Dual Tone Multi-Frequency (DTMF)
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or Multi-Frequency (MF) in Europe. In the U.S. MF means single
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tones used for system control.
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Pulse dialing is traditionally accomplished with a rotary
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dial, which is a speed governed wheel with a cam that opens and
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closes a switch in series with your phone and the line. It works
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by actually disconnecting or "hanging up" the telephone at
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specific intervals. The United States standard is one disconnect
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per digit, so if you dial a "1," your telephone is
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"disconnected" once. Dial a seven and you'll be "disconnected"
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seven times; dial a zero, and you'll "hang up " ten times. Some
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countries invert the system so "1" causes ten "disconnects" and
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0, one disconnect. Some add a digit so that dialing a 5 would
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cause six disconnects and 0, eleven disconnects. There are even
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some systems in which dialing 0 results in one disconnect, and
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all other digits are plus one, making a 5 cause six disconnects
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and 9, ten disconnects.
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Although most exchanges are quite happy with rates of 6 to
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15 Pulses Per Second (PPS), the phone company accepted standard
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is 8 to 10 PPS. Some modern digital exchanges, free of the
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mechanical inertia problems of older systems, will accept a PPS
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rate as high as 20.
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Besides the PPS rate, the dialing pulses have a make/break
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ratio, usually described as a percentage, but sometimes as a
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straight ratio. The North American standard is 60/40 percent;
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most of Europe accepts a standard of 63/37 percent. This is the
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pulse measured at the telephone, not at the exchange, where it's
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somewhat different, having traveled through the phone line with
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its distributed resistance, capacitance, and inductance. In
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practice, the make/break ratio does not seem to affect the
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performance of the dial when attached to a normal loop. Bear in
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mind that each pulse is a switch connect and disconnect across a
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complex impedance, so the switching transient often reaches 300
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Volts. Try not to have your fingers across the line when
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dialing.
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Most pulse dialing phones produced today use a CMOS IC and a
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keyboard. Instead of pushing your finger round in circles, then
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removing your finger and waiting for the dial to return before
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dialing the next digit, you punch the button as fast as you want.
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The IC stores the number and pulses it out at the correct rate
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with the correct make/break ratio and the switching is done with
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a high-voltage switching transistor. Because the IC has already
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stored the dialed number in order to pulse it out at the correct
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rate, it's a simple matter for telephone designers to keep the
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memory "alive" and allow the telephone to store, recall, and
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redial the Last Number Dialed (LND). This feature enables you to
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redial by picking up the handset and pushing just one button.
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Because pulse dialing entails rapid connection and disconnection
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of the phone line, you can "dial" a telephone that has lost its
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dial, by hitting the hook-switch rapidly. It requires some
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practice to do this with consistent success, but it can be done.
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A more sophisticated approach is to place a Morse key in series
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with the line, wire it as normally closed and send strings of
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dots corresponding to the digits you wish to dial.
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Touch tone, the most modern form of dialing, is fast and
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less prone to error than pulse dialing. Compared to pulse, its
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major advantage is that its audio band signals can travel down
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phone lines further than pulse, which can travel only as far as
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your local exchange. Touch-tone can therefore send signals
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around the world via the telephone lines, and can be used to
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control phone answering machines and computers. Pulse dialing is
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to touch-tone as FSK or AFSK RTTY is to Switched Carrier RTTY,
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where mark and space are sent by the presence or absence of DC or
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unmodulated RF carrier. Most Radio Amateurs are familiar with
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DTMF for controlling repeaters and for accessing remote and auto
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phone patches.
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Bell Labs developed DTMF in order to have a dialing system
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that could travel across microwave links and work rapidly with
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computer controlled exchanges. Each transmitted digit consists
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of two separate audio tones that are mixed together (see fig.3).
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The four vertical columns on the keypad are known as the high
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group and the four horizontal rows as the low group; the digit 8
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is composed of 1336 Hz and 852 Hz. The level of each tone is
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within 3 dB of the other, (the telephone company calls this
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"Twist"). A complete touch-tone pad has 16 digits, as opposed to
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ten on a pulse dial. Besides the numerals 0 to 9, a DTMF "dial"
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has *, #, A, B, C, and D. Although the letters are not normally
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found on consumer telephones, the IC in the phone is capable of
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generating them.
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The * sign is usually called "star" or "asterisk." The #
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sign, often referred to as the "pound sign." is actually called
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an octothorpe. Although many phone users have never used these
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digits - they are not, after all, ordinarily used in dialing
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phone numbers - they are used for control purposes, phone
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answering machines, bringing up remote bases, electronic banking,
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and repeater control. The one use of the octothorpe that may be
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familiar occurs in dialing international calls from phones in the
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United States. After dialing the complete number, dialing the
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octothorpe lets the exchange know you've finished dialing. It
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can now begin routing your call; without the octothorpe, it would
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wait and "time out" before switching your call.
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When DTMF dials first came out they had complicated cams and
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switches for selecting the digits and used a transistor
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oscillator with an LC tuning network to generate the tones.
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Modern dials use a matrix switch and a CMOS IC that synthesizes
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the tones from a 3.57MHz (TV color burst) crystal. This
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oscillator runs only during dialing, so it doesn't normally
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produce QRM.
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Standard DTMF dials will produce a tone as long as a key is
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depressed. No matter how long you press, the tone will be
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decoded as the appropriate digit. The shortest duration in which
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a digit can be sent and decoded is about 100 milliseconds (ms).
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It's pretty difficult to dial by hand at such a speed, but
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automatic dialers can do it. A twelve-digit long distance number
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can be dialed by an automatic dialer in a little more than a
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second - about as long as it takes a pulse dial to send a single
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0 digit.
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The output level of DTMF tones from your telephone should be
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between 0 and -12 dBm. In telephones, 0 dB is 1 miliwatt over
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600 Ohms. So 0 dB is 0.775 Volts. Because your telephone is
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considered a 600 Ohm load, placing a voltmeter across the line
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will enable you to measure the level of your tones.
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The Ringer
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Simply speaking this is a device that alerts you to an
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incoming call. It may be a bell, light, or warbling tone. The
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telephone company sends a ringing signal which is an AC waveform.
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Although the common frequency used in the United States is 20 HZ,
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it can be any frequency between 15 and 68 Hz. Most of the world
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uses frequencies between 20 and 40 Hz. The voltage at the
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subscribers end depends upon loop length and number of ringers
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attached to the line; it could be between 40 and 150 Volts. Note
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that ringing voltage can be hazardous; when you're working on a
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phone line, be sure at least one telephone on the line is off the
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hook (in use); if any are not, take high voltage precautions.
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The telephone company may or may not remove the 48 VDC during
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ringing; as far as you're concerned, this is not important.
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Don't take chances.
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The ringing cadence - the timing of ringing to pause -
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varies from company to company. In the United States the cadence
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is normally 2 seconds of ringing to 4 seconds of pause. An
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unanswered phone in the United States will keep ringing until the
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caller hangs up. But in some countries, the ringing will "time
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out" if the call is not answered.
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The most common ringing device is the gong ringer, a
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solenoid coil with a clapper that strikes either a single or
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double bell. A gong ringer is the loudest signaling device that
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is solely phone-line powered.
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Modern telephones tend to use warbling ringers, which are
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usually ICs powered by the rectified ringing signal. The audio
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transducer is either a piezoceramic disk or a small loudspeaker
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via a transformer.
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Ringers are isolated from the DC of the phone line by a
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capacitor. Gong ringers in the United States use a 0.47 uF
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capacitor. Warbling ringers in the United States generally use a
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1.0 uF capacitor. Telephone companies in other parts of the
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world use capacitors between 0.2 and 2.0 uF. The paper
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capacitors of the past have been replaced almost exclusively with
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capacitors made of Mylar film. Their voltage rating is always
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250 Volts.
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The capacitor and ringer coil, or Zeners in a warbling
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ringer, constitute a resonant circuit. When your phone is hung
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up ("on hook") the ringer is across the line; if you have turned
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off the ringer you have merely silenced the transducer, not
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removed the circuit from the line.
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When the telephone company uses the ringer to test the line,
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it sends a low-voltage, low frequency signal down the line
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usually 2 Volts at 10 Hz) to test for continuity. The company
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keeps records of the expected signals on your line. This is how
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it can tell you have added equipment to your line. If your
|
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telephone has had its ringer disconnected, the telephone company
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cannot detect its presence on the line.
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Because there is only a certain amount of current available
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to drive ringers, if you keep adding ringers to your phone line
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you will reach a point at which either all ringers will cease to
|
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ring, some will cease to ring, or some ringers will ring weakly.
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In the United States the phone company will guarantee to ring
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five normal ringers. A normal ringer is defined as a standard
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gong ringer as supplied in a phone company standard desk
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telephone. Value given to this ringer is Ringer Equivalence
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Number (REN) 1. If you look at the FCC registration label of
|
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your telephone, modem, or other device to be connected to the
|
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phone line, you'll see the REN number. It can be as high as 3.2,
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which means that device consumes the equivalent power of 3.2
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standard ringers, or 0.0, which means it consumes no current when
|
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subjected to a ringing signal. If you have problems with
|
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ringing, total up your RENs; if the total is greater than 5,
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disconnect ringers until your REN is at 5 or below.
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Other countries have various ways of expressing REN, and
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some systems will handle no more than three of their standard
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ringers. But whatever the system, if you add extra equipment and
|
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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.
|
||
|
||
End of text. Figures Follow
|
||
|
||
|
||
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
|
||
|
||
Fig 1. The Phone Line
|
||
|
||
|
||
A RELAY
|
||
200 Ohms Telephone . Subscriber
|
||
------- Exchange .
|
||
------- . TIP +
|
||
------~~~~~~~--o----------------------o
|
||
| 5 H | .
|
||
| | .
|
||
+| | .
|
||
--- | . No 22 AWG wire
|
||
--- 48V DC | . up to 10 Miles Long
|
||
- | .
|
||
--- A RELAY | .
|
||
-| 200 Ohms | .
|
||
| ------- | .
|
||
| ------- | . RING -
|
||
------~~~~~~~--|---------o------------o
|
||
5 H | | .
|
||
Audio 2uF | 2uF | .
|
||
coupling 250V --- 250V ---
|
||
Capacitors --- ---
|
||
| |
|
||
o----- \-------- |
|
||
|
|
||
A RELAY Contacts |
|
||
|
|
||
o----- \------------------
|
||
|
||
|
||
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
|
||
|
||
|
||
Fig 2. Telephone Speech Network.
|
||
|
||
Simplified U.S. Standard "425B". Component Values
|
||
may vary between manufacturers. Connections for Dials, Ringers
|
||
etc. not shown.
|
||
|
||
|-------------------|
|
||
..|...................|
|
||
. | .|
|
||
Sidetone balancing. | 0.047uF 250V .|
|
||
impedance & loop . | | | .|
|
||
compensation. >>> . o----| |-------o .|
|
||
. | | | | .|
|
||
. | | .|
|
||
. | |<| VR2 | .|
|
||
. o----| |-------o---.|
|
||
. | |>| |.|
|
||
. | |.|
|
||
. | 68 Ohms |.|
|
||
. o---\/\/\/-----| |.|
|
||
..|..............|..|.|
|
||
| | | |
|
||
| . | | |
|
||
-----)||(------|---------|
|
||
1)||(5 | | | |
|
||
Loop )||( | | | |
|
||
TIP Compensation 2)||(6 | | | |
|
||
o------ \------o---------)||(------o | | RX O
|
||
. | . || | | | |
|
||
. | || 1.5uF | | | |
|
||
. \ 180 || --- | | |
|
||
. / Ohms || --- | |----o
|
||
. \ || 250V | | |
|
||
. | || | | |
|
||
. VR1 --- . || . | | |
|
||
. ^ ^ ----)||(------o--- TX O
|
||
. --- | 3)||(7 |
|
||
. | | )||( |
|
||
RING . | | 4)||(8 22 Ohms |
|
||
o----- \-------o---------)||(---o----/\/\/----
|
||
| |
|
||
^ | |
|
||
Hookswitch ------------
|
||
|
||
|
||
|
||
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
|
||
|
||
Fig. 3. Standard DTMF pad and Frequencies
|
||
|
||
|
||
|
||
(Low ____ ____ ____ ____
|
||
Group)| | | | | | | |
|
||
697Hz >| 1 | | 2 | | 3 | | A |
|
||
|____| |____| |____| |____|
|
||
|
||
|
||
|
||
____ ____ ____ ____
|
||
| | | | | | | |
|
||
770Hz >| 4 | | 5 | | 6 | | B |
|
||
|____| |____| |____| |____|
|
||
|
||
|
||
|
||
____ ____ ____ ____
|
||
| | | | | | | |
|
||
825Hz >| 7 | | 8 | | 9 | | C |
|
||
|____| |____| |____| |____|
|
||
|
||
|
||
|
||
____ ____ ____ ____
|
||
| | | | | | | |
|
||
941Hz >| * | | 0 | | # | | D |
|
||
|____| |____| |____| |____|
|
||
|
||
^ ^ ^ ^
|
||
1209Hz 1336Hz 1477Hz 1633Hz
|
||
(High Group)
|
||
|
||
END
|
||
|
||
|
||
|
||
|
||
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||
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||
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||
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|
||
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|
||
X-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-X
|
||
Another file downloaded from: NIRVANAnet(tm)
|
||
|
||
& the Temple of the Screaming Electron Jeff Hunter 510-935-5845
|
||
Burn This Flag Zardoz 408-363-9766
|
||
realitycheck Poindexter Fortran 510-527-1662
|
||
My Dog Bit Jesus Suzanne d'Fault 510-658-8078
|
||
New Dork Sublime Demented Pimiento 415-864-DORK
|
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The Shrine Tom Joseph 408-747-0778
|
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|
||
"Raw Data for Raw Nerves"
|
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X-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-X
|