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                      _d_C_rawDATA_ -p-r-e-s-e-n-t-s-

          -Introduction to Lines, Losses, and Noise-

                            by Hybrid June'98

A transmission line can be considered as a conductor, or a group of 
conductors, with suitable insulating materials, whose function is to carry 
electronic information signals. The line can take various physical forms 
according to the type of information to be transmitted and the distance 
involved. 

Earth Return Circuits

Early morse code telegraph circuits used a single conductor or wire to 
connect two places together. 

                                 Single wire insulated from earth
                                  |
           Morse key              |       
    _______/. ____________________|______>______________________
   |         /                       Line current               |
   |                                                            |
   |                                                            |
   |                                                            |
   |                                                            |
 =====                                                    ______|_______
  ===   Signalling                                       |              |
   |     battery                                         |   Reciever   |
   |                                                     |______________|
   |                                                            |
   |                                                            |
=======                                                      =======
  === Earth connection                        Earth connection === 
   \                                                            /
    \                                                          /
     \_ _ _ _ _ _ _ _ _ _ _ _ _ < _ _ _ _ _ _ _ _ _ _ _ _ _ _ /
                       Earth return current

Are you paying attention?

The earth contains large amounts of different metals and can be used as a 
return conductor provided that a good connection with low resistance can 
be made with it. The main disadvantages of this arrangement, apart from the
problem of making a good electrical connection to the earth, are

a) The resistance (or opposition to current flow) of the insulated single 
wire is greater than the return path through the earth, so the line is 
unbalanced.

b) If other circuits also use the same arrangement, the earth is carrying 
return currents of all the different circuits, and mutual interference 
between the various circuits occur.

c) Power supply circuits which themselves do not carry information signals
can also produce interference to earth-return circuits.

Two-Wire Lines

The disadvantages of the earth-return system can be largely overcome by 
using two identical conductors insulated from earth other and from earth. 
The two conductors will now have the same resistance, and are not used by 
any other circuit. The simplest form of two-wire line is produced by using 
bare conductors suspented on insulators at the top of poles. Amother type of
two-wire line consists of conductors insulated from each other in a cable 
which also has an outer cover of insulation. Often the two insulated 
7conductors in the cable are twisted together along the length of the cable,
and are called 'a pair'. 

Multi-Pair Cables

It is often necessary to provide a number of two-wire lines between the same
two places, and this is done most convenietly by making a cable with a 
number of pairs of insulated wire inside it. Sometimes the wires are twisted
together in pairs, but sometimes they are provided in fours, or quads.
In order to identify the various wires, each one has a colouring on the 
insulating material around it in accordance with a standard colour code. 

Coaxial Cables

As the frequency of an alternating current is increased, the current tends 
to flow along the outer part of a conductor having a circlular cross-
section. This means that the centre part of the conductor is not carrying 
current and can be removed. The empty space can then be used for a second 
conductor, provided it is insulated from the outer conductor. This type of 
cable is called a coaxial cable. The two conductors can be insulated from 
each other either by a solid insulation along the whole length of the cable,
or by insulating 'spacers'fitted at regular intervals as supports for the 
inner conductor. The main insulation in this case is therefore the air 
between the two conductors.

Attenuation of Information Signals by Lines

What ever the type of cable used, the conductors must have some electrical 
resistance (or opposition to current flow). Furthermore, the insulating mate
rial used to seperate the conductors of a pair will have a value of 
insulation resistance which will allow a very small current to flow between
the conductors instead of flowing along the condcutor to the distant end.
Also the insulation between the conductors forms a capacitance which 
provides a conducting path between the conductors for alternating currents,
the conducting path becoming better as the frequncy of the alternating 
current increases. The capacitacnce also has the ability to strore 
electrical energy. This capacitive path therfore prevents part of the a.c. 
information signal from travelling along the conductors to the distant end 
of the line. Energy is used up to make the current flow against the 
resistance along the conductors, and against the insulation resistance 
between the conductors. Energy is also used in charging and discharging the
 capacitance between the conductors. In multi-pair cables there is 
capacitive and inductive coupling between pairs, so that some energy is 
passed from one pair to other pairs. This reduces the amount of energy that
 is transmitted along the origional pair, and so contributes to the loss.
In the case of an information signal, this energy is extracted from the sign
al source and so the energy available is gradually decreased as the signal 
travels along the line. This loss of energy along the line is called 
ATTENUATION. If the line is long, and the attenuation is large, eventually 
the signal energy available at the distant end is too small to operate a
receiving transducer. The attenuation generally increases, and this 
variation of attenuation with frequency is called ATTENUATION DISTORTION. 

Noise

In any telecommunication system, whether using line or radio links, there is
unwanted electrical energy present as well as that of the wanted information
signal. This unwanted electrical energy is generally called NOISE and 
arises from a number of different sources, which will now be considered very
briefly.

1) RESISTOR NOISE

A conductor is designed to carry current with minimum opposition, 
consistant with the size and cost. A resistor is a componment designed to 
have a paticular opposition to the flow of electric current in a particular
circuit. The opposition is called resistance in d.c. circuits, but in a.c. 
curcuits the term impendance is used because of added factors. In either 
case the unit used is the OHM. An electrical current is produced by the 
movement of electrons dislodged by an externally applied voltage from the 
outer shells of the atoms making up the conductor material or resistor 
material. The movement or agitation of atoms in conductors and resistors is
somewhat random, and is determined by the temperature of the conductor or 
resistor. The random movement of electrons broght about by thermal agitation
of atoms tends to have increased energy as temperature increases. The random
movement of atoms gives rise to an unwanted electrical voltage which is 
called resistor noise, circuit noise, Johnson noise or thermal noise. This 
unwanted signal spreads over a wide range of frequencies, and the noise 
present in given bandwidth required for a particular information signal is 
very inportant. 

2) SHOT NOISE

This is the name given to noise generated in active devices (energy sources)
, such as valves and transistors, by the random varying velocity of electron
movement under the influence of externally applied pottentials or voltages 
at appropriate terminals or electrodes.

3) PARTITION NOISE

This occurs in multi-electrode active devices such as transistors and valves
and is due to the total current being devided between the various 
electrodes.

4) FLUCTUATION NOISE

This can be natural (thunderstorms etc) or man made (car ignition systems, 
electrical apparatus, your neibours sex toys etc) and again spreads over a 
wide range of frequnencies. Such noise can be picked up by active devices 
and conductors forming transmission lines.

5) STATIC

This is the name given to noise encountered in the free space transmission 
paths of radio links, and is due mainly to ionospheric storms causing 
fluctuations of the earth's magnetic field. This form of noise is affected 
by the rotation of the sun (27.3 day cycle) and by the sunspot acrivity that
prevails.

6) COSMIC OR GALATIC NOISE

This type of noise is also most troublesome to radio links, and is mainly 
due to nuclear disturbances in all the galaxies of the universe.

7) In multi-pair cables there is capacitive and inductive coupling between 
diferent pairs which produces an unwanted noise signal on any pair because 
signals are transmitted to other pairs. This is called CROSSTALK between 
pairs and can be reduced to some extent by twisting the conductors of each
pair or by changing the realtive positions of pairs along the cable during 
manufacture or by balancing the pairs over a particular route after 
installation.

8) FLICKER NOISE

The cause of this is not well understood but it is noise which predominates
at low frequencies below 1 kHz, with the level decreasing as frequency 
increases. It is sometimes known as "excess noise."

In any telecommunications system, therefore, there will be a certain level 
of noise power arising from all or some of the sources described, with the 
noise power generaly being of a resonably steady level, except for some 
noise arising from impulsive sources such as car ignition systems and 
lightning. Noise which is sensibly constant mean level over a particular 
frequency bandwidth is generally called "white noise."
In order that a wanted information signal can be detected and reproduced 
satisfactorily at the receiving end of a system, it is essential that the 
power of the wanted signal is greater than the noise power present by at 
least a specified minimum value. This introduces the very important concept
of signal-to-noise ratio, or more commonly it is expressed in decibels (dB). 

For any type of information signal there is a minimum acceptable value of 
signal-to-noise ratio for the system to operate satisfactorily. Typical 
minimum signal-to-noise ratios for different systems are as follows:

1) Private land mobile radio telephone systems require 10 dB.
2) Ship-to-shore radio telephone services require 20 dB.
3) Telephone calls over the public network require 35 to 40 dB.
4) Television systems require 50 dB.

Now returning to the problem of sending an information signal along a line,
valve or transistor amplifiers can be used to increase the signal level to 
compenasate for the attenuation of the line. Each amplifier will generate 
noise internally, so the output of each amplifier will contain the wanted 
signal and unwanted noise with a certain signal-to-noise ratio.
There will also be Johnson noise present on the line because of the
resistance of the line conductors, and also crosstalk noise from other 
lines. One amplifier could be placed at the sending end, with sufficaint 
amplifing properties or gain to compensate for the line attenuation, so that
the information signal reaching the other end of the line has sufficiant 
power to operate the recieving transducer satisfactorily. This could result
in a large signal power at the sending end which would cause excessive 
interference to other circuits in the same cable due to mutual inductance 
and capacitive coupling between different pairs. To avoid this problem there
is a maximum permissible signal power laid down for application to pairs in
different types of cable.
Another way to othercome attenuation would be to put one amplifier at the 
recieving end with sufficient gain to compensate for the line attenation. 

a) SINGLE HIGH-GAIN AMPLIFIER PLACED AT SENDING END

                     O------------------Interference to other 
Sending transducer   O--------------------- cable pairs
 ________       ___                                           ________
|        |=====|   |===O->-----------------------------O-----|        |
|________|=====|___|===O->-----------------------------O-----|________|
              Sending end  Large signal power> >              Receiving
              amplifier                                       transducer

b) SINGLE HIGH-GAIN AMPLIFIER PLACED AT RECEIVING END

Sending transducer                                     Receiving transducer
 ________                                             ___       ________
|        |=====O------------------->-------------O===|   |=====|        |
|________|=====O------------------->-------------O===|___|=====|________|
                   Low power similar to amplifier   Recieving end
                   noise level and line noise       amplifier

c) LIMITED-GAIN AMPLIFIERS PLACED AT REGULAR INTERVALS ALONG A LINE

                    Output signal limited to avoid
                    interference to other cable pairs  Receiving transducer
Sending transducer    |                |                          |
 ________      ___             ___             ___              ________
|        |====|   |==O-->--O==|   |==O-->--O==|   |============|        |
|________|====|___|==O-->--O==|___|==O-->--O==|___|============|________|
          Sending end    Signal power well above     Receiving end
          amplifier      amplifier noise level and   amplifier
                         line noise

However, if the line is long with a resricted permissible power at the 
sending end, the attenuation could be such that the information signal power
at the reciever is low enough to give an inadequete signal-to-noise ratio 
when the line noise and noise genderated by the receiver are considered.
To overcome these problems, amplifiers must be placed at regular points 
along the line where the information signal power is still large enough to 
give an adequate signal-to-noise ratio compared with the amplifier noise and
line noise. Since an amplifier is generally a one way device with definate 
output connections, the arangement considered illustrated above needs to be
duplicated to enable information signals to be transmitted in the opposite 
direction. However it has previously been seen that simple telephone 
communication circuits carry information in both directions over a single 
pair of wires. To meet this requirement, it is therefore neccessary to 
arrange that when amplification is needed over the telephone circuits, the 
normal simple two-wire connection is changed into a four-wire connection to
one pair for transmitting signals in each direction. 
It should be added here that there are certain types of amplifier that can 
be inserted into a two-wire line to give amplification in both directions, 
but the use of these in the public telephone network is limited.

                 ,       ,
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