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__________ ____________________ ________ << \________ __ ! / __ ___ \____________/ ____ | \ /\ | \| / / \ /| \ ___ > \ | |/__\ |__/|= | \ / | | /___\ _________ __ \_________|___/ \| \| \ | ! |___/ \____________/ >> \ < \ ! \ \___ / ____ _____ \______________________________________/ \_________/ _________DarkCyde_____________________ Communications __UK/USA_ / ___ \ ____/ \_____ __/ ___<_________ / í / / \ ___ ____________ / | \ __ /|__/| / | í | \ /___ \___>>____ ____/ | |\ / | \|= | / \ | | \___/ ____|___/ \/ |__/| \ \__ / \|___/ ________>____ __<<______/ í \____________________/ \________ ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !!==================================================!! !Hy3ridWarl0ck+PUBLiC_NEiSENCE+Nitrous_OxCyde+Red_LED! !!================================================!! _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 conductors 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. , , /( )` \ \__ / | /- _ `-/ ' (/\/ \ \ /\ / / | ` \ O O ) | `-^--'`< ' (_.) _ ) / `.___/` / `-----' / <----. __ / __ \ <----|====O)))==) \) /====Hybrid <----' `--' `.__,' \ | | g0d@deathsdoor.com \ / ____( (_ / \______ www.darkcyde.org ,' ,----' | \ `--{__________) \/