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Wire, coaxial cables, waveguides, optical fiber, and free space transmission havecharacteristics which vary as frequency changes.. The frequency characteristics of a BST 26-gauge non-loa

TELECOMMUNICATION TRANSMISSION MEDIA

In this chapter the characteristics of the media in which the transmission of signalstakes place will be discussed It so happens that we humans basically communicatethrough speech=hearing and by sight Human hearing is from 20 Hz to 20 kHz and

we can see only the portion of radiation spectrum from about 4:3
1014Hz(infrared; l ¼ 7
107m) to approximately 7:5
1014Hz (ultraviolet;

l ¼ 4
107m) These communication channels occupy only small portions ofthe detectable frequency spectrum which has no lower boundary but has an upperboundary of about 1022Hz (gamma rays) Acoustic radiation in the frequency range20–20 kHz is attenuated quite severely in our environment even when attempts aremade to guide it along a conduit It is therefore quite inefficient to transmit anacoustic signal over any distance which would qualify as telecommunication Thesame observation can be made about visible light To communicate over distancesgreater than what we can bridge by shouting, or see reliably, it is necessary toconvert the signal into another form that can be guided (by wire, waveguide, oroptical fiber) or which can be radiated efficiently in free space

Wire, coaxial cables, waveguides, optical fiber, and free space transmission havecharacteristics which vary as frequency changes A medium may be efficient in onefrequency range but quite unsuitable for another frequency range But efficiency isnot the sole criterion for the choice of the frequency to which audio and videosignals have to be translated for transmission To help keep some order and tominimize interference among the various users of communication services, it isnecessary to assign various frequency bands for specific uses and governmentsarrogate to themselves the right to demand a licensing fee for the use of these bands.For example, satellite communication has been assigned 4–6, 12–14, and 19–

29 GHz but there is no technical reason why they cannot operate at frequencies inbetween these frequencies or indeed outside them

367

Telecommunication Circuit Design, Second Edition Patrick D van der Puije

Copyright # 2002 John Wiley & Sons, Inc ISBNs: 0-471-41542-1 (Hardback); 0-471-22153-8 (Electronic)

12.2 TWISTED-PAIR CABLE

This consists of two insulated wires twisted together to form a pair Several to manyhundred pairs may be put together to form a cable When this is done it is usual touse different pitches of twist in order to limit electromagnetic coupling betweenthem and hence cross-talk The conductor material is copper, usually numbers 19,

22, 24, and 26 American Wire Gauge (AWG), and the insulation is usuallypolyethylene Wax-treated paper insulation was used in the past but the ingress ofmoisture into the cable was a problem in most applications; it is still a problem evenwith polyethylene insulated cables which are sometimes filled with grease-likesubstances to take up all the air spaces and thus discourage moisture from entering.Such cables may be suspended from poles where they are easy and inexpensive toservice but are aesthetically undesirable, or buried which make them expensive anddifficult to repair

The frequency characteristics of a BST 26-gauge non-loaded cable terminated in

900 O are shown in Figure 12.1 It can be seen that the twisted pair has a low-passcharacteristic It should be noted that, contrary to expectation, the primary constants

of the twisted pair (series resistance, shunt capacitance, series inductance and shuntconductance, all per unit length) change with frequency The bandwidth of thetwisted pair can be extended to a higher frequency by inductive loading of the line.Lumped inductances are connected in series with the line at specified distances Thebest results are obtained when the interval is kept short and the value of the lumpedinductance is kept low, thus minimizing the discontinuities introduced by loading.The frequency responses of a 12,000 ft (3.7 km) number 26-gauge with 900 Oterminations for the loaded and unloaded cases are shown in Figure 12.2

900 O.

It can be seen that, while loading solves the problem of limited bandwidth for thetypical subscriber loop voice channel, it is quite inadequate for analog (the basicsupergroup requires 552 kHz bandwidth) and digital (DS-1 requires 1.5 MHzbandwidth) carrier applications, for which it is used In both these cases, the linehas to be equalized by placing amplifiers or repeaters at specific distances along itslength that emphasize the high-frequency response or regenerate the pulses Linesused for digital transmission require phase equalization as well, otherwise pulsedegradation due to dispersion takes place Dispersion causes the rate of rise of theleading and trailing edges of the pulse to slow down and the base to spread out over amuch longer time than the original pulse.

It can be seen from Figure 12.1 that there is a flat loss at lower frequencies, so it isusual to combine the equalizer with an amplifier An amplifier used for this purpose

is called a repeater A repeater can take a number of forms In a two-wire systemwhere signals flow in both directions, a negative impedance converter is coupled inseries and=or in shunt with the line through a transformer The configuration of thenegative-impedance converter and its connection to the line are shown in Figure12.3

Measures have to be taken to ensure that the negative impedance does notoverwhelm the line impedance resulting in oscillation The introduction of repeaters

terminated in 900 O and 2 mF Reprinted with permission from Transmission Systems for Communications, 5th Ed., AT&T, Bell Labs, 1982.

12.2 TWISTED-PAIR CABLE 369

into the cable causes an impedance mismatch at the point of connection and this cancause echo problems Severe echo on the cable can impair the speech of mosttelephone users There are circuits built into the cable or at the terminations to cancelthe echo.

12.2.1 Negative-Impedance Converter

The negative-impedance converter is a two-port which converts an impedanceconnected to one port into the negative of the impedance at the other port Considerthe two-port shown in Figure 12.4 terminated at port 2 by an impedance ZL

If the two-port is a negative impedance converter (NIC) then

where k is a constant

Reprinted with permission from Transmission Systems for Communications, 4th Ed., AT&T, Bell Labs, 1970.

Such a two-port is best described by a chain matrix equation

Both matrices satisfy the condition for a negative-impedance converter, namely

This is called the voltage negative impedance converter or VNIC [5]

From the second matrix,

This is called the current negative-impedance converter, INIC or CNIC

Without loss of generality, we can make k1¼k2¼1 so that

The transistors Q1and Q2may be represented by the low-frequency T-equivalentmodel shown in Figure 12.6.

Assuming that C1 and C2 are short-circuits at the frequency of operation, theequivalent circuit of the VNIC is as shown in Figure 12.7

The hybrid k parameters of the circuit in Figure 12.7 are

37

7 ð12:2:16Þ

circuit.

12.2 TWISTED-PAIR CABLE 373

When R1¼R2¼R and re is small compared to R and a1¼a2 1, the circuitbehaves like a VNIC The practical version of the VNIC circuit is shown in Figure12.8.

12.2.2 Four-wire Repeater

In a four-wire system, the forward and return paths are different and ordinaryamplifiers may be used This is shown in Figure 12.9 Again precautions have to betaken to counteract the possibility of instability through the hybrid-to-hybrid feed-back path

As frequency increases, the twisted pair has the tendency to lose signal powerthrough radiation Ultimately, its usefulness is limited by cross-talk between pairs

Reprinted with permission from Transmission Systems for Communications, 4th Ed., AT&T, Bell Labs, 1970.

The structure of the coaxial cable ensures that, at normal operating frequencies,the electromagnetic field generated by the current flowing in it is confined to thedielectric Radiation is therefore severely limited At the same time, the outerconductor (normally grounded) protects the cable from extraneous signals such asnoise and cross-talk.

The primary constants of the coaxial cable are much better behaved than those ofthe twisted pair The inductance, L, capacitance, C, and conductance, G, per unitlength are, in general, independent of frequency The resistance, R, per unit length is

a function of frequency due to skin effect; it varies as a function of ffiffiffi

f

p.The frequency characteristics of a 0.375 inch (9.5 mm) coaxial cable are shown inFigure 12.10 As expected, the coaxial cable has a much larger bandwidth than thetwisted pair However, it still requires repeaters and frequency equalizers for analoglines and phase equalization for digital signal transmission The characteristics of therepeaters are usually adaptively controlled to correct for changes in temperature andother operating conditions

Coaxial cable is used for transmitting data at 274.176 Mbit=s in the LD-4 Canada) and T4M (Bell System in the USA) systems They have 4032 voice

with permission from Transmission Systems for Communications, 4th Ed., AT&T, Bell Labs, 1970.

12.3 COAXIAL CABLE 375

channels or the equivalent video or digital data traffic Its regenerators are spaced at1.8 km intervals and the total length of the line can be 6500 km [1].

Specially constructed coaxial cables with repeaters of very high reliability areused for submarine cable systems Because of the very high cost of these cables, theyare used to transmit messages in both directions by assigning separate frequencybands to each direction In spite of the development of satellite communicationchannels, submarine cables are still viable for trans-Atlantic and trans-Pacific traffic.Because of the propagation delay involved in the signal travelling to the satellite andback, most trans-Atlantic telephone conversations use the satellite link in onedirection only; cable is used in the opposite direction In 1976, the TAT-6 (SG)trans-Atlantic cable system was installed It used a 43 mm diameter coaxial cablewith a 4200 voice channel capacity over a distance of 4000 km [2]

A waveguide may be viewed as a coaxial cable with the central conductor removed.The outer conductor guides the propagation of the electromagnetic wave In its mostcommon form it has a rectangular cross section with an aspect ratio of 2 : 1 Thewider dimension must be about one-half the wavelength of the wave which it willtransmit Therefore the waveguide has a low-frequency cut-off There are a number

of modes in which the wave can propagate but in every case the electric andmagnetic fields are orthogonal When the electric field is at right angles to the axis ofthe waveguide, it is described as transverse electric (TE) mode When the magneticfield is at right angles to the axis it is called transverse magnetic (TM) mode.The mechanical structure of the waveguide disqualifies it from being used forlong-haul transmission Irregularities on the walls, such as projections, holes, lack of

a perfect match at joints, bends, twists and imperfect impedance matching at theterminations, can cause reflection and spurious modes to be generated, all of whichresult in signal loss

Waveguides are used mainly as feedlines to antennas in terrestrial microwaverelay systems and for frequencies above 18 GHz they are superior to all other media

in terms of loss, noise and power handling

(2) A low-loss glass fiber which can be used as a waveguide for the light.(3) A detector for the signal at the receiving end

The laser can be modulated at a rate in the range of 109bit=s while the LED canoperate at 108bit=s The information bearing capacity of the system is enormous.On-going research continues to increase the bit rate limits.

The optical fiber is essentially a high quality glass rod of about 50 mm diameterfor multi-mode propagation and 8 mm for single-mode propagation The mechanicalproperties of a glass rod that small will make the system impracticable In practice, asecond layer of glass concentric with the optical fiber proper is deposited in theoutside, bringing the overall diameter to 125 mm The outer glass sheathing, referred

to as cladding, has a different refractive index and the signal is therefore confined tothe core The core may be of uniform refractive index or it may be graded Thesetechniques have resulted in optical fibers that have attenuation less than 0.2 dB=km.Various types of protective covering may be put on the fiber and several fibers puttogether to form a cable

The optical receiver is a reverse-biassed semiconductor junction diode and it iscoupled to the fiber so that the incoming light falls on the junction The energy in thelight is transferred to the electrons in the semiconductor lattice, causing them tobreak away and move into the conduction band The high electric field sweeps theelectrons out of the junction into the external circuit where they can be detected as acurrent Silicon diodes have been used for 1 mm wavelength detectors For longerwavelengths, such as 1.3 and 1.5 mm, germanium, InAs and InSb are used [3]

In 1988, a trans-Atlantic optical fibre communication system went into service(TAT-8) It spans a distance of 6500 km and provides the equivalent of 40,000telephone channels It operates on the 1.3 mm wavelength; repeater separation is

35 km and the bit rate is 274 Mbit=s

The transmission media discussed earlier had one thing in common; the propagation

of the signal was guided by a twisted pair, a coaxial cable, a waveguide, or an opticalfiber We now consider transmission systems which rely on propagation through freespace

In 1873 Maxwell showed that electromagnetic waves can propagate through freespace It took three decades to demonstrate experimentally that this was possible,when Hertz constructed the first high-frequency oscillator – this was the famousspark-gap apparatus

A large number of factors have to be taken into account when designing a freespace propagation communication system including the following:

(1) the distance between transmitter and receiver,

(2) the carrier frequency of the transmission,

(3) the physical size of the antenna to be used,

(4) the power to be radiated

(5) the effect of the transmission on other users of the same and adjacentchannels

12.6 FREE SPACE PROPAGATION 377

Figure 12.11 shows different paths by which a signal radiated from a transmittercan reach a receiver.

12.6.1 DirectWave

As the name suggests, the signal travels directly from the transmitter antenna to thereceiver antenna This requires that the two antennas be in line-of-sight The directwave is the major mode of propagation for medium wave AM radio (540–

1600 kHz), commercial FM broadcasting (88–108 MHz), terrestrial microwaverelay systems, used for long-distance telephone and television signals (2, 4, 6, 11,

18, and 30 GHz), and satellite transmission systems (4, 6, 8, 12, 14, 17–21, and 27–

12.6.3 Troposphere-Reflected Wave

At a distance of approximately 10 km above the Earth’s surface, there is an abruptchange in the dielectric constant of the atmosphere This portion of the upperatmosphere is called the troposphere High-frequency signals, such as those used interrestrial microwave relay systems, may be reflected from the troposphere and havethe same effect as the Earth-reflected wave at the receiver On the other hand, thetroposphere may be used as part of the communication channel in cases where directline-of-sight conditions are not possible This is called tropospheric scatter propaga-tion and the best frequencies for this are 1, 2, and 5 GHz It is most commonly used

by the military for communication over difficult terrain, typically over a distance of300–500 km

12.6.4 Sky-Reflected Wave

Surrounding the Earth at an elevation of approximately 70–400 km is a layer ofionized air caused by constant bombardment of ultraviolet, a, b and g radiation fromthe Sun as well as cosmic rays It is called the ionosphere and it consists of severallayers which have different reflective, refractive, and absorptive effects on radiowaves

When a radio signal reaches the ionosphere, a number of things can happendepending on the frequency and the angle of incidence The wave may be reflectedback to Earth, it may undergo refraction and eventually be returned to the Earth or it

12.6 FREE SPACE PROPAGATION 379