12 TELECOMMUNICATION TRANSMISSION MEDIA 12.1 INTRODUCTION In this chapter the characteristics of the media in which the transmission of signals takes place will be discussed. It so happens that we humans basically communicate through 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 Â10 14 Hz (infrared; l ¼ 7  10 À7 m) to approximately 7:5 Â10 14 Hz (ultraviolet; l ¼ 4 Â10 À7 m). These communication channels occupy only small portions of the detectable frequency spectrum which has no lower boundary but has an upper boundary of about 10 22 Hz (gamma rays). Acoustic radiation in the frequency range 20–20 kHz is attenuated quite severely in our environment even when attempts are made to guide it along a conduit. It is therefore quite inefficient to transmit an acoustic signal over any distance which would qualify as telecommunication. The same observation can be made about visible light. To communicate over distances greater than what we can bridge by shouting, or see reliably, it is necessary to convert the signal into another form that can be guided (by wire, waveguide, or optical fiber) or which can be radiated efficiently in free space. Wire, coaxial cables, waveguides, optical fiber, and free space transmission have characteristics which vary as frequency changes. A medium may be efficient in one frequency range but quite unsuitable for another frequency range. But efficiency is not the sole criterion for the choice of the frequency to which audio and video signals have to be translated for transmission. To help keep some order and to minimize interference among the various users of communication services, it is necessary to assign various frequency bands for specific uses and governments arrogate 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 in between 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 many hundred pairs may be put together to form a cable. When this is done it is usual to use different pitches of twist in order to limit electromagnetic coupling between them and hence cross-talk. The conductor material is copper, usually numbers 19, 22, 24, and 26 American Wire Gauge (AWG), and the insulation is usually polyethylene. Wax-treated paper insulation was used in the past but the ingress of moisture into the cable was a problem in most applications; it is still a problem even with polyethylene insulated cables which are sometimes filled with grease-like substances 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 to service but are aesthetically undesirable, or buried which make them expensive and difficult 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-pass characteristic. It should be noted that, contrary to expectation, the primary constants of the twisted pair (series resistance, shunt capacitance, series inductance and shunt conductance, all per unit length) change with frequency. The bandwidth of the twisted 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. The best results are obtained when the interval is kept short and the value of the lumped inductance 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 O terminations for the loaded and unloaded cases are shown in Figure 12.2. Figure 12.1. Frequency characteristics of 26 gauge BST non-loaded cable terminated in 900 O. 368 TELECOMMUNICATION TRANSMISSION MEDIA It can be seen that, while loading solves the problem of limited bandwidth for the typical subscriber loop voice channel, it is quite inadequate for analog (the basic supergroup requires 552 kHz bandwidth) and digital (DS-1 requires 1.5 MHz bandwidth) carrier applications, for which it is used. In both these cases, the line has to be equalized by placing amplifiers or repeaters at specific distances along its length that emphasize the high-frequency response or regenerate the pulses. Lines used for digital transmission require phase equalization as well, otherwise pulse degradation due to dispersion takes place. Dispersion causes the rate of rise of the leading and trailing edges of the pulse to slow down and the base to spread out over a much 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 is usual 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 system where signals flow in both directions, a negative impedance converter is coupled in series and=or in shunt with the line through a transformer. The configuration of the negative-impedance converter and its connection to the line are shown in Figure 12.3. Measures have to be taken to ensure that the negative impedance does not overwhelm the line impedance resulting in oscillation. The introduction of repeaters Figure 12.2. Comparison of loaded and unloaded 12,000 feet (3.7 km) number 26 gauge cable 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 can cause echo problems. Severe echo on the cable can impair the speech of most telephone users. There are circuits built into the cable or at the terminations to cancel the echo. 12.2.1 Negative-Impedance Converter The negative-impedance converter is a two-port which converts an impedance connected to one port into the negative of the impedance at the other port. Consider the two-port shown in Figure 12.4 terminated at port 2 by an impedance Z L . If the two-port is a negative impedance converter (NIC) then Z in ¼ÀkZ L ð12:2:1Þ where k is a constant. Figure 12.3. The connection of the negative impedance converter (NIC) to the telephone line. Reprinted with permission from Transmission Systems for Communications, 4th Ed., AT&T, Bell Labs, 1970. Figure 12.4. A two-port with its defining voltages and currents. 370 TELECOMMUNICATION TRANSMISSION MEDIA Such a two-port is best described by a chain matrix equation V 1 I 1  ¼ AB CD  V 2 ÀI 2  : ð12:2:2Þ This gives V 1 ¼ AV 2 À BI 2 ð12:2:3Þ and I 1 ¼ CV 2 À DI 2 : ð12:2:4Þ From the termination we have ÀI 2 Z L ¼ V 2 ð12:2:5Þ Substituting into Equations (12.2.3) and (12.2.4) gives V 1 ¼ AV 2 þ BV 2 Z L ð12:2:6Þ and I 1 ¼ CV 2 þ DV 2 Z L : ð12:2:7Þ But for a two-port Z in ¼ V 1 I 1 ¼ AZ L þ B CZ L þ D : ð12:2:8Þ For Z L to be equal to ÀkZ L , B ¼ C ¼ 0, then Z in ¼ AZ L D so that k ¼À A D : ð12:2:9Þ There are two possibilities: AB CD  ¼ Àk 1 0 0 k 2  or k 1 0 0 Àk 2  : ð12:2:10Þ 12.2 TWISTED-PAIR CABLE 371 Both matrices satisfy the condition for a negative-impedance converter, namely Z in ¼ Àk 1 k 2 Z L ¼ÀkZ L ð12:2:11Þ where k ¼ k 1 =k 2 From the first matrix, V 1 ¼Àk 1 V 2 : ð12:2:12Þ This is called the voltage negative impedance converter or VNIC [5]. From the second matrix, I 1 ¼ k 2 ðÀI 2 Þð12:2:13Þ This is called the current negative-impedance converter, INIC or CNIC. Without loss of generality, we can make k 1 ¼ k 2 ¼ 1 so that AB CD  ¼ À10 01  or 10 0 À1  ð12:2:14Þ The NIC is an example of what is described as a degenerate two-port, that is, it cannot be described by open-circuit impedance [Z] nor short-circuit admittance [Y ] parameters. However, it has chain and hybrid parameters (both ½h and ½k). The VNIC may be described in terms of its hybrid k parameters as follows: k 11 k 12 k 21 k 22  ¼ 0 À1 10  ð12:2:15Þ The basic form of the VNIC is shown in Figure 12.5. Figure 12.5. The basic form of the voltage negative-impedance converter. 372 TELECOMMUNICATION TRANSMISSION MEDIA The transistors Q 1 and Q 2 may be represented by the low-frequency T-equivalent model shown in Figure 12.6. Assuming that C 1 and C 2 are short-circuits at the frequency of operation, the equivalent circuit of the VNIC is as shown in Figure 12.7. The hybrid k parameters of the circuit in Figure 12.7 are k 11 k 12 k 21 k 22  ¼ 1 Àa 2 R 1 þ r e2 þ r b2 ð1 Àa 2 Þ Àa 1 Àa 2 R 2 R 1 þ r e2 þ r b2 ð1 Àa 2 Þ r e1 þðr b1 þ R 2 Þð1 Àa 1 Þ 2 6 6 6 4 3 7 7 7 5 ð12:2:16Þ Figure 12.6. The T-equivalent model of the bipolar transistor. Figure 12.7. The basic VNIC when the transistors have been replaced with the T-equivalent circuit. 12.2 TWISTED-PAIR CABLE 373 When R 1 ¼ R 2 ¼ R and r e is small compared to R and a 1 ¼ a 2 % 1, the circuit behaves like a VNIC. The practical version of the VNIC circuit is shown in Figure 12.8. 12.2.2 Four-wire Repeater In a four-wire system, the forward and return paths are different and ordinary amplifiers may be used. This is shown in Figure 12.9. Again precautions have to be taken 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 power through radiation. Ultimately, its usefulness is limited by cross-talk between pairs. Figure 12.8. The circuit of the voltage negative-impedance converter. Figure 12.9. The use of ordinary amplifiers on the telephone line with 2-to-4 wire hybrid. Reprinted with permission from Transmission Systems for Communications, 4th Ed., AT&T, Bell Labs, 1970. 374 TELECOMMUNICATION TRANSMISSION MEDIA 12.3 COAXIAL CABLE In a coaxial cable, one conductor is in the form of a tube with the second running concentrically along the axis. The inner conductor is supported by a solid dielectric or by discs of dielectric material placed at regular intervals along its length. A number of these cable are usually combined together with twisted pairs to form a multi-pair cable. 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 the dielectric. Radiation is therefore severely limited. At the same time, the outer conductor (normally grounded) protects the cable from extraneous signals such as noise and cross-talk. The primary constants of the coaxial cable are much better behaved than those of the twisted pair. The inductance, L, capacitance, C, and conductance, G, per unit length 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 in Figure 12.10. As expected, the coaxial cable has a much larger bandwidth than the twisted pair. However, it still requires repeaters and frequency equalizers for analog lines and phase equalization for digital signal transmission. The characteristics of the repeaters are usually adaptively controlled to correct for changes in temperature and other operating conditions. Coaxial cable is used for transmitting data at 274.176 Mbit=s in the LD-4 (Bell- Canada) and T4M (Bell System in the USA) systems. They have 4032 voice Figure 12.10. The insertion characteristics of a terminated 0.375 inch coaxial cable. Reprinted 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 at 1.8 km intervals and the total length of the line can be 6500 km [1]. Specially constructed coaxial cables with repeaters of very high reliability are used for submarine cable systems. Because of the very high cost of these cables, they are used to transmit messages in both directions by assigning separate frequency bands to each direction. In spite of the development of satellite communication channels, 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 and back, most trans-Atlantic telephone conversations use the satellite link in one direction 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 cable with a 4200 voice channel capacity over a distance of 4000 km [2]. 12.4 WAVEGUIDES 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 most common form it has a rectangular cross section with an aspect ratio of 2 : 1. The wider dimension must be about one-half the wavelength of the wave which it will transmit. 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 and magnetic fields are orthogonal. When the electric field is at right angles to the axis of the waveguide, it is described as transverse electric (TE) mode. When the magnetic field 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 for long-haul transmission. Irregularities on the walls, such as projections, holes, lack of a perfect match at joints, bends, twists and imperfect impedance matching at the terminations, can cause reflection and spurious modes to be generated, all of which result in signal loss. Waveguides are used mainly as feedlines to antennas in terrestrial microwave relay systems and for frequencies above 18 GHz they are superior to all other media in terms of loss, noise and power handling. 12.5 OPTICAL FIBER The use of optical fiber as a medium for telecommunication was made possible by a coincidence of the development of a number of technologies. (1) The laser which is a coherent frequency source of the order of 10 14 Hz and it can be modulated. A light emitting diode (LED) which produces non- coherent light can also be used. (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. 376 TELECOMMUNICATION TRANSMISSION MEDIA [...]... cost of launching them and the difficulty of making repairs should anything go wrong dictate very high levels of reliability To avoid the complexity of tracking the satellite from rising to setting and to maintain continuous communication, it is best to ‘‘park’’ the satellite at a point directly above the equator and to choose the correct speed in the direction of rotation of the Earth so it stays in a... 1.5 MHz over a twisted-pair cable 12.2 Derive an expression for the input impedance of the circuit shown in Figure P12. 1, assuming that the operational amplifier is ideal How can this circuit be used to improve the transmission characteristics of the twisted-pair or coaxial cable? Figure P12. 1 12.3 Describe two techniques which can be used for the compensation of gain loss on a twisted-pair and=or coaxial... multiple hop and can be used to reach places beyond the single-hop distance 12.6.5 Surface Wave At very low frequencies (VLF: 3–30 kHz), the ionosphere and the Earth’s surface form two parallel conducting planes and can act as a waveguide VLF signals are used for worldwide communication and navigational aids At higher frequencies, temperature inversions and other local phenomena can generate a surface... upconversion to microwave frequencies Frequency modulation is the preferred method 12.7.1.1 Terminal Transmitter and Receiver The block diagram of the transmitter is shown in Figure 12.12(a) The modulating signal, 70 MHz, itself modulated by 3600 voice-frequency telephone channels (jumbo group) or its equivalent, is amplified and used to drive the modulator, mixer or upconverter The other input to the... mixed with the output of the local oscillator to produce an intermediate-frequency signal After amplification, equalization and the application of AGC, it goes to a demodulator where the original modulating signal (70 MHz) is recovered It should be noted that, in the case of the 3600 voice-frequency telephone channels, several levels of demodulation have to be carried out before the voice frequency signals... the functional blocks of the analog radio shown in Figure 12.13 are present in some form or another in the digital radio The baseband signal to be transmitted may be the output of a digital switch operating at a rate of 1.544, 6.312, 44.736 or 274.176 Mbit=s The binary output of the switch is used in a modulation scheme similar to that of the modem discussed in Section 9.4.1 The digit 1 is assigned a... height, it can cover a large area of the globe, making it possible to cover the entire surface with three geostationary satellites It has applications in point-to-point communications as well as broadcasting and it can reach remote parts of the Earth where other systems cannot reach without large expenditures of money and effort All that is required is the satellite and the terminal equipment in the Earth... SPACE PROPAGATION 377 The laser can be modulated at a rate in the range of 109 bit=s while the LED can operate at 108 bit=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 diameter for multi-mode propagation and 8 mm for single-mode propagation The mechanical properties... perigee) The two foci can be at the same 385 Figure 12.14 The regenerative repeater used in digital radio 386 TELECOMMUNICATION TRANSMISSION MEDIA point, in which case the orbit is circular There are distinct advantages to having a satellite in a circular orbit since this fixes the distance travelled by the message and hence the delay To satisfy these conditions, an Earth satellite has to be 42,230 km... wheel in it which spins The gyroscopic effect of this helps to further stabilize the satellite If the satellite itself spins, it is necessary to spin the antenna in the opposite direction to keep it pointing at the Earth at all times For about 30 minutes a day for several days around the equinoxes (March 21 and September 21) the Sun is directly behind the satellite and the electrical noise generated by . adaptively controlled to correct for changes in temperature and other operating conditions. Coaxial cable is used for transmitting data at 274.176 Mbit=s in the LD-4 (Bell- Canada) and T4M (Bell System. of reliability. To avoid the complexity of tracking the satellite from rising to setting and to maintain continuous communication, it is best to ‘‘park’’ the satellite at a point directly above. have to be taken to ensure that the negative impedance does not overwhelm the line impedance resulting in oscillation. The introduction of repeaters Figure 12.2. Comparison of loaded and unloaded