Thông tin thiết kế mạch P12

TELECOMMUNICATION TRANSMISSION MEDIAIn 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 Â 1014 Hz (infrared; l ¼ 7 Â 10À7 m) to approximately 7:5 Â 1014 Hz (ultraviolet; l ¼ 4 Â 10À7 m). These communication channels occupy only small portions of the detectable frequency spectrum which has no lower boundary but has an upper...
Nội dung trích xuất từ tài liệu:
Thông tin thiết kế mạch P12 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 TELECOMMUNICATION TRANSMISSION MEDIA12.1 INTRODUCTIONIn 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 andwe can see only the portion of radiation spectrum from about 4:3 Â 1014 Hz(infrared; l ¼ 7 Â 10À7 m) to approximately 7:5 Â 1014 Hz (ultraviolet;l ¼ 4 Â 10À7 m). These communication channels occupy only small portions ofthe detectable frequency spectrum which has no lower boundary but has an upperboundary of about 1022 Hz (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. 367368 TELECOMMUNICATION TRANSMISSION MEDIA12.2 TWISTED-PAIR CABLEThis 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 in900 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 constantsof 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.Figure 12.1. Frequency characteristics of 26 gauge BST non-loaded cable terminated in900 O. 12.2 TWISTED-PAIR CABLE 369Figure 12.2. Comparison of loaded and unloaded 12,000 feet (3.7 km) number 26 gauge cableterminated in 900 O and 2 mF. Reprinted with permission from Transmission Systems forCommunications, 5th Ed., AT&T, Bell La ...