Figure 2-8 Directional couplers used to branch A, B, and C
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dB Losses 750 MHz 50 MHz Device 40 10 200' cable 12 03 32-dB tap 05 26-dB tap 13 15 07 20-dB tap 10 17-dB tap 18 17 14-dB tap 26 41 30 11-dB tap 15 DC-8 (8dB) 28 57 3-way splitter 72
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port level of at least 10 dBmV at the 750-MHz frequency Proper drop cable shielding effectiveness is also necessary to control signal leakage and ingress You will often have several choices for branching within a service area design If a manual design method is being used, the designer can ponder the various choices, taking signal level and cost factors into consideration Signal splitters and directional couplers are the passive devices used for system street branching Figures 2-8 and 2-9 illustrate two methods serving the same residential street layout One method employs a balanced output splitter and the other one uses directional couplers Figure 2-8, using two direction couplers, has two short branches with one four-port tap each and one long branch with two taps before a line extender is required Figure 2-9, on the other hand, could use two taps in three directions, with the last tap a terminating tap before a line extender ampli er is required to extend the system Computer-aided design and drafting programs keep a running tabulation of signal levels When this level approaches a predetermined level,
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3-way splitter 72 72 dB / 750 MHz 57 57 dB / 50 MHz Even Split An 11-dB tap here will cause input levels to be less than 20 dBmV for LE input 110 320 115 295 200' 200'
Figure 2-9 Balanced three-way splitter for branches A, B, and C
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280 262 285 275
All signal levels are +dBmV Top figure is @ 750 MHz Bottom figure is @ 50 MHz Legs A, B, and C are identical
then an ampli er is needed at that point to continue the cable run Where branching is required, the designer operating the CAD system can make the branching choices When an ampli er is inserted, the program then annotates the system map with such parameters as input /output level, pad value, equalizer value, and carrier-to-noise and carrier-to-distortion calculations These parameters can be very useful when compared to actual system measurements at initial proof of performance time Trunk system splitting and coupling is equally important in determining the system network topology A system designer should have a good general idea where any areas of future expansion might be proposed Designing in some additional trunk during an initial build can often pay off down the road when a new area is developed Only local people living in the area will know this information
222 Cable System Electronic Equipment
Cable system electronics have come a long way from where they started, from vacuum tubes to transistors and now to integrated circuits (ICs) Today s cable ampli ers have more gain, higher output operating levels, improved noise and distortion gures, and less power consumption Equally important is the high construction quality that gives the industry a superior, more reliable device
The industry has also become more knowledgeable about the metallurgy involved in cable systems, resulting in corrosion-resistant and weather-tight ampli er housings Modular construction of the ampli er sections has increased the exibility and serviceability of the present-day cable ampli ers Most cable ampli ers can be operated at more than one power supply voltage Most systems operate at 60 vac, more progressive systems operate at 90 vac Jumpers in the power supply section can be set to select the proper system voltage Also, the stability of the ampli ers has been improved With proper thermal compensation circuits, along with automatic gain and slope systems, present-day ampli ers can provide many years of service In many cases, a properly designed and constructed cable system will cause few problems, so service and technical personnel do not get enough service experience When things do break down, there can be a lot of head scratching, as workers try to remember where to start and what to do 2221 The theory of cascaded ampli ers has been well known for many years Radio communications people worked with short cascades to solve antenna preampli er and strip ampli er signal distribution problems Receiver designers used the theory to design multistage IF ampli ers Telephone company engineers used cascaded repeater ampli ers in long haul or long trunk lines where telephone voice channels were multiplexed on a coaxial cable Double sideband AM techniques were used where one voice channel occupied the upper sideband and another voice channel occupied the lower sideband of a carrier The carriers were in the 10- to 100-KHz range, and groups of channels were formed according to CCITT international speci cations Telephone traf c was routed through coaxial cable cascaded ampli er systems for much of the transcontinental runs This technique was also used in microwave radio relay systems where the channel groups operating in the KHz bands were frequently converted to microwave carriers and were relayed by microwave hops The microwave carriers could be converted to the lower-frequency ranges used on the coaxial cable system Many of the early cable television engineers and technicians found a lot of information about cascaded ampli er techniques in the technical papers published by the telephone companies Cascaded ampli er fundamentals appear in Appendix E Most people familiar with the theory use the mathematical formulas shown in Figure 2-10 and often work with them, designing system additions and extensions using a scienti c pocket calculator As the upper frequency limits of cable systems increased, and performance speci cations as well, the number of ampli ers in cascade had to