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Table 1-2 mV Equivalents in dBmV
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1V Voltage (in mV) 1000 100 10 1 01 001 0001 dBmV 60 40 20 0 20 40 60
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Note: dBmV
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20 log V (mV)/1 (mV) 0 dBmV
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1 mV across a resistance of 75 ohms
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Figure 1-3 The unity gain building block
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+20 dB gain +30 dBmV 20 dB +30 20 =10 dBmV +30 dBmV 20 dB
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+20 dB gain +30 dBmV
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Unity gain Unity gain
nal level at the low-frequency end of the spectrum has to be corrected by the internal equalizer network at the ampli er input section Such equalizers come in a variety of values and are plug-in, so the correct value can be selected for the speci c cable section Present-day ampli ers still have selectable pads and equalizers to adjust the input signal level for each ampli er station The standard procedure is to measure the ampli er input signal level at both the low and high end (usually pilot frequencies) and then select the appropriate pad and equalizer to facilitate the setting of the output gain and slope This technique will be treated in more detail later in the book This procedure is known as ampli er or system balancing and is extremely important for the ampli er cascade to perform with optimum
Introduction to Cable Communications Systems
distortion and noise speci cations For a study of ampli er cascade noise see Appendix E
122 Development of Coaxial Cables
Coaxial cable has undergone many developments over the years The rst type of coaxial cable had a braided copper wire shield and was covered by a plastic (polyethylene) jacket Shielding effectiveness was poor because of the weaving of the braided shield, which allowed the cable to be quite exible Connecting to such a shield was a problem in itself The center conductor was often a copper wire or copper-plated steel wire The insulation was often a harder polyethylene Early coaxial cable was usually found on the surplus market left over from World War II This type of cable had high attenuation, poor shielding, and was easily damaged by moisture With the start of the cable television industry, cable manufacturers were quick to realize the need for more and better cable 1221 The solid-aluminum-sheath cable was introduced in the 0412-in and 0500-in sizes with a nominal impedance of 75 2 ohms The dielectric was either Styrofoam or polyethylene foam, and the center conductor was either solid copper or copper-clad aluminum A polyvinyl chloride (PVC) jacket was available and was recommended in coastal climates The solid-aluminum sheath (tubing) vastly improved the shielding effectiveness of the cable In a great many areas, the bare aluminum cable worked well and became widely used A whole series of connectors appeared on the market Some were better than others and eventually the best became even better as the cable television industry grew The solid aluminum sheath cable was dif cult to handle; it was a lot stiffer and could be easily kinked, thus destroying the transmission capability Proper construction procedures, tools, and equipment were developed to correctly handle and install the cable in an expeditious manner With the development of this cable, the cable television industry could extend its plants, further serving more subscribers with more channels and improved pictures 1222 Various techniques were used to try to improve the loss-versusfrequency characteristics of coaxial cable Since dry air has a velocity constant of 095, a cable with pure dry air as a dielectric should provide about the best attenuation-versus-frequency characteristic Because some means has to be employed to keep the center conductor in the center (air
1
itself just does not do the job), one cable manufacturer used polystyrene disks placed a few inches apart, sealing in cells of air as well as supporting the center conductor in a concentric condition Such cable did provide superior loss-versus-frequency characteristics, but it was dif cult to manufacture because fusing the disks in place was a critical process Some of this cable is most likely still in service and an improved version is available today Coaxial cable used today uses the polyfoam technique, where the plastic foam forms bubbles of air that control the dielectric constant This cable has proven to be very rugged and maintains its characteristics over most climatic conditions 1223 The attenuation of cable as a function of frequency is a wellknown characteristic to the cable television industry Essentially, the loss, or attenuation, versus frequency is a logarithmic function, as shown in Figure 1-4 Most manufacturers publish a chart of attenuation versus frequency so system designers can calculate the cable span loss between ampli ers The usual design technique is to calculate the span loss at the highest operating frequency that exhibits the greatest attenuation This assures that each ampli er has the proper input and output signal levels at the upper frequency limit The lower-frequency end will have to use some equalization so the ampli ers are not overdriven As far as cable performance, there have not been any large breakthroughs in cable loss characteristics, but several manufacturers have improved their manufacturing techniques, which have produced cable that is more rugged, has a lower return loss characteristic, and demonstrates improved environmental performance Essentially, the loop resistance of coaxial cable depends on the conductivity of the center and outer conductors Since the solid aluminum sheath cable has high conductivity for the shield, most of the loop resistance is attributed to the center conductor Making the center conductor out of aluminum and cladding it with copper produces a lower resistance at the RF operating frequencies due to the well-known skin effect However, the resistance at 60 Hz is appreciable enough to limit cable powering of the repeater ampli ers By using a solid-copper center conductor, the loop resistance of the cable span is decreased, resulting in possibly fewer 60-Hz power supplies This fact has to be taken into account when working out the cost per mile gures Also, the larger the cable size, the larger diameter center conductor, which results in lower loop resistance values Again, lower loop resistance caused by either choice, solid-copper center conductor or larger
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