tensity modulation characteristics of the fiber-optic source. Instead of digital modulation of the source, amplitude modulation (analog) is employed. The most common method takes the entire CATV spectrum as it would appear COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION) Fiber system Amplifier cascade Headend Figure 17.7 A model showing the concept of hybrid fiber-coaxial cable CATV system. TX, fiber-optic transmitter; RC, fiber-optic receiver. Figure 17.8 HFC system layout for optimal performance (one-way). on a coaxial cable and uses that as the modulating signal. The second method also uses analog amplitude modulation, but the modulating signal is a grouping of subcarriers that are each frequency modulated. One off-the-shelf system multiplexes in a broad FDM configuration, eight television channels, each on a separate subcarrier. Thus a 48-channel CATV system would require six fibers, each with eight subcarriers (plus 8 or 16 audio subcarriers). 17.4.1.1 Link Budget for an AM System. We will assume a model using a distributed feedback laser (DFB) with an output of +5 dBm coupled to a pigtail. The receiver is a PINFET with a threshold of -5 dBm. This threshold will derive approximately 52-dB S/N in a video channel. The C/N required is about 49.3 dB [see formulas (17.4) and (17.5)]. This is a very large C/N value and leaves only 10 dB to be allocated to fiber loss, splices, and link margin. If we assign 2 dB for the link margin, only 8 dB is left for the fiber/splices loss. At 1550-nm operation, assuming a conservative 0.4-dB/km fiber/splice loss, the maximum distance from the headend to the coax hub or first fiber-optic repeater is only 8/0.4 or 20 km. Of course, if we employ an EDFA (erbium-doped fiber amplifier) with, say, 20 dB gain, the distance can be extended by 20/0.4 or 50 km. 17.4 HYBRID FIBER-COAX (HFC) SYSTEMS Typical design goals for the video/TV output of the fiber-optic trunk are C/N (carrier-to-noise ratio) 58 dB, Composite second-order (CSO) products = -62 dBc (dB down from the carrier level), CT