n one tap to the TV set on the other tap. For example, a line voltage signal level is +34.8 dBmV entering a tap. The tap insertion loss is 0.4 dB SO the level of the signal leaving the tap to the next tap or extender amplifier is +34.4 dBmV. The tap is two-port. We know we want at least a +10.5 dBmV at the port output. Calculate +34.8 dBmV - X dB = +10.5 dBmV. Then X = 24.3 dB, which would be the ideal tap loss value. Taps are not available off-the-shelf at that loss value, the nearest value being 23 dB. Thus the output at each tap port will be +34.8 dBmV - 23 dB = 11.8 dBmV. 17.4 HYBRID FIBER-COAX (HFC) SYSTEMS The following advantages accrue by replacing the coaxial cable trunk system with optical fiber: Reduces the number of amplifiers required per unit distance to reach the furthest subscriber Results in improved C/N and reduced CTB and Xm levels Also results in improved reliability (i.e., by reducing the number of active components) Has the potential to greatly extend a particular CATV serving area One disadvantage is that a second fiber link has to be installed for the reverse direction, or a form of WDM is required, when two-way operation is required and/or for the CATV management system (used for monitoring the health of the system, amplifier degradation, or failure). The concept is illustrated in Figure 17.7. Figure 17.8 shows an HFC system where there are no more than three amplifiers in tandem to reach any subscriber tap. Also note that with this system layout there cannot be a catastrophic failure. For the loss of an amplifier, only one-sixteenth of the system is affected in the worst-case scenario; with the loss of a fiber link, the worst case would be one-sixth of the system. 17.4.1 Design of the Fiber-Optic Portion of an HFC System Before proceeding with this section, it is recommended that the reader turn back to Chapter 9 for a review of the principles of fiber-optic transmission. There are two approaches to fiber-optic transmission of analog CATV signals. Both approaches take advantage of the in