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),
= -65 dBc.
One technique used on an HFC system is to employ optical couplers (a form of power
splitter), where one fiber trunk feeds several hubs. A hub is a location where the optical
signal is converted back to an electrical signal for transmission on coaxial cable. Two
applications of optical couplers are illustrated in Figure 17.9. Keep in mind that a signal
split not only includes splitting the power but also the insertion loss
of the coupler. The
values shown in parentheses in the figure give the loss in the split branches (e.g., 5.7 dB,
. FM systems are much more expensive than their AM counter-
parts but provide improved performance. EIA/TIA-250C (Ref. 5), discussed in Chapter 16,
specifies a signal-to-noise ratio of 67 dB for short-haul systems. With an AM fiber-optic
system it is impossible to achieve this S/N, whereas a well-designed FM system can
conform to EIA/TIA-250C. AM systems are degraded by dispersion on the fiber link;
FM systems much less so. FM systems can be extended further than AM systems. FM
systems are available with 8, 16, or 24 channels, depending on the vendor. Of course,
channel capacity can be increased by increasing the number of fibers.
Figure 17.10 shows an eight-channel-per-fiber frequency plan, and Figure 17.11 is a
transmit block diagram for the video portion of the system. Figure 17.12 illustrates a
Two-way and three-way splits of a light signal transporting CATV.
Insertion loss (IEEE, Ref. 10) is the total optical power loss caused by the insertion of an optical component
such as a connector, splice, or coupler.