Satellite & Cable TV

The third part of this series of articles takes a closer look at the practical aspects of Fiber Optic System design. It concludes with an actual system design request that we have received from a reader. Infact, this article was inspired by an actual query that we received from one of our readers in Southern India. This query will be discussed in detail at the end of this article, as a typical case study. BASIC SYSTEM DESIGN: OPTICAL POWER As explained in the earlier articles, the Optical Signal is generated by a laser diode inside the fiber Optic transmitter. The amount of light output available is measured in dBmW. This indicates the extent to which the optical output is greater than a 1 milli Watt Optical Output, which is taken as the 0 dB reference. An output signal of 3 dBmW indicates a power twice that of the 0 dB signal. Hence, 3 dBmW = 2 milli Watts. An output of 6 dBmW will be = 4 milli watts. Similarly, 9 dBmW = 8 milli Watts and 12 dBmW = 16 milli Watts. The output power keeps doubling for every 3 dB increase. Hence, as you can imagine a 12 dBmW Fiber Optic Transmitter will be substantially more expensive than a 9 dBmW transmitter. CABLE LOSSES: As explained in the earlier part of this article, fiber optic cable has a loss of 0.35 dB per KM @ 1310nm. However, there are other factors that add in a small way to the theoretical cable loss. Let us examine each of these in detail. SAG LOSS: When cable is laid overhead, even when supported by messenger wires, it sags hence the actual length of cable between two points is longer than the theoretical measurement. This amount is referred to as Cable Sag. System designers often build in an extra 2% to 5% length to compensate for cable sag.