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Frequency Modulation


Frequency modulation, called FM, is a more sophisticated modulation scheme than AM modulation. It is well-suited to the inherent properties of optical fiber since proper recovery of the encoded signals only requires measurement of timing information, one of fiber's strengths. FM is also immune to amplitude variations caused by optical loss, one of fiber's weaknesses. The heart of the FM modulator revolves around a high-frequency carrier. Now, instead of changing the amplitude of the carrier, the frequency of the carrier is changed according to differences in the signal amplitude. Part of the advantage of FM systems is buried in mathematical analyses that show that the signal-to-noise ratio at the receiver can be improved by increasing the deviation of the carrier. FM also has the advantage of eliminating the need for highly linear optical components that are required for AM systems. Often optical systems employing FM encoding refer to the technique as pulse-frequency modulation (PFM). This simply means that the FM signal is limited (converted to digital 0's and 1's) before it is transmitted over the fiber. The result is the same. In multiple channel FM systems, each video signal is modulated on a separate carrier. These carriers are then combined to modulate one light source. Each FM video modulator is adjusted to balance distortion products, achieve best signal-to-noise/bandwidth plus compensate for the non-linearity of the light source. As more video channels are added, the overall modulation index of the combined signal increases, which degrades the per channel signal-to-noise ratio (i.e., there is less optical power available per channel.) FM optical systems almost always require more complex electronic circuitry than AM optical systems, but often the total cost is comparable since lower-cost optical components can be used in the FM system. Figure 1 shows the block diagram of an FM video link. Assume that a transmitted video signal has a 5 MHz bandwidth. Using a 70 MHz carrier frequency and applying the video signal to produce a 5 MHz deviation, the receiver achieves about a 5 dB enhancement in its signal-to-noise ratio, compared to an AM system. If we increase the deviation of the carrier frequency to 10 MHz, then the improvement increases to 15.6 dB. Unlike AM, FM eliminates the need for highly linear optical components, another important advantage.

Figure 1 - FM Video Link

FM Video Link


Three techniques for FM video transmission include sine wave FM, square wave FM, and pulse-frequency modulation. The presence of harmonics yields the notable difference between sine wave FM, square wave FM, and pulse-frequency modulation, as shown in Figure 2. The square wave FM spectrum signal contains only odd-order harmonics. The pulse-frequency modulation spectrum contains all odd- and even-order harmonics yielding a cluttered spectrum poorly suited for multiple-channel stacking; however, it retains its value as a single-channel transmission scheme.

Figure 2 - Frequency Spectra of Three FM Modulation Techniques

Three FM Modulation Techniques


Sine wave FM offers an effective means of transmitting multiple channels. In this technique, multiple channels are each assigned a separate carrier frequency, such as illustrated in Figure 3. In this example, four channels of video have been assigned frequencies of 70 MHz,90 MHz, 110 MHz, and 130 MHz.

Figure 3 - Four Channel Video Transmission Using Sine Wave FM Modulation

Sine Wave FM


Often optical systems employing FM encoding refer to the technique as pulse-frequency modulation (PFM). This limits the FM signal by converting it to digital 0's and 1's before transmission. Generally the modulator is designed so that the pulse frequency increases as the input voltage increases. Regardless of the FM technique used, however, these optical systems almost always require more complex electronic circuitry than AM optical systems, but often the total cost is comparable since lower-cost optical components can be used in the FM system.