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Amplitude Modulation


Amplitude modulation (AM) uses the same scheme as AM radio. This simple technique often requires very low-cost hardware. Two basic types of AM techniques include baseband and RF carrier. In a baseband system, the input signal directly modulates the strength of the transmitter output, in this case light. In the RF carrier AM technique, a carrier, with a frequency much higher than the encoded information, varies according to the amplitude of the information being encoded. In a fiber optic system, the magnitude of the voltage input signal directly translates into a corresponding light intensity. Figure 1 illustrates the operation of an AM system.
Figure 1 - AM System Operation

AM System Operation
In order to use AM transmission over optical fiber, there must be compensation for optical link loss budget. This problem may be solved two ways: by taking advantage of some special property of the input waveform (for video, the sync pulse is an invariant that can be used to distinguish optical loss from signal level variation) or using a technique that allows the signal level to be interpreted independently from optical loss. To accomplish this, one could send a pilot tone at a high frequency that is above or below the frequency of the information being encoded. Three techniques for AM video transmission include baseband AM, RF carrier AM, and Vestigial-sideband AM. Studying the corresponding frequency spectra of these modulation schemes, illustrated in Figure 2, allows for a clearer understanding of their differences. Simple baseband AM occupies the region from DC to about 5 MHz and requires the least bandwidth (assuming we are talking about uncompressed digital encoding techniques). The RF carrier modulation spectrum is similar; it has been shifted to a non-zero frequency (F). This approach requires additional bandwidth and offers no advantage over baseband operation in a single channel per fiber system. However, it does allow multiple channels to be combined onto a single fiber. With vestigial-sideband AM, the spectrum again shifts to a non-zero frequency (F), and filtering removes the lower sideband. It allows for more efficient use of the spectrum as compared to straight RF carrier AM, requiring half the bandwidth per channel.
Figure 2 — Frequency Spectra of Three AM Transmission Techniques

Frequency Spectra of AM Transmission Techniques


AM modulation has two main drawbacks. First, the system requires highly linear components to prevent signal distortion as it travels through the communication link. Second, because varying the light intensity encodes the signal, the receiver cannot necessarily differentiate between intended signal level variations and the optical loss that occurs naturally in the fiber itself. For instance, providing a 100% maximum signal into the optical transmitter with 10 dB of optical loss between the transmitter and receiver, the receiver would indicate that the signal level is 10%. The receiver cannot readily distinguish between changes in signal level and changes in optical loss due to the fiber. Automatic gain control can compensate for these losses in an AM link. One approach uses a sophisticated circuit that analyzes the input waveform and sets the sync pulse level to the required 40 IRE units. Figure 3 illustrates this.

Figure 3 — Sync Pulse Level AGC

Sunc Pulse Level AGC


The need for highly linear components can erase much of AM's advantage over other techniques because of the expense associated with obtaining highly linear LEDs. In spite of the difficulties mentioned above, AM systems represent the simplest and least expensive approach to encoding information for fiber optic transmission.