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Chapter 11: Digital Upconversion and Downconversion - Pg. 113

CHAPTER 11 Digital Upconversion and Downconversion Previously, we discussed complex modulation and baseband signals. The whole point of this discussion is to create a signal that can be used to carry the information bits from one location to another, whether over copper wire, a fiber-optic cable, or electromagnetically through the air. In nearly all cases, the signal needs to ride on a carrier frequency to be efficiently sent from one location to another. To do this, the signal frequency spectrum needs to be able to be moved up and down the frequency axis at will. This is a process of (frequency) upconversion and downconversion. All early methods used analog circuits to accomplish this task. In the past couple of decades, digital circuits, particularly FPGAs, have developed the computational capacity to perform these functions in many cases and offer important advantages over analog methods. These methods are known as digital upconversion and downconversion (also known as DUC and DDC, respectively, within the industry). The process of upconversion is to take a signal that is at baseband (the frequency representation of the signal) and move or shift that frequency spectrum up to a carrier frequency. The width of the signal's frequency spectrum does not change; it is just moved to