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Application Notes
Digital modulation has created a revolution in RF and microwave communications. Cellular systems have moved from the old analog AMPS and TACS systems to second generation NADC, CDMA, and GSM standards. Today, the cellular industry talks of moving to third generation (3G) digital systems such as W-CDMA and cdma2000. Outside of cellular communications, the broadcast industry is also moving into the era of digital modulation. High Definition Television (HDTV) systems will use 8-level Vestigial Side Band (8VSB) and Coded Orthogonal Frequency Division Multiplexing (COFDM) digital modulation formats in various parts of the world.
The move to 3G systems will yield signals with higher peak-to-average power ratios (crest factors) than previously encountered. Analog FM systems operate at a fixed power level: analog double-sideband large-carrier (DSB-LC) AM systems cannot modulate over 100% (a peak value 6 dB above the unmodulated carrier) without distortion. Third generation systems, on the other hand, combine multiple channels, resulting in a peak-to-average ratio that is dependent upon not only the number of channels being combined, but also which specific channels are used. This signal characteristic can lead to higher distortion unless the peak power levels are accounted for in the design of system components, such as amplifiers and mixers.
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