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Digital Analysis (QAM View OPT4): Constellation Mode Common Impairment Types Figures 9-12 through 9-14 show constellation displays depicting three common impairments – thermal (system) noise, phase noise, and coherent interference. Gain compression and I/Q imbalance (not shown here) are examples of two other impairment types with specific graphic signatures. Thermal noise impairments cause the dots on the graph to spread out toward the grid lines (decision boundaries), indicating significant errors (see Fig. 9-12). The zoom feature would show more detail about the position of these dots in relation to the grid lines. Fig. 9-12 Thermal Noise Impairment Phase noise impairments cause the dots on the graph to form a circular pattern (see Fig. 9-13). Phase noise is best seen on the grid without the use of the zoom feature. 9 167
