VeEX VePal CX350, Руководство пользователя

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The diagram is an X/Y plot of the I (In-phase) and Q (Quadrature) axis components of the QAM signal and is particularly useful for viewing impairments due to ingress
and/or modulation problems present on the digital signal.
A symbol (essentially a waveform representing one or more bits) should ideally appear as a compact or clean dot in the center of each symbol box. Decision
boundaries exist within the constellation so that if the symbol falls within the boundary, the correct data is received.
If due to noise or other interference, the symbol falls outside the boundary, the data is in error. Error detection and and correction (e.g. FEC) will attempt to the
correct these errors, thus avoiding video impairments.
Observing the symbol's shape and location relative to their ideal positions allows you to draw some conclusions about the nature of an impairment
Understanding the Constellation Diagram
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The following impairments can be assessed visually:
Gain compression
- Pulls the symbol clusters into the center while the middle ones are unaffected. Gain compression can result from poor or bad amplifiers, IF
equalizers and up/down converters.
Gain Compression
Broadband or system noise
- Causes symbol clusters to enlarge, which increases the probability of errors. An error occurs when the dot is pulled across a decision
boundary.
System Noise - Incoherent Interference
CX350 e-Manual D07-00-037 RevC01 Page 27 of 81