Cisco GS7000 Installation And Operation Manual page 81

Shape
4037424 Rev A
Focus Impairment
Individual cells Coherent
Interference
Individual cells Gaussian Noise
Entire QAM Phase Noise
constellation
Entire QAM Gain Compression
constellation
Entire QAM I-Q Imbalance in the
constellation Modulator
Entire QAM Quadrature
constellation Distortion
RF Constellation Page
Description
Individual cells of QAM
constellation contain diffused
hollow circles or
―doughnuts‖. This indicates
an interfering carrier and
shows the effect of not
allowing the carrier to ever
reach the proper point in the
target range.
Individual cells contain a
complete and fairly uniform
smear up to all decision
boundaries, and is usually
caused by improper system
setup, too many amplifiers in
a cascade, damaged/
overheated hardware, and/or
low power.
QAM constellation consists of
smeared, concentric, circular
patterns.
QAM constellation looks
uniformly square, but the
outside corners appear to be
―smashed‖ toward center of
grid (compression in the RF
plant).
Overall appearance of QAM
constellation is rectangular
rather than the desired square
shape (square inequality).
Overall appearance of QAM
constellation has a twisted or
skewed parallelogram shape.
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