User's Manual Part 2

Tech-X Flex
®
(NG2) Tech-X Flex User Guide - Firmware v06.50
8-21
Intro
Overview
Wi-Fi
Ethernet
System
IP/Video
MoCA
RF
Specs
In this case, the variation of the actual amplitudes from the expected levels for perfect modulation would
cause the symbol to be plotted off-center. Because the I and Q signals both deviate from the expected
amplitude, the plot is pushed away from center on both axes:
Figure 8-16 Imperfect symbol plot
When the unit draws the constellation graph, it samples a small subset of symbols on the selected
channel and plots them in the respective square. In summary, then:
• The more closely the symbol plots are clustered at the center of each square, the better the
performance of the QAM signal overall. As real-world perfection is not possible, a tight cluster close
to the center generally represents the best performance possible.
• If the amplitude of one or both signals deviates so much the original binary number is misinterpreted
as another, bit errors occur. A constellation diagram that shows widely dispersed points indicates a
signal where this is more likely to occur. A diagram with closely-clustered points indicates the
opposite, where the reliability of the signal is strong and bit errors/ambiguities are less likely.
• The overall amount of deviation from the ideal is a key factor in the calculation of MER. An MER
measurement, therefore, is fundamentally a numerical summarization of a constellation graph.
Figure 8-17 Symbol plot examples
Any nature of impairment (such as noise) can cause degradation of an amplitude-modulated symbol and
cause scattered plots on a constellation graph. Certain types of impairments tend to show a recognizable
signature on the graph. The following figures show some of these common impairments:
Actual plot
Ideal location
Symbol cluster
on a good signal
Symbol cluster
on a poor signal
Preliminary issue - Limited distribution only!