Tektronix OM4006D, Руководство пользователя

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Operating basics
Constellation diagrams Many types of constellation diagrams can be chosen by clicking on the
constellation icon. Once the laser phase and frequency
fl uctuations are removed,
the resulting electric fi eld can be plotted in the complex plane.
When only the values at the symbol centers are plotted, this is called a
Constellation Diagram. When continuous traces are also shown in the complex
plane, this is often called by the more generic term of IQ Diagram. Since the
continuous traces can be turned on or off in the OUI, we refer to both as the
Constellation Diagram.
The scatter of the symbol points indicates how close the modulation is to ideal.
The symbol points spread out due to additive noise, transmitter eye closure, or
fi ber impairments. The scatter can be measured by symbol standard deviation,
error vector magnitude, or mask violations.
Constellation measurements. Measurements made on constellation diagrams are
the most comprehensive in the OUI. Numerical measurements are available on the
fl yout panel associated with each graphic window. The measurements available
for constellations are described in the following text.
Elongation: The mean inter-symbol spacing of the quadrature signals
divided by the mean inter-symbol spacing of the in-phase signals. “Tall”
constellations have Elongation > 1.
Real Bias: The real part of the mean value of all symbols divided by the
magnitude; expressed as a percent. A positive value means the constellation
is shifted right.
42 OM4000D Series Coherent Lightwave Signal Analyzer