EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 49)

Customizing the Graph and Data Layout
Optical Modulation Analyzer 41
Constellation Chart
Constellation Chart
Useful when characterizing advanced modulation schemes such as
DP-QPSK, the constellation chart is a representation of a signal modulated
in phase and/or in amplitude.
It shows valid symbols (amplitude and phase relative to the carrier) of a
modulation format on a polar graph. Since each symbol is encoded with n
bits (depending on the modulation scheme), there will be 2
n
permitted
symbols.
The “I” axis (in-phase) represents the “real” part of the complex field,
while the “Q” axis (quadrature) represents the “imaginary” part.
The length of the vector to the symbol in the chart is the amplitude of the
signal (normalized in the Optical Modulation Analyzer application), and its
angle from the “I” axis is the phase of the signal.
Note: There is one constellation chart per polarization state (X and Y).
}
}
“Q”
“I”
Symbol
I-Value
Q-Value

Polar-to-Rectangular Conversion
Project signal to
“I” and “Q” axes