EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 263)

Measurement Definitions
Optical Modulation Analyzer 255
Measurements for Constellation Charts
Quadrature-Phase Error
The quadrature-phase error (EQ) is the rms deviation of the Q component
of the error vector, denoted Q in the graph. It is calculated according to:
given in percent to the magnitude of the longest reference vector.
IQ Offset
The IQ offset measures the signal constellation DC offset from origin of the
constellation chart. Its value is given in dB units according to:
The following diagram illustrates a signal with IQ offset and the
corresponding ideal reference vectors.
EQ
rms
1
N
---- Q
meas
n ( ) Q
ref
r n ( )
–  
2
n 1 =
N 
100  %rms   =
IQ
offset
20
1
N
---- I
meas
n ( )
2
n 1 =
N  1
N
---- Q
meas
n ( )
2
n 1 =
N 
+ log dB   =