EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 273)

Measurement Definitions
Optical Modulation Analyzer 265
Measurements for Eye Diagrams
IQ Skew
The IQ skew is the time difference between I and Q tributaries of the signal
for each polarization. It is defined from the mean crossing points T
cross
of I
and Q, respectively, according to
The global skew takes both polarizations into account, according to
where X and Y indexes refer to X and Y polarization states.
IQ skew is not applicable to binary modulation formats (BPSK, OOK).
IQSkew
T
cross1 Q 
T
cross1 I 
– T
cross2 Q 
T
cross2 I 
– +
2
----------------------------------------------------------------------------------------------------------------- =
GlobalSkew
Max T
cross1
( ) Min T
cross1
( ) – Max T
cross2
( ) Min T
cross2
( ) – +
2
----------------------------------------------------------------------------------------------------------------------------------------------------- =
T
cross1
T
cross1 I X  
T ,
cross1 Q X  
T ,
cross1 I Y  
T ,
cross1 Q Y  
=
T
cross2
T
cross2 I X  
T ,
cross2 Q X  
T ,
cross2 I Y  
T ,
cross2 Q Y  
=