EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 264)

Measurement Definitions
256 PSO-200
Measurements for Constellation Charts
Quadrature Error
Quadrature error appears when the I and Q channels are not operating
precisely at 90 degrees relative to each other. The quadrature error is
referenced to the average vector of the demodulated symbols:
where r =1..M (for M level signals) refers to the related symbol of the
normalized measured I and Q component. The quadrature error is
computed as the rms for all constellation symbols and expressed as the
angular deviation from the ideal 90 degrees, according to:
Quadrature error is not computed for binary modulation formats.
The following diagram illustrates a signal with quadrature error and the
corresponding ideal reference vectors:
S
avg
r
I
avg
r
Q
avg
r
 
 
 
1
N
---- I
meas
n ( )
n 1 =
N 
1
N
---- Q
meas
n ( )
n 1 =
N 
 
 
 
 
= =
QE
r
90
1
2
---
I
avg
r
 
2
I
ref
2
–   Q
avg
r
 
2
Q
ref
2
–   +
I
ref
Q
ref

------------------------------------------------------------------------------------ 
 
 
 
acos – =
QE
rms
1
M
----- QE
r
 
2
r 1 =
M 
deg   =