Exfo PSO-200 Optical Modulation Analyzer Manuel d'utilisateur Page 264

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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=
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