EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 265)

Measurement Definitions
Optical Modulation Analyzer 257
Measurements for Constellation Charts
Signal-To-Noise Ratio
The signal-to-noise ratio (SNR) computed in the constellation is the ratio of
the symbol amplitudes over the two-dimensional noise distribution. The
presented value is the average SNR of all symbols, r = 1..M, according to
where 
I,Q
is the standard deviation of the I
meas
and Q
meas
, respectively.
XY Imbalance
The XY imbalance measurement is the ratio of the amplitudes of the X and
Y polarization tributaries of the signal:
Power Level
The power level is an approximation of the power level on polarization X, Y
and global.
where K is the total number of acquired samples of the waveform
(transitions and center points).
SNR 10
1
M
-----
I
avg
r
2
Q
avg
r
2
+

I
2

Q
2
+
---------------------------------------
 
 
 
 
r 1 =
M

dB   log =
XYimbal
S
avg X 
S
avg Y 
------------------ =
Power
X
1
K
---- S
X
k ( )
2
k 1 =
K 
= Power
Y
1
K
---- S
Y
k ( )
2
k 1 =
K 
=
Power
TOT
Power
X
Power
Y
+ =