EXFO PSO-100 Series Optical Sampling Oscilloscopes - Manual (Page 119)

Measurement Definitions
Optical Sampling Oscilloscope 111
Signal-to-Noise Ratio
Signal-to-Noise Ratio
The eye signal-to-noise ratio (SNR) is a figure of merit of an eye diagram
indicating the eye opening relative to the noise in both the one and zero
levels. The eye SNR is defined as:
The noise in the one and zero levels, and , are the standard
deviations of the histogram at each level.
Note: The eye SNR, sometimes referred to as Q-value, is different from the SNR
used in vertical histograms and the definition of signal sensitivity (that is
P
2
one
/σ
2
one
).
Crossing
The crossing percentage is a measure of the location of the eye crossing
points relative to the separation between the one and zero levels.
Horizontal time histograms are constructed, and the histogram having the
smallest variance determines the crossing point.
EyeSNR 20log
10
P
one
P
zero
– ( )
σ
one
σ
zero
+ ( )
----------------------------------- =
σ
one
σ
zero
Histogram windows
100 %
Crossing = x %
0 %
Smallest histogram