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Use of the Q-factor Calculations
Comparison of the Results
Agilent J-BERT N4903B High-Performance Serial BERT
This graph shows two straight lines. The intersection of these lines marks the Q-
factor and the Q optimum Threshold.
Mathematically, the standard deviation and mean values are calculated as:
1
s = -
B
-A
m =
B
This calculation leads to the values of μ
Notes on the Q-Results
You can specify the range of data points used for these calculations by means of
the Min BER for Q and BER Threshold parameters (see also
274).
For example, you can calculate the Q-factor from relatively few data points with
measured bit error rates between 1e-10 and 1e-5. The calculation tells you the
optimum decision threshold and also the BER expected when the error detector
uses this threshold.
To measure such small bit error rates, you have to compare an adequate number
of bits. Depending on the DUT characteristics, you may also have to use a very fine
step size for the threshold variation.
On the other hand, such a comparison may make it easier to understand the
dependencies.
If you set the BER Threshold to a position that gives you only a few data points,
you will find:
•
The Q-factor is relatively high.
Advanced Analysis
and σ
.
1,0
1,0
"View Tab" on page
6
285
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