Agilent Technologies N4906B Serial BERT User Manual page 107

Sampling Point Setup
Agilent N4906B Serial BERT, User's Guide, October 2005
BER Threshold
In this list, select an alignment BER threshold that is appropriate for
your application.
The alignment BER threshold is the pre-defined threshold used by the
Data Center, 0/1 Threshold Center, and Auto Align functions to define
the edges of the data input eye in the time and voltage axes. You may
wish to change the threshold for the following reasons:
• Choosing smaller alignment BER thresholds will cause the auto-
search functions to set more accurate sampling points. However, if
the BER threshold is set lower than the residual BER of the
measurement, the auto-search functions will fail. 1E-7 is the
smallest BER threshold available.
• Running the auto-search functions and viewing eye results at
different BER thresholds will allow you to see the eye margin at
specific BERs. You can also construct basic eye contour
information.
Avg. 0/1 Threshold
Activate this checkbox to make the error detector continuously set the
0/1 threshold level to the average DC voltage level of the incoming data
signal.
NOT E
Manual 0/1 threshold changes are impossible while this function is
selected.
This function is useful for providing a "starting point" for sampling
somewhere within the data eye on the vertical voltage axis. It is
recommended to use this prior to clock/data alignments.
When used with the following patterns, this function may provide a
0/1 threshold that requires no further adjustment:
• 2^n-1 PRBS patterns
• Patterns with 50% mark densities (large amplitudes and no
excessive noise)
Setting up the Error Detector
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