Agilent Technologies N4906B Serial BERT User Manual page 113

Pattern Synchronization
Further Considerations
Agilent N4906B Serial BERT, User's Guide, October 2005
The detect word on which the error detector attempts to resync is
chosen strictly by chance. So if there are two instances of the detect
word in the pattern, the error detector has a 50 % chance of selecting
the correct one.
The more instances of the detect word exist in the pattern, the higher
are the chances for incorrect synchronization. The software attempts
in any case to identify a 48-bit pattern that occurs as seldom as
possible in the pattern. For very large patterns, this can unfortunately
take a very long time, and the software ends the search if it expects
that it would take longer to find an adequate detect word than it would
to attempt to synchronize. If the search for a detect word is ended, the
most unique detect word identified is used.
Patterns must always be synchronized in order to do accurate BER
testing. If patterns are out of alignment by just one bit, errors can be as
high as 50% (5E-1) for PRBS patterns, and 100% (1E+0) for custom
patterns.
By default, the error detector is in automatic sync mode with a sync
threshold of 1E-3. This setting is recommended for most applications,
and usually allows the synchronization function to be "transparent",
requiring no attention. However, for special applications, changes can
be made to the sync mode and sync threshold.
What Type of Synchronization Should You Use?
The type of synchronization you use affects how errors are measured
and displayed. A Sync Loss is recognized when the BER is greater than
the sync threshold. This can be caused by a high error rate, pattern
misalignment, or clock loss. Choose the sync mode setting that is
appropriate for the type of errors you anticipate.
• Automatic Sync with a sync threshold BER of 1E-3 is recommended
for most applications.
With this mode selected, the synchronization algorithm starts
whenever the BER exceeds the threshold. However, it is not possible
to make accurate BER measurements higher than the sync
threshold.
Setting up the Error Detector
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