242 Keysight M8000 Series of BER Test Solutions User Guide
4 User Interface - M8020A Display Views
Differences to a hardware based error detector
Due to the post processing style of error counting, there are some
differences that need to be taken into account when measuring BER.
1 The measurement duration to achieve a certain target BER with a given
confidence level, or simply to compare a given number of bits, is
significantly longer than with a hardware based error detector.
Therefore a meaningful target BER for a real-time oscilloscope based
setup is 1e-6 or higher.
2 Elapsed time cannot be used to estimate or define the number of
compared bits.
This is having impact on how measurements, especially the error ratio
measurement, have to be configured when specifying the
measurement duration. Always use either a pass/fail criteria or specify
the number of bits to measure. Do not specify the measurement
duration in terms of time.
3 Single error events may not get recognized.
The smallest recognizable BER per acquisition is defined by the
number of bits per acquisition under DataIn1 -> Acquisition. Error
events that occur with a lower probability will not be detected reliably
if these are single events. However, when running the error ratio
measurement in pass/fail mode, or until a specified number of bits has
been compared, the error information will be statistically correct for
randomly distributed error events. Increasing the number of bits per
acquisition will not only lower the smallest detectable BER of the
instantaneous BER result, but also reduce the probability that recurring
error events are missed.
4 Inserting single errors, or using an error insertion ratio that is less than
the detectable BER, will most likely result in not detecting the error
events.
5 Auto-alignments do stop the acquisition. This can have side effects in
controlling software if this is monitoring the status bits.
6 The configured pattern sequence is absolutely uncorrelated to any
other data channel or sequence in M8070A. This is even true, if
multiple error detectors are using the same sequence.
7 The instantaneous BER updates only after a complete acquisition cycle
has finished. Depending on the configured number of bits per
acquisition, symbol rate, clock recovery and equalizer configuration,
this can be in the range of every second to several minutes.
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