Keysight M8000 Series of BER Test Solutions User Guide 239
User Interface - M8020A Display Views 4
Additionally the standard Error Ratio and Jitter Tolerance measurements
of M8070A can be used, which is not possible with Infiniium's built-in
BER/SER measurements.
Integrating an oscilloscope into M8070A as an error detector does
introduce a few oscilloscope related terms that did not exist in the context
of error detectors in the past. One important term to understand is
acquisition.
A regular error detector is processing the received data in real-time and is
comparing and counting each and every bit that it receives. An
oscilloscope as error detector, or better digitizer, is not capable of doing
this.
The oscilloscope is capturing the sampled input signal into its acquisition
memory and then starts processing of the captured data in order to
improve signal shape, apply clock recovery or equalizer algorithms. While
the oscilloscope is doing this processing, it does not capture the received
input signal. Which means that only a relatively small number of bits is
being captured and finally compared with the expected pattern, relative to
the time that it takes to process the bits in software.
The steps that it takes from capturing the received signal in the acquisition
memory to finally updating the error counters in M8070A are summarized
under the term acquisition.
In M8070A the acquisition is normally enabled, thus BER is getting
measured. But it can be disabled. This leaves the error detector
parameters still under control of M8070A, but additionally enables
changing or tweaking oscilloscope settings locally. Therefore settings that
are not directly offered by M8070A can still be optimized for the specific
measurement task. For example, selecting a type of clock recovery that is
not offered by M8070A, or manually fine tuning the threshold values.
While acquisition is enabled, the local user interface of the oscilloscope is
locked and no local changes are possible.
The most important parameter in the context of acquisition is the number
of bits that shall be processed in one acquisition cycle. This will define the
required acquisition memory size on the oscilloscope. Setting the number
of bits to a larger value will lower the detectable BER per acquisition. And
since a single acquisition directly corresponds to an error counter update
in M8070A, this defines the lowest BER value greater than 0.0 that the
instantaneous BER will report.
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