EXFO PSO-100 Series Optical Sampling Oscilloscopes - Manual (Page 131)

Asynchronous Sampling Principle
Optical Sampling Oscilloscope 123
Pattern Effects
Pattern Effects
The software synchronization principle also results in another
phenomenon. If there is no external clock present and you must rely on the
internal signal synchronization, there may be pattern depending effects in
some situations.
For example if you sample a 2
6
-1 = 63 bit PRBS eye diagram and chose to
visualize 3 eyes. Since 63 is evenly divisible by 3 you would see that not all
combinations of transitions occur for every eye.
Besides, there is no way for the software to discriminate between different
eyes from layer to layer. As a result, the eyes may jump around and look
different from layer to layer. This may look strange the first time you see it,
but now when you are aware of it, you know where it comes from. A way
to get rid of this effect is of course to visualize for example 4 eyes instead of
the 3 eyes shown here.