EXFO PSO-100 Series Optical Sampling Oscilloscopes - Manual (Page 130)

Asynchronous Sampling Principle
122 PSO-100
Frequency Holes
Directly after startup of the instrument the sampling frequency drifts
relatively fast until it settles. During this time, you may pass a number of
frequency holes. If you still have ended up in a situation with recurrent
frequency holes, a possibility is to tune the signal frequency by only a few
100 Hz to jump out of the frequency hole. To reduce the probability for
frequency holes, you can increase the number of samples/layers and the
number of accumulated layers in the Display menu. We have calculated
that the probability for frequency holes is less than 0.1 % for 128 k samples,
if you characterize a frequency hole event by having less than 100 distinct
points over one bit period.
Note: If we visualize patterns instead, these events become more frequent as you
increase the total time window.