EXFO PSO-100 Series Optical Sampling Oscilloscopes - Manual (Page 127)

Optical Sampling Oscilloscope 119
D Asynchronous Sampling
Principle
Since the instrument has no hardware trigger, there is no way for the
system to control the sampling frequency relative to the signal frequency,
that is, the frequencies are asynchronous, and the sampling points will
appear at seemingly random positions in time. In order to synchronize the
data, the instrument relies on software synchronization. As a consequence,
there are some forbidden signal frequencies that will be sampled at the
same position of the eye, referred to as frequency holes.
Trigger-Free Synchronization
The figure below illustrates the optical data at bit rate B sampled at a
sampling frequency of f
s
. Samples of the signal with energy proportional to
the signal power at the overlap with the sampling pulses are taken.
Eye diagram synchronization requires that the offset frequency, Δf,
between the signal bit-rate, B, and the sampling frequency, f
s
, related by
B=Mf
s
+Δf, is known. Assuming that the bit-rate is known within the
correct f
s
/2 interval, then the integer M, which is the number of bit slots
between the samples, is well defined.
1/f
s
= M - T +
Δ
t
Δ
t
Optical
data (B)
Sampling
pulses (f
s
)
T= 1/B
Time
Time