EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 249)

Coherent Detection and Sampling Methods
Optical Modulation Analyzer 241
Sampling Methods
Real-Time Sampling
Real-time sampling means that a signal is acquired with high-speed
sampling in order to track the instantaneous amplitude variations.
Real-time sampling enables measurements of single events and
transitions, but requires high-speed electronics, since accurate
reconstruction of a digitized signal requires a sampling rate that exceeds
twice the signal bandwidth, as dictated by the Nyquist criterion.
An insufficient sampling rate gives rise to the “aliasing” phenomenon, that
is, high-frequency components are misinterpreted as lower frequency
components and corrupt the reconstruction of the signal. It is therefore
common practice to implement anti-aliasing filters to limit the analog
bandwidth of the sampling to below half the sampling rate. To this date,
commercially available electric real-time sampling systems have analog
bandwidths typically less than 20 GHz and sampling rates up to around
50 samples per second.
Equivalent-Time Sampling
The PSO-200 Optical Modulation Analyzer uses equivalent-time sampling,
which is the most common technique to achieve sampling with high
bandwidth. The sampling rate is lower than in real-time sampling, which
eliminates the need for high-speed acquisition electronics.
The signal waveform is built-up by sweeping the sampling point over the
signal; hence signal reconstruction using equivalent-time sampling is
restricted to measurements of periodic signals and studies of deterministic
events. Constellation and eye diagrams of random pattern data sequences
with fixed symbol rate can also be acquired.