EXFO PSO-200 Optical Modulation Analyzer User Manual
Page 251
Coherent Detection and Sampling Methods
Optical Modulation Analyzer
243
PSO-200 Principles of Operation
When the LO pulses overlap the input signal, mixing occurs, and optical
samples are generated. These samples are detected, captured in an ADC in
long batches, processed, and finally displayed, for example in the form of a
constellation chart.
The sampling frequency is determined by the pulse repetition frequency
(f
s
=1/T) of the LO pulse source, whereas the resolution of the sampling
gate is determined by the LO pulse width (t). If the bandwidth of the
sampling gate is much larger than the symbol rate (that is, the signal phase
and amplitude are approximately constant over the LO pulse duration t),
the set of optical samples are given by the equations below.
In order to achieve undistorted constellation charts, the pulses should be
characterized by low noise, low timing jitter, short temporal duration, and
be chirp-free. The sampling frequency f
s
is independent of the symbol rate
B, however, advantageously, f
s
can be tuned to scan over the signal.
Ch1 n
I
x
nT
(
)
4 E
s x
nT
(
)
E
LO x
IF x
nT
s x
nT
(
)
LO x
nT
(
)
–
+
cos
=
=
Ch2 n
Q
x
nT
(
)
4 E
s x
nT
(
)
E
LO x
IF x
nT
s x
nT
(
)
LO x
nT
(
)
–
+
sin
=
=
Ch3 n
I
y
nT
(
)
4 E
s y
nT
(
)
E
LO y
IF y
nT
s y
nT
(
)
LO y
nT
(
)
–
+
cos
=
=
Ch4 n
Q
y
nT
(
)
4 E
s y
nT
(
)
E
LO y
IF y
nT
s y
nT
(
)
LO y
nT
(
)
–
+
sin
=
=
n
0 1 2
N
=