Tektronix OM4006D, User Manual

Page: 95 / 205

Taking measurements
Like the other multicarrier plots, the Multicarrier Spectrum plot saves data from
each channel analyzed to form a composite view of the channel group. In the
example shown above, this means that the Input Signal spectrum calculated
while analyzing channel 1 is plotted in green together with that calculated
while analyzing channel 2 in red. This scan was done with an absolute channel
de
fi nition table.
Because a different LO frequency was speci fi ed for each channel, the red and
green plots cover different spectral regions. When the LO was tuned to channel
number 1, the green spectrum was found. When it was tuned to channel 2 it was
the red spectrum. Since the channels are close together relative to the optical
bandwidth of the system, there is substantial overlap between red and green. The
red spectrum cuts off sharply on the left side corresponding to the lower limit of
the optical system bandwidth whereas the green spectrum cuts off sharply on
the right side corresponding to the upper edge of the optical system bandwidth
for that LO tuning.
Ignoring the sharp cut-offs, the union of the red and green curves gives the true
optical spectrum which is wider than what the 23 GHz receiver used here could
achieve in a single LO tuning.
OM4000D Series Coherent Lightwave Signal Analyzer 71