Tektronix OM4006D, User Manual

Page: 96 / 205

Taking measurements
Figure 9: Multicarrier spectrum plot details
The fi gure above shows some of the other features of the Multicarrier Spectrum
plot. Here the X-pol signal after front-end processing is shown together with the
digital
fi lter used. Notice that the X-pol signal (purple or orange) show much
deeper nulls than the total power spectrum (red or green). The nulls are the result
of using a split and delay technique to generate the electrical I and Q modulator
input signals from one data stream. There is a null at integer multiples of one over
the difference in cable delays.
Since different cables are used on the X and Y inputs of the modulator, the nulls
do not perfectly align and so wash out when looking at the total power spectrum.
The nulls are clearly visible once the transmitter polarization signals have been
separated.
Multicarrier Constellation Plots. The Multicarrier Constellation plots are accessed
by clicking on the constellation icon button on the Home tab. These plots behave
in a similar fashion to the existing constellation plots except that there are regions
reserved for each channel. The layout is controlled by the Multicarrier Setup
window described above.
72 OM4000D Series Coherent Lightwave Signal Analyzer