Agilent Technologies 86038B, Руководство пользователя

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Agilent 86038B Photonic Dispersion and Loss Analyzer, Second Edition
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For an RF modulation frequency of 1000 MHz, the sideband
separation is 2000 MHz, which gives an approximate sideband
separation of 16 pm.
The effects of imaging cannot be completely eliminated, but they
can be reduced to acceptable levels by choosing a sufficiently
small RF modulation frequency.
To achieve a high-resolution measurement of group delay ripple,
the modulation frequency should be low enough that the
modulation sidebands track up and down the group delay ripple
as a pair, as illustrated in Figure 13 .
Figure 13 Selecting the modulation frequency for measurement of GD
ripple
Relationship of setup parameters to DGD and PDL accuracy
As with GD measurements, increasing the modulation
frequency reduces the influence of phase noise on DGD.
Because typical DGD values below 10 ps are much smaller than
the modulation period, even at the highest modulation
frequency, phase wraps are usually not a problem. However at
high DGD, the lower and upper sidebands of the modulated
signal experience different Jones matrices from each other and
from the carrier wavelength and are converted into differing
output polarization states. This distorts the signal. Therefore at
quite high DGD levels, the modulation should be reduced to
keep the sidebands closer together. For example, 2 GHz
modulation should only be used up to about 25 ps DGD. Also as
with GD, if the DGD must be measured with high wavelength
resolution, the frequency should be reduced so that the signal
width including the sidebands does not exceed this resolution.