Agilent Technologies 86038A User Manual page 48

Measurement Concepts Description of the 86038A
the device under test has variations of interest. If the RF modulation
sideband separation is more than the wavelength increment, these two
different sample points can cause imaging.
Consider an example of a narrow band-pass filter being measured with a
very small wavelength increment and an RF modulation frequency so large
that each sideband is stepped through the filter pass band while the other
sideband is well rejected. For this example the pass band would appear at
two wavelengths, each incorrect in wavelength by half the sideband
separation, and each attenuated in amplitude.
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 6.
Figure 6 Selecting the modulation frequency for measurement of GD ripple
Relationship of setup parameters to ripple amplitude accuracy
Assuming a sufficiently small wavelength increment has been chosen to
reveal ripple across wavelength without aliasing, the peak-to-peak
amplitude of the ripple for insertion loss or group delay will appear slightly
less than the true amplitude.
One attenuation effect is caused by not sampling the phenomena at its
extremes. To reduce this effect, use a smaller wavelength increment.
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Agilent 86038A Optical Dispersion Analyzer, Third Edition