Agilent Technologies 86038A User Manual page 47

Description of the 86038A Measurement Concepts
incorrect representation. When the result is aliased, it appears at a false
period, longer than the actual period. An aliased measurement often
appears as an unusual shape, but can appear with the expected shape at a
multiple of the expected period.
To eliminate aliasing, use a smaller wavelength increment. If the RF
modulation sideband separation is less than the wavelength increment,
and the wavelength increment can be reduced without changing the
period or shape of the measurement, then aliasing is not present.
Relationship of setup parameters to resolution
Two measurement parameters affect resolution: the wavelength
increment and the RF modulation frequency. In most cases both are
reduced to improve wavelength resolution or amplitude resolution. The RF
modulation frequency may be constrained by phase wrapping, noise,
imaging, or ripple amplitude accuracy. The wavelength increment may be
constrained by aliasing, phase wrapping, ripple amplitude accuracy, or
measurement speed. Since these have overlapping constraints, it may be
necessary to compromise on acceptable settings.
The three-dimensional graph in Figure 5 shows the selection of
wavelength step and modulation frequency to achieve high wavelength
resolution. Reduction of only modulation frequency may fail to achieve the
desired resolution and simply raise the noise floor.
Figure 5 Adjusting wavelength step and mod. freq. to achieve desired resolution
Relationship of setup parameters to imaging
Since we are using AM modulation, we are sampling at the two AM
sidebands simultaneously and reporting the average result. Again there is
an implication of the performance between the sampled points. Even if the
wavelength increment is arbitrarily small, the RF modulation sideband
separation must be small compared to the wavelength range over which
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Agilent 86038A Optical Dispersion Analyzer, Third Edition