Agilent Technologies 86038A User Manual page 52

Measurement Concepts Description of the 86038A
Control of resolution and noise
When measuring low-dispersion, narrowband devices at high resolution,
we face the noise limit implied by Equation 5. Figure 9 provides an intuitive
guide to the management of this noise. The brick walls represent limits.
The left-right axis of the figure represents the trace noise of the group
delay measurement. The upper half of the figure shows the modulation
frequency. We reduce into achieve better resolution (movement to the
right in the figure), but eventually the trace becomes noisier.
Figure 9 Managing noise in high-resolution measurements
The bottom half of Figure 9 shows three setup parameters that can be
adjusted to push our measurement back toward the low noise, high
repeatability side of the figure. We can increase the number of samples
taken at each wavelength (in the stepped mode), achieving a phase noise
reduction through the averaging of all of these phase samples. We can
also decrease the IF bandwidth. Finally, in extreme cases, we may elect to
repeat the measurements and average the traces to reduce noise. These
three noise control approaches all lengthen the measurement time.
Selecting setup conditions for a new type of narrowband device
Identifying the best setup conditions is a process that should be performed
once before testing a new class of device. Once the appropriate setup
conditions are determined, they can be saved and used for all future
measurements of devices that have similar characteristics.
The simplest approach is to use the 86038A's Quick Setup feature, which
provides two slider-type controls that in turn adjust a variety of setup
parameters. Assuming stepped mode operation, you may wish to start by
choosing a relatively small wavelength increment and setting the Quick
Setup slider controls for less speed but more resolution and sensitivity.
After viewing the measurement results, you may wish to speed up the
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Agilent 86038A Optical Dispersion Analyzer, Third Edition