Agilent Technologies 86038B User Manual page 111

Agilent 86038B Photonic Dispersion and Loss Analyzer, Second Edition
In the Swept measurement mode the samples per point is
always 1. In Stepped measurement modes, you select how many
samples to take at each measurement wavelength. The multiple
samples are averaged to generate a single trace point. A larger
number of points means more averaging, suitable for reducing
trace noise in cases of high insertion loss test devices. Note that
a large number of samples increases susceptibility to drift.
Tips on Number of Samples
• In stepped mode, use a larger number of samples to improve
signal to noise. Expect to trade-off measurement speed.
• 101 samples per point is often a good compromise, collecting
enough data to average out noise, and keeping the
measurement time short enough to avoid drift effects.
Drift Correction This system function reduces measurement
error contributed by gradual, continuous temperature changes
in the test device, especially long fibers. It is appropriate for use
when the temperature of the room or device is gradually
changing. This function may not help against rapid or stepped
changes, as due to movement of air over the test path.
In swept measurements, the elapsed time between wavelength
points is very short, but drift may occur between the sweeps for
each polarization state. Selecting Drift Correction in
GD/CD+DGD/PDL Swept mode causes the sweep at the first
polarization state to be repeated after the four (or six) sweeps
are finished. Any drift between the first sweep and this
repetition is then used to correct the data, assuming a
continuous rate of drift. This function is neither needed nor
available for swept measurements of GD/CD only.
In stepped measurements, the elapsed time between
polarization states is usually short but temperature drift in very
long links may still influence the DGD results. This is corrected
in the same way as for swept measurements, with a repeated
measurement at the first polarization state.
In stepped measurements, the elapsed time between wavelength
points is longer and drift may also distort the GD and CD
spectra. To correct this, in Stepped mode the Drift Correction
feature also causes the spectrum to be measured twice, once
stepping with increasing wavelength and once with decreasing
wavelength and the result is formed by averaging the two
measurements. This doubles both the number of samples used
for averaging and the time required for the measurement.
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