Normalization And Test - Agilent Technologies 86038A User Manual

Description of the 86038A Measurement Concepts

Normalization and Test

Loss and Group Delay Normalization
The measurement results of the 86038A are actually the results of two
measurements, taken with and without the DUT inserted in the path. A
'normalization' measurement, taken without the DUT in the path, captures
the loss and group delay characteristics of the measurement system and
test path. The normalization measurement establishes a loss and group
delay baseline for later measurements of the DUT itself.
Optical Modulator Bias Adjust
In actual operation, the normalization measurement is accompanied by
one or more additional automatic performance optimizations, all under the
broad heading of 'normalization'. The 86038A contains an automated
modulator bias adjustment function that is implemented when PDA is
enabled during a normalization, or by itself through the menu item
Measure > Optimize Optical Modulator.
Amplitude modulation in the 86038A is accomplished using a lithium
niobate Mach Zehnder modulator. In this type of modulator, the light is
split into two paths which experience different phase shifts in response to
a co-propagating RF wave. The two light paths rejoin at the output, where
interference between the differentially phase shifted lightwaves
determines the instantaneous optical output level.
The modulator is biased by an adjustable DC voltage in order to select the
optimum operating point.
The automatic bias adjustment routine fixes the wavelength and the
modulation frequency, then measures the variation in modulated optical
output while changing the modulator bias voltage. From the resulting
curve of output level versus bias voltage, the optimum bias voltage is
chosen. For more information, refer to "Optimize Optical Modulator Bias"
on page 110.
Phase Dynamic Accuracy
A second optimization corrects for phase dynamic accuracy errors. This
correction is optional but is recommended in cases where the DUT is
expected to exhibit a significant variation of insertion loss or gain over the
measurement wavelength range.
Phase dynamic accuracy (PDA) is a performance attribute of the
performance network analyzer (PNA) that describes the extent to which a
change in the input RF power level causes an apparent (erroneous) change
in RF phase. The PDA error is highest at very high and very low RF power
levels. The 86038A reduces the impact of the PDA error by means of a user
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Agilent 86038A Optical Dispersion Analyzer, Third Edition