Agilent Technologies 86038A User Manual page 70

Making Measurements
Narrowband Filter or DWDM Wavelength Routing Device
70
Characteristics
• Very narrow band
• Low dispersion, DGD, PDL with possible exception of band edges
• Fine in-band structure to some attributes, possible including group delay ripple.
• Steep filter slopes
• High rejection outside the passband
Measurement Tips
The combination of narrow passband, fine group delay structure, and low
dispersion calls for a special strategy
• Swept mode rapidly gathers a large number of points to smooth, taking advan-
tage of the smoothing function. The step mode can take and average a large
number of samples at each wavelength, reducing the noise but increasing
measurement time over the swept case.
• Start by measuring group delay and loss in the swept mode. This measurement
is fast and will usually reveal the fine loss and group delay structure quite well.
• Use a low enough modulation frequency to reveal the fine structure.
• In swept mode group delay measurements, reduce trace noise by means of the
built-in smoothing functions. Use a smoothing interval wide enough to reduce
noise but not so wide as to smooth away the group delay features of the device.
• In stepped mode, manually select the modulation frequency. The automatic
modulation frequency selection feature is intended only for transmission fiber.
• Always select the Phase Dynamic Accuracy (PDA) correction when performing
a swept or stepped normalization for filter testing. This correction reduces the
degree to which the large variations in device loss with wavelength are trans-
lated into small errors in group delay through RF amplitude-to-phase effects in
the performance network analyzer (PNA).
• To measure DGD, select the stepped mode. Set the IF bandwidth to 70 Hz to fil-
ter out noise while not filtering out the group delay variations caused by polari-
zation scanning. Select 201 samples to allow a sufficiently long polarization
scanning time. Set the modulation frequency to 2000 MHz to achieve best signal
to noise at the expense of wavelength resolution. Experiment with lower mod-
ulation frequency values and correspondingly smaller wavelength increments
to reveal fine structure in the DGD response.
Choosing Setup Parameters for Your Device
Agilent 86038A Optical Dispersion Analyzer, Third Edition