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Making Measurements
Measuring Signal-to-Noise Ratios
Automatic interpolation
User- entered
wavelength
When the signal-to-noise “user” function is selected, the
Agilent 86120C uses only one wavelength to measure the noise power
for all signals. This wavelength is set by the user and all signals are
compared to the noise level at this wavelength to determine their cor-
responding signal-to-noise ratios.
Noise bandwidth When measuring noise power, the Agilent 86120C must account for the
noise bandwidth used during the measurement. Because noise band-
width varies with measurement bandwidth (a wide bandwidth allows
more noise to the Agilent 86120C’s detector than a narrow bandwidth),
the Agilent 86120C normalizes all noise power measurements to a
bandwidth of 0.1 nm. The annotation 0.1 nm is displayed to show
that the noise bandwidth is being normalized to a 0.1 nm bandwidth.
Repetitive data formats
The Agilent 86120C signal-to-noise application works best when the laser being
tested is not modulated, or modulated with non-repetitive data formats. With
repetitive data formats, such as PRBS data and SONET formats, there is signifi-
cant low-frequency amplitude modulation of the laser. This modulation raises the
noise floor of the Agilent 86120C significantly. The signal-to-noise measured can
be limited to about 15 dB while measuring lasers modulated by repetitive data for-
mats. For improved performance when the laser is modulated with repetitive data
formats, use the Signal-to-Noise with Averaging application.