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User-entered
wavelength
Noise bandwidth
1 Press the front-panel Preset key.
2 Press List by WL or List by Power.
3 Press Appl's and then S/N.
When the signal-to-noise "user" function is selected, the Keysight 86120B 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 deter-
mine their corresponding signal-to-noise ratios.
When measuring noise power, the Keysight 86120B must account for the noise
bandwidth used during the measurement. Because noise bandwidth varies with mea-
surement bandwidth (a wide bandwidth allows more noise to the Keysight 86120B's
detector than a narrow bandwidth), the Keysight 86120B 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 Keysight 86120B 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
significant low-frequency amplitude modulation of the laser. This modulation
raises the noise floor of the Keysight 86120B significantly. The signal-to-noise
measured can be limited to about 15 dB while measuring lasers modulated by
repetitive data formats. For improved performance when the laser is modulated
with repetitive data formats, use the Signal-to-Noise with Averaging applica-
tion.
To measure signal-to-noise
Measurements Applications
Measuring Signal-to-Noise Ratios
71
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