Keysight Technologies 86120B, User Manual

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Measurements Applications
Measuring Signal-to-Noise Ratios
Measuring Signal-to-Noise Ratios
Signal-to-noise measurements provide a direct indication of system performance.
Signal-to-noise measurements are especially important in WDM systems because
there is a direct relation between signal-to-noise and bit error rate. The
Keysight 86120B displays signal-to-noise measurements in the third column. For
example, the selected signal in the following figure has a signal-to-noise ratio of
30.0 dB.
Signal-to-noise display.
During a signal-to-noise measurement, the absolute power of the carrier, in dBm, is
compared to the absolute power of the noise at the carrier wavelength. See the fol-
lowing figure. The noise power at the carrier must be determined by interpolation
because the carrier, in most cases, can not or should not be turned off.
You can select one of two methods used to determine the wavelength where the
noise is measured: automatic interpolation or a user-entered wavelength. In the fig-
ure above, notice that “
S/N AUTO
” is displayed to indicate that automatic interpo-
lation is selected.