Agilent Technologies 86121A, User Manual, Page: 55 / 240

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18 Measuring Signal-to-Noise Ratios with Averaging
Measuring Signal-to-Noise Ratios with
Averaging
Signal-to-noise measurements provide a direct indica-
tion of system performance. Signal-to-noise measure-
ments are especially important in WDM systems
because there is a direct relation between signal-to-
noise and bit error rate. The WDM channel analyzer dis-
plays 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 abso-
lute power of the noise at the carrier wavelength. See
the following figure. The noise power at the carrier
must be determined by interpolation because the car-
rier, in most cases, can not or should not be turned off.
Noise
bandwidth When measuring noise power, the WDM channel ana-
lyzer must account for the noise bandwidth used during
the measurement. Because noise bandwidth varies with
measurement bandwidth (a wide bandwidth allows
more noise to the detector of the WDM channel ana-
lyzer than a narrow bandwidth), the WDM channel ana-
lyzer normalizes all noise power measurements to a
bandwidth of 0.1 nm. The annotation
0.1 nm
is dis-
played to show that the noise bandwidth is being nor-
malized to a 0.1 nm bandwidth.