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3-20 Measuring Signal-to-Noise Ratios with Averaging
Averaging
When the lasers being measured are modulated, espe-
cially with repetitive data formats such as SONET or
PRBS, the noise floor is raised. Averaging reduces the
noise floor and allows an improvement of greater than
10 dB in a signal-to-noise measurement. In general,
averaging will decrease the noise floor caused by modu-
lation until the true optical noise level is reached. The
displayed signal-to-noise will improve with each aver-
age until the true optical noise level is reached, and
then the displayed signal-to-noise will remain approxi-
mately constant. If, however, the true signal-to-noise is
below the instrument sensitivity of approximately 40 dB
(in a 0.1 nm noise bandwidth), it will not be measured.
Averaging is performed on the noise, not on the wave-
length or power of the laser signals.
The display indicates S/N A xx, where A indicates
averaging and xx is the number of averages taken so far.
The maximum number of averages is 900, the minimum
number of averages is 10, and the default (Preset) value
is 100 averages. A measurement with 100 averages
takes about 2 minutes to complete. When the measure-
ment is complete, the instrument will stop. Then, press-
ing the Run key will start a completely new
measurement. During a measurement and before the
number of averages has been reached, pressing the Stop
key will stop the measurement. Then, pressing the Run
key will continue with the current measurement.
The number of averages can be changed. As long as the
new number of averages is greater than the number of
averages taken so far, the measurement continues. If
the new number of averages selected is less than the
number of averages taken so far, the measurement
stops. Then, pressing the Run key will start a completely
new measurement.
To measure signal-to-noise with averaging
1 Press the front-panel S/N key.
2 To change the number of averages, press NUM AVG. The
default value is 100 averages.
3 To stop the measurement at the current number of
averages shown, press the front-panel Stop key. Then
press the front-panel Run key to continue the present
measurement.
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