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Using the Instrument
Setting Up Measurements
Setting the Sensitivity
Setting sensitivity requests the lowest amplitude signal that can be measured
relative to the highest amplitude signal displayed. It is defined as the signal
that is six times the RMS noise. The minimum setting is –100 dB. An error will
be reported for values outside of this range and the sensitivity will round to
the nearest valid sensitivity. Refer to “Sensitivity” on page 3-81.
Manual allows manual input of sensitivities and enables auto gain ranging.
The “top of screen” and the sensitivity setting determines the requested
dynamic range. The system will sweep once per gain stage and may require up
to three sweeps to achieve the requested dynamic range.
Auto automatically chooses a sensitivity and a single gain range based on “top
of screen”. This will result in approximately 40 dB of dynamic range.
The sweep time that is displayed in the lower portion of the display is the time
for the OSA to sweep over one gain stage. The OSA may take up to three
sweeps in three different gain stages to make the measurement. This depends
on the settings for sensitivity, reference level, auto range and also the particu-
lar device being measured. The final data trace is a blended composite of each
trace taken in the different gain stages.
An increase in sensitivity may also require a narrower video bandwidth, which
will slow the sweep speed. Normally, the optical spectrum analyzer selects the
greatest sensitivity possible that does not require amplification changes dur-
ing the sweep. If you manually increase the sensitivity level, the sweep pauses
to allow this change in gain.
The settings for sensitivity, video bandwidth and sweep time interact. If the
sensitivity is set to manual, the video bandwidth and sweep time may be
forced to Auto mode. If the video bandwidth is set to manual, the sensitivity
and sweep time may be forced to Auto. If the sweep speed is set to manual
and is set too fast, the over sweep indicator will come on in the display area.
Since these settings interact, it is recommended that only one of the settings
be changed, whichever setting is most important to the measurement task
being performed.
Press Amplitude, Sensitivity, toggle to manual, and enter a value.