Agilent Technologies 86140B Series, User Manual

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2-7
Using the Instrument
Setting Up Measurements
Setting Video Bandwidth
Video bandwidth filtering occurs after the detection of the light. In the auto
coupled mode, the video bandwidth has an extremely wide range. This allows
the instrument to avoid unnecessary filtering that would reduce the sweep
speed more than required.
Refer to “Video BW” on page 3-99 .
The instrument has two detection techniques: peak (auto mode) and sample
(manual mode). Peak detection is beneficial for maintaining the fastest sweep
times and displaying narrow aspect ratio signals. Sample detection is benefi-
cial for obtaining best measurement accuracy and measuring low level signals.
Peak detection finds and displays the maximum signal level present during
each trace point interval. Peak detection is used if video filtering is not
required to achieve the desired level of sensitivity. However, there is one
exception: if an auto-coupled sweep time is limited by either maximum motor
speed or a 50 ms auto-coupling limit, then sample detection is used with as
narrow a digital video bandwidth as possible in order to achieve maximum
sensitivity for the chosen sweep time. This exception only applies when both
sweep time and video bandwidth are auto-coupled.
Sample detection displays a filtered version of the sampled data at the end of
each trace point interval. The filter function is varied with the video band-
width function from 100 mHz to 3.0 kHz, or the bandwidth of the currently
selected transimpedance amplifier, whichever is less. Increased filtering pro-
vides greater sensitivity.
The detection mode is automatically determined by the instrument. You can
adjust the settings of video bandwidth, sensitivity, or sweep time to obtain the
desired detection mode. Sample detection can be forced at any time by put-
ting video bandwidth in manual. Peak detection can usually be obtained by
placing sensitivity and video bandwidth in auto.
The following functions affect video bandwidth:
• changing the sensitivity value
• changing the reference level
• turning auto ranging on or off
The range of video bandwidths available in auto mode is much greater than
can be set manually from the front panel. A lower video bandwidth value
requires a longer sweep time. Because of the interdependence between the