Agilent Technologies 8614 B Series User Manual page 46

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 3-91.
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 beneficial for obtaining best measurement accuracy and mea-
suring low level signals.
Peak detection finds and displays the maximum signal level present during each trace point inter-
val. Peak detection is used if video filtering is not required to achieve the desired level of sensitiv-
ity. 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 digi-
tal 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 bandwidth function from 100 mHz to 3.0 kHz,
or the bandwidth of the currently selected transimpedance amplifier, whichever is less. Increased
filtering provides 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 detec-
tion can be forced at any time by putting 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 video bandwidth and sensitivity, it is recommended that either
the sensitivity or the video bandwidth be changed, whichever is the most important to the mea-
surement task being performed.
To reduce noise, you can select a narrower video bandwidth to improve repeatability and sensi-
tivity or select a wider video bandwidth to shorten overall measurement time. This selection
allows the choice between repeatability and measurement time based on your measurement
requirements. The narrower the video bandwidth, the longer the sweep time.
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