6
Section 2
Principles of Operation
This section contains a brief overall functional description
of the Model 840-C and its detector measurement consid-
erations. Information pertaining to the Model 840-AC wall
mount Adaptor operation is also included. The Model 840
Hand Held Optical Power Meter System typically includes a
photodetector, a memory module, and an electronic
display. Some systems have the photodetector combined
with the memory module (such as those with the fiber
input module).
The photodetector is a sensor which is connected to its
matched
Detector Cal Module through a cable with a
standard BNC connector. The photodetector is a semi-
conductor photodiode which generates current propor-
tional to the total amount of light incident upon the
detector. The photodetector is operated without voltage
bias to reduce dark currents and shot noise. The current
is then measured by the 840-C power meter, and con-
verted into optical power as described in Section 2.4.
The memory module is an in-line device that supplies
calibration information about its
matched
photodetector
to the 840-C power meter. It is connected to the photode-
tector via a BNC connector and to the electronic display
by a sub-miniature eight-pin DIN connector. The memory
module is separate from the power meter so that
photosensor and memory modules can be quickly
changed without opening up the electronic display. The
memory module may be separated from the photodetec-
tor to allow the photodetector to be used as an input
sensor to other common power meters or an oscillo-
scope.
Introduction
2.1
Photodetector
2.2
Memory
Module
2.3
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