Chapter 5 _________________________________________________________ Theory of Operation
VAISALA _______________________________________________________________________ 135
Hardware Description
Measurement Unit FSM102
The Measurement Unit consists of three electronics parts: the transmitter
and receiver units and the controller board. These parts are described in
detail in the next sections.
Transmitter Unit FST102
The transmitter unit consists of an infrared LED, control and triggering
circuits, a LED intensity stabilizer, a backscatter receiver, and window
contamination measurement circuitry.
The transmitter unit electronics cause the infrared LED to pulse at a
frequency of 2.2 kHz. A PIN photodiode monitors the transmitted light
intensity and automatically adjusts the light level of the LED to the preset
value. This compensates for the temperature and aging effects of the light
emitting diode.
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