Vaisala FD12P - Manual (Page 99)

Chapter 5 _______________________________________________________ Functional Description
VAISALA ________________________________________________________________________ 99
The lock-in circuits take two samples of the background level and one
sample of the active signal level while the transmitter LED is lit. The
difference between the sampled voltages is amplified and then
converted into frequency.
The frequency signal is buffered by a differential line driver and sent
to the CPU board for accurate counting.
An ambient light level as high as 30 kcd/m
2
does not influence the
detection of the photo diode, neither does it saturate the A4 pre-
amplifier. The Ali signal (proportional to the ambient light) is led to
the CPU for monitoring.
An extra IR-LED is needed for backscatter or contaminant
measurement. The light level is sampled and converted into frequency
using the same method of detection described with the scattering
signal measurement.
Additional Measurements
General
The FD12P includes the DRD12 Rain Detector for estimating the
water content of precipitation and the DTS14B Temperature Sensor
for measuring the sensor crossarm temperature (TS). Both additional
sensors are measured using the DRI21 Interface Board, which is
coupled on the FD12P PICOBUS.
DRI21 Interface Board
The DRI21 is a Vaisala general-purpose sensor interface with several
analog and digital input channels. One of the DRI21 temperature input
channels (Pt100) is used to measure the crossarm temperature
(DTS14B). One 10-bit analog input channel is used to measure the
DRD12 analog signal. In addition, the DRI21 controls the DRD12
heating and reads the rain ON/OFF status.