Vaisala LT31, Руководство пользователя, Страница: 50 / 294

User's Guide ______________________________________________________________________
50 __________________________________________________________________ M210667EN-C
Under the control of the measurement CPU, the light source is modulated
with a frequency of approximately 1 kHz. A monitor photodiode
measures the emitted light intensity and the main transmitter module uses
a closed loop regulator in combination with a LED driver to achieve a
normalized, constant transmitter intensity.
The intensity signal and a derating signal, proportional to the applied
LED forward current, are connected to the ADC of the measurement
CPU for self-diagnostics.
The main transmitter module is also equipped with a temperature
measurement and EEPROM. These components are connected to the
measurement CPU through the IIC-Bus interface. Individual calibration
parameters are stored in the EEPROM and are used to adapt individual
settings, especially after module exchange.
The basic principle of LTL112 is shown in Figure 17 below.
Transmitter
LED
Intensity
Measurement
Regulator
+Driver
Temperature
Measurement
EEPROM
CPU
Transmitter Clock
CPU
Derating
CPU
Transmitter Intensity
CPU
CPU
0401-034
Figure 17 Block Diagram LTL112
Main Receiver Module LTD112
The main receiver module LTD112 (see Figure 18 on page 51) is
mounted to the optics tube of the optics unit and is secured by a screw. It
is connected to the measurement CPU using two connectors.
Additionally, a BNC connection is used to forward the main receiver
signal to the measurement CPU.