Ceilometer CT25K
User’s Guide CT25K-U059en-2.1
70
backscatter signal detected by the photodiode in the receiver is sampled in
real-time in this subassembly at the speed equal to that of the light beam in
the atmosphere sent from the laser transmitter. All Range Gates are sampled
at each laser pulse and digitized by means of the 8-bit flash A-to-D converter.
They are stored in a First-In First-Out (FIFO) memory according to the height
profile (Figure 5-13).
For improving the Signal-to-Noise Ratio a large number of laser pulses is
utilized and the samples are added to the FIFO memory in digital format. This
way, a complete backscatter profile is then available at the end of a complete
measurement cycle. The raw values are sent via a data bus directly to the
Random Access Memory on the processor board to be algorithmically
evaluated. A Digital-to-Analog converter outputs control signals to the laser
transmitter and receiver. The D-to-A converter is also controlled through the
data bus of the processor board.
A multiplexer contributes to the monitoring of different analog signals from
the optics monitor, for temperature measurement of the laser and for angle
measurement of the tilt angle. Controlled by the processor, it connects the
analog signals to the A-to-D converter of the processor board.
The timing and control circuit takes care of the triggering and length of the
laser pulse and simultaneously for the selection of receiver gain and
bandwidth. The system parameters, as they are defined in the configuration
and factory settings, are stored in a special EEPROM on the DCT51 board.
The data transmission to and from the EEPROM is carried out through an I
2
C
Bus which is independent of the actual Data Bus.
5.3.9 Line and Power Interface Subassembly CTP241
Line AC power is connected to the Line Power and Interface Subassembly
CTP241, where it gets transformed down to low-voltage AC for supplying the
DC Converter DPS52. Further, the CTP241 contains relays for controlling the
Internal Heater (CT25039) and the external Window Conditioner blower
motor and airflow heater. Internal heating and window conditioning are under
software control via DC Converter DPS52.
WARNING
Line Power and Interface Subassembly CTP241, Internal
Heaters’ Subassembly CT25039 and Window Conditioner
CT2614 or CT2688 contain Line AC voltage, which may
be lethal.
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