Ceilometer CT25K
User’s Guide CT25K-U059en-2.1
60
small control circuit with a temperature sensor and a heating element ensures
the correct laser temperature. The laser temperature is also routed to the
Ceilometer Interface Board DCT51 and Processor Board DMC50B for
internal monitoring.
WARNING
Dangerous voltages are present in this instrument. Use
extreme caution when handling, testing, and adjusting.
Laser safety
To guarantee the eye safety of the laser equipment, the cable connecting the
subassembly is short, permitting operation only when the installation is
complete. For removing the laser transmitter subassembly, the cable between
the transmitter and the laser interface board must be disconnected first.
5.3.3 Receiver CTR21
Figure 5-7 CTR21 Block Diagram
The Receiver subassembly CTR21 consists of two identical avalanche photo
diodes (APDs). The second APD compensates most of the optical cross-talk
appearing in the lens system through signal reflection by means of a half-
bridge connection. The first APD receives the actual measuring signal and
sends the pulse current through a transimpedance amplifier with selectable
gain. At the output of this amplifier the signal is a voltage signal transferred
further through a filter with a frequency limit of 15 MHz to a second signal
amplifier. According to the operating mode the signal goes further to a new
amplifier stage or to a combined additional filter with a frequency limit of 3
MHz and an amplifier.
The amplifier gain is selected by the system automatically so that the smallest
noise signal appearing in the measurement signal is bigger than the resolution
of the flash AD converter of the Ceilometer Interface Board DCT51. The
biggest measurement signal must not exceed the full measurement range of
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