Chapter 3 ______________________________________________________ Functional Description
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Figure 18
Main Receiver Module LTD112
The main receiver module uses a PIN photodiode as a light receiver. An
optical filter is mounted in front of the photodiode to adapt the spectral
receiver sensitivity to the spectral response of the human eye.
The raw receiver signal, which is generated by the detector and the
preamplifier is amplified using a narrow-band amplifier. Afterwards the
signal is directed to a synchronous demodulator, which is controlled by
the original main transmitter modulation clock, transmitted to the
measurement CPU of the receiver using RS-422 technology. The
measurement CPU circuitry converts the signal to TTL and then it is sent
to the main receiver module. With this technology it is possible to
achieve an optimal signal to noise ratio. The resulting main receiver
signal is sent to one channel of the 20-bit ADC of the measurement CPU
through a coaxial cable.
In parallel to the AC signal processing, the DC component of the receiver
signal is amplified and brought to the ADC of the measurement CPU.
This main receiver DC signal is used for self-diagnostic purposes, to
detect possible saturation.
The main receiver module is also equipped with a test LED that the
measurement CPU controls.
The main receiver module contains a temperature measurement and an
EEPROM. These components are connected to the measurement CPU
via the IIC-Bus interface. Individual calibration parameters are stored in
the EEPROM and are used to adapt individual settings, especially after
module exchange.