Synchronous Demodulator (Sync/Demod) Assembly - Teledyne 300E Instruction Manual

Model 300E Carbon Monoxide Analyzer
Instruction Manual
Schmidt Triggers: To ensure that the waveforms produced by the Segment Sensor
and the M/R Sensor are properly shaped and clean, these signals are passed
through a set of Schmidt Triggers circuits.
IR Photo-Detector
The IR beam is converted into an electrical signal by a cooled solid-state
photo-conductive detector
piece of lead-salt crystal whose electrical resistance changes with temperature, and a
two-stage thermo-electric cooler.
When the analyzer is on, a constant electrical current is directed through the detector,
The IR beam is focused onto the detector surface, raising its temperature and lowering
its electrical resistance that results in a change in the voltage drop across the detector.
During those times that the IR beam is bright, the temperature of the detector is high;
the resistance of the detector is correspondingly low and the its output voltage output is
high. During those times when the IR beam intensity is low or completely blocked by
the GFC Wheel mask, the temperature of the detector is lowered by the two-stage
thermo-electric cooler, increasing the detectors resistance and lowering the output
voltage.

10.3.4. Synchronous Demodulator (Sync/Demod) Assembly

Overview
While the Photo-Detector converts fluctuations of the IR beam into electronic
signals, the Sync / Demod Board amplifies these signals and converts them into
usable information. Initially the output by the photo-detector is a complex and
continuously changing waveform made up of Measure and Reference pulses (see
Figure 6-10). The Sync/Demod Board demodulates this waveform and outputs two
analog DC voltage signals, corresponding to the peak values of these pulses. CO
MEAS and CO REF are converted into digital signals by circuitry on the
Motherboard then used by the CPU to calculate the CO concentration of the sample
gas.
Additionally the Synch/Demod Board contains circuitry that controls the photo-
detector's thermoelectric cooler as well as circuitry for performing certain diagnostic
tests on the analyzer.
P/N 04288 Rev: A
The detector is composed of a narrow-band optical filter, a
THEORY OF OPERATION
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