Teledyne GFC-7000T - Trace CO2 Analyzer - manual - Manual (Page 220)

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 202
Reference
Pulses
Measurement
Pulses
IR Beam
Pulses
Segment Sensor
Pulses
MR Sensor
Pulses
Figure 9-12: Segment Sensor and M/R Sensor Output
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.
9.3.3.4. IR Photo-Detector
The IR beam is converted into an electrical signal by a cooled solid-state photo-conductive detector. The
detector is composed of a narrow-band optical filter, a 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 low. 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 raising the
output voltage.
9.3.4. Synchronous Demodulator (Sync/Demod) Assembly
9.3.4.1. 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.
The sync/demod board demodulates this waveform and outputs two analog DC voltage signals,
corresponding to the peak values of these pulses. CO2 MEAS and CO2 REF are converted into digital
signals by circuitry on the motherboard then used by the CPU to calculate the CO
2
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.