Teledyne GFC-7000E - Trace CO2 Analyzer - Manual (Page 176)

Model GFC7000E Instruction Manual THEORY OF OPERATION
04584 Rev A1 165
synchronous / demodulation circuitry of the analyzer to latch onto the most stable part of each
measurement and reference IR pulse.
Reference
Pulses
Measurement
Pulses
IR Beam
Pulses
Segment Sensor
Pulses
MR Sensor
Pulses
Figure 10-11: 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.
10.3.3.4. 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.
10.3.4. Synchronous Demodulator (Sync/Demod) Assembly
10.3.4.1. 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.