Teledyne GFC-7000T - Trace CO2 Analyzer - manual - Manual (Page 209)

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 191
IR shining through both cells is effected
equally by interfering gas in the Sample
Chamber
M/R
is Shifted
Figure 9-5: Effects of Interfering Gas on CO2 MEAS & CO2 REF
Thus, the difference in the peak heights and the resulting M/R ratio is only due to CO
2
and not to
interfering gases. In this way, Gas filter correlation rejects the effects of interfering gases and so that the
analyzer responds only to the presence of CO
2
.
To improve the signal-to-noise performance of the IR photo-detector, the GFC wheel also incorporates an
optical mask that chops the IR beam into alternating pulses of light and dark at six times the frequency of
the measure/reference signal. This limits the detection bandwidth helping to reject interfering signals
from outside this bandwidth improving the signal to noise ratio.
The IR Signal as the Photo-Detector sees it
after being chopped by the GFC Wheel
Screen
CO
2
REF
CO
2
MEAS
Figure 9-6: Chopped IR Signal