Teledyne M300EM operation manual Summary Interference Rejection, 6:Chopped IR Signal

Teledyne API – Technical Manual - Model 300E Family CO Analyzers Theory of Operation

Interference and Signal to Noise Rejection:

If an interfering gas, such as H2O vapor is introduced into the sample chamber, the spectrum of the IR beam is changed in a way that is identical for both the reference and the measurement cells, but without changing the ratio between the peak heights of CO MEAS and CO REF. In effect, the difference between the peak heights remains the same.

M/R

is Shifted

IR shining through both cells is affected equally by interfering gas in the Sample Chamber

Figure 11-5: Effects of Interfering Gas on CO MEAS & CO REF

Thus, the difference in the peak heights and the resulting M/R ratio is only due to CO and not to interfering gases. In this case, GFC rejects the effects of interfering gases and so that the analyzer responds only to the presence of CO.

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

S

CO MEAS

CO REF

Figure 11-6: Chopped IR Signal

11.2.1.3.Summary Interference Rejection

The basic design of the M300E/EM rejects most of this interference at a 300:1 ratio. The two primary methods used to accomplish this are:

∙The 4.7μm band pass filter just before the IR sensor which allows the instrument to only react to IR absorption in the wavelength affected by CO.

∙Comparison of the measure and reference signals and extraction of the ratio between them.

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