|Sensitivity Issues in Step-Scan FT-IR Spectrometry
Now the stability of the instrument can be checked
with and without the temperature/convection control elements installed. The measurements
in this slide were made with the interferometer mirror phase modulating at the same fringe
of the centerburst. These plots represent 4000 data points accumulated over 20 minutes.
Ideally, these lines would be flat, indicating stability of the detector signal. The 1.7%
variation of the signal (top trace) was so serious that a test of the electronics was
conducted with the use of a very stable HP 3336B signal generator as a false detector
signal. The signal generator frequency could not be exactly matched to the phase
modulation frequency, so much of the residual 0.1% variation is due to a beat frequency
between the two. The short-term stability of the detector signal with the insulation
installed (bottom trace) was improved by a factor of 15. There is still a slow drift in
the intensity, but it does not go through a complete cycle during the interferogram
acquisition time, and consequently would not appear as noise.