Page 551 - Sensors and Control Systems in Manufacturing

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dark noise in SpectRx systems remains a negligible contributor to
system noise.
10.7.11 Jitter Noise
Jitter noise is noise caused by jitter during sampling of the interfero-
gram. Sampling jitter is mainly due to ADC jitter, laser intensity fluc-
tuations, and the noise from the electronics that trigger the ADC.
Jitter noise can become troublesome at elevated scanning speeds.
This is because at high speeds, a given jitter becomes non-negligible
as compared to the sampling interval. Jitter noise is especially annoy-
ing because it is proportional to the measured signal; therefore, the
SNR due to jitter cannot be improved by observing a warmer object
view. Because of this, rapid scanning systems require carefully
designed electronic circuits to trigger the ADC. The reader is referred
to (RD1) for more details.

10.7.12 Scanning Instability Noise
Scanning instability noise can be attributed to scanning speed insta-
bilities together with uneven spectrometer response and/or a delay
mismatch between the ADC triggering and the IR signal. Mainly
because of the analog filter response at high frequencies, the SpectRx
spectroradiometer is operated with a non-zero slope gain. This is not
a problem if the scanning speed that converts optical speeds to elec-
trical speeds is constant. This is not the case, however, and some
noise-equivalent current results from the peak amplitude of the speed
variation and the analog filter slope. Filter slope noise is often a non-
negligible contributor to FTIR system noise. This is the main reason
why analog filter design is carefully analyzed.
Speed variations can also introduce noise if the ADC triggering
circuitry is not perfectly synchronized with the IR signal. Because it
has a larger bandwidth than the IR channel, the sampling laser fringe
detection circuitry usually has a smaller associated delay. If this
delay-mismatch is not corrected, the spectroradiometer will sample
the interferogram with a constant negative time offset: –Δt. This is not
a problem if the scanning speed is constant, but as this is not the case,
the uneven sampling resulting from the delay-mismatch and speed
variations converts into a noise-equivalent current.
Like jitter noise, these two sources of noise are of great concern,
especially as they are proportional to the measured signal, so that the
SNR due to these sources cannot be improved by observing warmer
object views. Careful design of the analog filter cut-off frequency as
well as delay-mismatch compensation is therefore applied.

10.7.13 Instrument Efficiency
The modulation efficiency of the interferometer depends on many
factors, such as the imperfections of the beam-splitter, and the shear