Composite signal fluctuation limits

Composite Signal Fluctuation and Background Fluctuation Limits... 

Fig. 2.19. Minimum detectable monochromatic power as a function of wavelength for composite of signal fluctuation limit (SFL) and background fluctuation limit (BFL) for two detector areas and electrical bandwidths. Background temperature is 290 K and field ofview is 2x steradians. Detector long wavelength limit is assumed equal to source wavelength...

The intersections of three pairs of curves for which the bandwidths of the signal and background fluctuation limits are equal are emphasized in Fig. 2.19. Note that all lie between 1.0 pm and 1.5 pm. To illustrate the composite, that for an area of Icm (applicable to the background fluctuation limit) and a... 

Figure 2.20 illustrates the minimum detectable power from a 500 K black body as a function of wavelength, illustrating the composite of the signal fluctuation and background fluctuation limits. The same values of the param-... 

Detector sensitivity is one of the most important properties of the detector. The problem is to distinguish between the actual component and artifact caused by the pressure fluctuation, bubble, compositional fluctuation, etc. If the peaks are fairly large, one has no problem in distinguishing them however, the smaller the peaks, the more important that the baseline be smooth, free of noise and drift. Baseline noise is the short time variation of the baseline from a straight line. Noise is normally measured "peak-to-peak" i.e., the distance from the top of one such small peak to the bottom of the next. Noise is the factor which limits detector sensitivity. In trace analysis, the operator must be able to distinguish between noise spikes and component peaks. For qualitative purposes, signal/noise ratio is limited by 3. For quantitative purposes, signal/noise ratio should be at least 10. This ensures correct quantification of the trace amounts with less than 2% variance. The baseline should deviate as little as possible from a horizontal line. It is usually measured for a specified time, e.g., 1/2 hour or one hour and called drift. Drift usually associated to the detector heat-up in the first hour after power-on. 

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