Detectors thermal noise from

The phase-sensitive amplifier has a certain response bandwidth and will therefore measure a signal due to the thermal noise from the mixer over that bandwidth, limiting the ultimate signal to noise ratio of the system. The nature of noise in these mixers and detectors is discussed in Section 3.5. 

The problem of measuring infrared radiation by thermal means is compounded by thermal noise from the surroundings. For this reason, thermal detectors are housed in a vacuum and are carefully shielded from thermal radiation emitted by other nearby objects. To further minimize the effects of extraneous heat sources, the beam from the source is generally chopped. In this way, the analyte signal, after transduction, has the frequency of the chopper and can be separated electronically fromextraneous noise, which usually varies slowly with time. 

In interferometric detectors thermal noise mainly contributes far from resonances, since care is taken to shift most of them outside the detection band. In bars the resonance provides the detection mechanism. The resonance can also be excited from noise, including thermal noise. Thermal noise has a power spectral density Sx =k T, thus the expected rms amplitude is Xm = The recipe... 

Optical receiver noise can arise partly from fundamental photon noise and partly from thermal noise in the receiver circuit. For CO2 detection, it was assumed that the optical receiver was an extended InGaAs detector, followed... 

The optimum operating temperature for minimum noise from the FET was found to be slightly higher than the operating temperature of the Ge(Li) detector. A thermal heater made from a 100-ohm carbon resistor was therefore installed next to the FET and operated at a current that heated the transistor to a temperature just above that of liquid nitrogen. 

Noise associated with the detector elements themselves (dark or thermal noise) can be reduced by decreasing the temperature, typically from —20 to —70° C although cooling to liquid nitrogen temperatures essentially eliminates dark noise, it is typically impractical for applications outside of the laboratory. The theoretical value for relative signal-to-noise reduction is a factor of 2 improvement for every 6.3°C reduction in temperature, although this can be... 

With phototubes and photomultiplier-type detectors (photoemissive detectors, ultraviolet to visible range), thermal noise becomes insignificant as compared to shot noise. Shot noise is the random fluctuation of the electron current from an electron-emitting surface (i.e., across a junction from cathode to anode), and in PM mbes that is amplified and becomes the noise-limiting fluctuation. In instruments with these detectors, the absolute error is not constant at all values of T, and the expressions for the spectrophotometric error become more complicated. It has been calculated that, for these cases, the minimal error should occur at 0.136 or A = 0.87. These instruments have a working range of about 0.1 to 1.5 A. 

In an FM MMW spectrometer the spectral source frequency is modulated at a certain rate /, typically 1 kHz. This gives rise to sidebands of the spectral source frequency above and below the carrier frequency. The frequency modulated MMW carrier has in its modulation envelope phase and amplitude relationships to the carrier. Mixing in the non-linear junction of the detector yields the modulation signals altered by their interaction with the cavity and gas inside it, with their preserved amplitude and phase relationship to the original modulation signals. Those properties are measured by passing the heterodyne mixer output and the thermal noise contribution from the mixer, to a filtered phase-sensitive detection system, with the original modulation as reference. 

Rather than the noise equivalent power as a figure of merit, the common descriptor for a sampling detector is the noise equivalent electron count, NEE. The output voltage is a measure of the apparent charge transferred to the capacitor, and the three sources of the charge fluctuation may each be categorized as a fluctuation in the electron count, N. The photon noise term is simply (NEE) yot = VN = (rjPlhv)T, from the Poisson statistics of the photoexcitation process. The NEE count associated with the capacitor thermal noise is (NEE)c = = kTC/q, and that of the ampli-... 

Now if it is assumed that a concentration of one part per million of carbon tetrachloride is just detectable, i.e., the thermal noise must be half the detectable signal, then the thermal fluctuations must be maintained at a level below 1.8 x IQ- °C. Such temperature stability can be extremely difficult to maintain in practice and thus, the temperature control can place a severe limit on the sensitivity that can be obtained from such a detector. Even the heat of adsorption and desorption of the solute from a column packed with silica gel can easily result in temperature changes of the order of 1.8 x 10 °C. 

In order to eliminate thermal noise and flow sensitivity, the inlet flow of solvent from the column must be brought to the same temperature as the detector cell before it actually enters the cell. The heat exchanger used for this is shown in Figure 8 which embodies many of the standard features of modern LC detector heat exchangers. The inlet tube is first coiled tightly round the exit tube to make good thermal contact and then wrapped round an... 

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