Noise typical sources

Environmental noise — is related to the transfer of energy from the environment to the measuring system. Although this kind of noise typically occurs at specific frequencies, such as the electric and magnetic fields produced by the electrical transmission lines, environmental sources can also originate from random sources. -> Shielding the electronic circuits can reduce or eliminate this kind of noise [ii]. 

Fnvirrmincntat noise is a composite of different f<>rms of noise that arise from the surioundings. Figure suggests typical sources of environmemal noise in n university lalH ralory. 

A second strategy is, of course, to avoid the generation of synchronous noise in the system. Typical sources of coherent noise are cavity dumpers, pulse pickers, and picosecond diode lasers. For the shielding of these devices the same rules should be applied as for detector shielding. 

Figure 2(a) depicts a typical photodiode detector circuit, in this case ac coupled to an amplifier. The equivalent circuit, in Figure 2(b), shows a current source proportional to incident optical power, a parallel dark current, //), a noise current source which represents the shot or photon noise associated with these currents, and a resistance and capacitance attributable to both the detector and the electrical amplifier. For the vacuum or semiconductor diode, the responsivity, DI, determined for a given wavelength, X, and quantum efficiency, rj, is... 

The variety of powered tools and equipment used on construction sites make many jobs easier, but at a price. That price is the noise they generate and the vibrations they set up in use. Typical sources of noise and vibrations include jack hammers, compactors, pneumatic drills, powered percussion tools, etc. The exhausts from internal combustion engines, whether in vehicles or stationary plant, adds to the level of noise. 

Noise is airborne sound energy within a broad range of frequencies that has the potential to cause discomfort and/or hearing loss. The following are typical sources of high noise levels in the process industries ... 

Noise may be generated by processes within the seismometer or by external processes acting on the seismometer. Some noise is expected or unavoidable, such as the manufacturer s characterized self-noise, or may be indicative of instrument defects such as persistent spurious transient noise event ( pops ) or installation deficiencies such as thermally driven noise or the seismometer shifting or tilting. It is first useful to review some typical sources of internal and external noise. 

For the amplifier pulse to be recognized in the ADC, it must exceed the lower level set by a discriminator, which is used to prevent noise pulses from jamming the converter. Once the pulse is accepted it is used to charge a capacitor that is discharged through a constant current source attached to an address clock typically... 

In nearly all cases noise control at source is the best option. Typical examples are ... 

It is important to understand that this material will not be presented in a theoretical vacuum. Instead, it will be presented in a particular context, consistent with the majority of the author s experience, namely the development of calibrations in an industrial setting. We will focus on working with the types of data, noise, nonlinearities, and other sources of error, as well as the requirements for accuracy, reliability, and robustness typically encountered in industrial analytical laboratories and process analyzers. Since some of the advantages, tradeoffs, and limitations of these methods can be data and/or application dependent, the guidance In this book may sometimes differ from the guidance offered in the general literature. 

Now that the parameters of typical prototype systems have been considered, it seems worthwhile to consider further variations that could be used to enhance the power and/or versatility of the basic apparatus. An obvious improvement would be to use a completely tunable IR probe source. As mentioned above, a globar source is completely tunable but with a lower than desired number of photons per unit bandwidth (60,73). There are a number of other means of producing completely tunable IR probe radiation with enough intensity to pull the detector out of the blackbody-noise regime. 

Stability may not be as much of a problem as with a diode source. However, there are problems with this method as well. The range of tunability is limited by the absorption properties of the nonlinear crystal which generates the difference frequency. At present, tunability is limited to wavenumbers >2500 cm-1 and conversion efficiencies are low. Typical laser powers in the CH2 experiments (82) were 20 n W (compared to the power of the CO lasers, 10 mW-1 W). This produces a situation where IR detectors, particularly fast ones, may be close to or background noise limited. However, it is clear that more applications of this technique will appear in the future. 

Noise can be also introduced by biochemical heterogeneity of the specimen. This can be a major cause of uncertainty in biological imaging. The high (three-dimensional) spatial resolution of fluorescence microscopy results in low numbers of fluorophores in the detection volume. In a typical biological sample, the number of fluorophores in the detection volume can be as low as 2-3 fluorophores for a confocal microscope equipped with a high NA objective at a fluorescent dye concentration of 100 nM. This introduces another source of noise for imaging applications, chemical or molecular noise, related to the inherent randomness of diffusion and the interaction of molecules. 

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