Other Noise Sources

The sensitivity of forthcoming laser interferometers is limited by seismic noise at low frequencies (from a few hertz to tens of hertz according to the isolation strategy), by thermal noise up to some hundreds of hertz, and by shot noise above. Other sources of noise, however, can contribute to the total noise budget  

The lasers are stabilized using high finesse reference cavities, typically a Fabry-Perot cavity in one arm. 

FIGURE 5 Schematics of an advanced interferometer using power recyciing and signal recycling. 

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We have seen how the presence of shot noise dictates some key choices minimum laser power, beam and mirror diameter, necessity to use Fabry-Perot cavities in the arms. Other noise sources will fix other important optical parameters. 

This source of noise is not usually called noise in most technical contexts it is more commonly called error rather than noise, but that is just a label since it is a random contribution to the measured signal, it qualifies as noise just as much as any other noise source. So what is this mystery phenomenon It is the quantization noise introduced by the analog-to-digital (A/D) conversion process, and is engendered by the fact that for... 

Fuel-cell vehicles contribute to reduced noise pollution, since the drive system is nearly noiseless. This is especially important in urban areas, where the share of noise coming from the drive system is dominant, compared to other noise sources from... 

We defined S as the signal, in electrons, resulting from the Raman scattering of the analyte of interest, as described in Chapter 3. In the absence of other noise sources, the standard deviation of S is determined by the shot noise limit ... 

SNRrf scales with but is always less than SNR. As (Po aDaKC) increases to the point where it significantly exceeds 0, analyte shot noise will exceed dark noise (cr cr ), and Eq. (4.22) will revert to Eq. (4.16). As always, the best one can do is the analyte shot noise limit, when other noise sources become insignificant. 

In an ideal imaging system, the only source of noise in the image should be quantum noise. Quantum noise is fundamental and unavoidable in an X-ray image, but for a given exposure, its effect should be minimized by ensuring that the quantum efficiency of the detector is as close to 100% as possible. In real imaging systems, there are also other noise sources, mainly associated with the detector, and efforts should be made in the system design to ensure that these are much smaller than the quantum noise. 

In detectors employing an electrical readout mechanism, electrical noise fluctuations (Johnson noise plus possibly other noise sources such as low frequency contact noise in some cases) must be considered. There will be a Johnson noise source associated with the output impedance of the detector. In thermopiles and bolometers the output impedance is predominantly resistive so that the calculation of the Johnson noise is straightforward. The output impedance o f the pyroelectric detector is predominantly capacitative. In this case the resistive component associated with the dielectric loss factor of the... 

Note that in a practical system there is also an additional constant noise component Nth due to Johnson noise, and other noise sources in the receiver electronics however this is not pertinent to the present analysis of the noise introduced by the amplifier. 

The intensity I(t) of a cw laser is not completely constant, but shows periodic and random fluctuations and also, in general, long-term drifts. The reasons for these fluctuations are manifold and may, for example, be due to an insufficiently filtered power supply, which results in a ripple on the discharge current of the gas laser and a corresponding intensity modulation. Other noise sources are instabilities of the gas discharge, dust particles diffusing through the laser beam inside the resonator, and vibrations of the resonator mirrors. In multimode lasers, internal effects, such as mode competition, also contribute to noise. In cw dye lasers, density fluctuations in the dye jet stream and air bubbles are the main cause of intensity fluctuations. 

From the results of the present work, it can be concluded that LDV approach is very efficient for measuring natural frequencies from ambient and forced vibrations spectra for the bridge test. As far as the LNG experiment is concerned, only the first frequency has been measured by the LDV. A noticeable difference between the two approaches is appearing because mainly of the set up of the experiment, the noise background and other noise sources as well as the relative vibration of the insfrument head support. 

Because space is almost always severely limited offshore, workers and accommodation areas are generally close to equipment and other noise sources. Hence noise management and control often plays an important role in the design of a facility—particularly with regard to layout. 

Amplifier systems generate electrical noise which adds to the signals being processed. The source of this noise is the passive elements in the circuitry and the active elements themselves. Other noise sources are electrodes and environmental electrical noise from switchgear, motors, fluorescent lamps, and the like. 

A less direct estimate of and is obtained from the amplitude of the power spectra of current fluctuations. Basically, power spectra contain more information than the simple variance, as shown by eqn.(7). The additional information is very useful to ascertain to what extent the measured current fluctuations can be attributed to the flickering of ion specific channels between open and closed states rather than to other noise sources. This control was particularly desirable in the early studies of nerve membrane noise, which revealed the presence of large 1/f spectral components of still unclear origin (16). Indeed, the first unequivocal characterizations of sodium and potassium channel noise in the squid axon membrane (13) and of sodium channel noise in frog nodes (14,15) were obtained from the fitting of measured spectra with the superposition of Lorentzian-like spectra plus 1/f components. From the low frequency asymptote and the cut-off frequency of the Lorentzian components estimates of Y =12 pS and were obtained for the ionic channels of quid... 

Enclosure surrounding the machine or other noise source with sound-absorbing material is of limited use unless total enclosure is achieved. 

However, this does not apply for small detectors operating at very low or very high frequencies, since other noise sources dominate. In this case the detectivity will be maximized by maximizing Fy in equation (5.30). 

An FT-IR spectrometer is used optimally when detector noise exceeds all other noise sources and is independent of the signal level. This is the usual case for mid-infrared spectrometry but may not be so for shorter wavelengths. The sensitivity of mid-infrared detectors is commonly expressed in terms of the noise equivalent power (NEP) of the detector, which is the ratio of the root mean square (rms) noise voltage, P , in V Hz to the voltage responsivity, R, of the detector, in V W . It is effectively a measure of the optical power that gives a signal equal to the noise level thus, the smaller the NEP, the more sensitive is the detector. The NEP is proportional to the square of the detector area, Ao, with the constant of proportionality being known as the specific detectivity, D that is. 

At the sort of signal levels used by analog multipliers, the ion statistical noise is small compared to other noise sources and especially the ICP source. This means that fast analog data acquisition is likely to be noise limited by the ICP source noise. 

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