S/N ratio

Minimum exposure times must be observed in order to reach the requisite S/N ratio. As per EN 1435 and EN 584-1, for the different ranges of utilization (energy, wall thickness), definite film elasses are prescribed. They are characterized by the minimum gradient-to-noise ratios. Based on this, one can calculate the minimum values for the S/N ratio based on the IP systems. The exposure time and the device parameter sensitivity and dynamics (latitude) must be adjusted accordingly, with an availability of an at least 12 bit system for the digitalization. 

Due to the conversion process an absorbed photon give rise to less than one electron generated in the CCD. This phenomenon, also called a "quantum sink" shows that the detector is degrading the S/N ratio of the image. The quality of an image being mainly limited by the quantum noise of the absorbed gamma this effect is very important. 

Acrylonitrile reacts with the sodium salt of 4.5-dimethvl-A-4-thiazoline-2-thione (73J (R4 = R5 = Me) to yield 3-(2-cyanoethyl)-4.5-dimethyl-A-4-thiazoline-2-thione (74) (R4 = R, = Me) (Scheme 35 (160). Humphlett s studies of this reaction showed that the size of the R4 substituent is a determinant factor for the S versus N ratio (161. 162). If R4 == H, 100% of the N-substituted product (74) is obtained this drops to 50% when R4 = methyl, and only the S-substituted product (75) is obtained when R4 = phenyl. The same trend is observed with various CH2 = CH-X (X = C00CH3. COCH3) reagents (149). The S/N ratio also depends on the electrophilic center for CH2 = CH-X systems thus S-reaction occurs predominantly with acrylonitrile, whereas N-substitution predominates with methvlvinvlketone (149). 

Signals generated by high-speed maehinery are very eomplex in nature and are generated by several forees with a net effeet that masks the pure tones. The random portion of the signal, whieh is blended with the pure tones, is ealled noise. The ratio of the total amplitude (area under speetrum) to that of the noise is ealled the signal-to-noise (S/N) ratio. Sometimes this ratio is expressed in deeibels, or db, as follows ... 

If the S/N ratio is less than 10 db, it beeomes diffieult to differentiate the periodie part of the speetrum from noise. 

The second limitation of this pickup is illustrated by an example. Acceleration of one g at 0.5 represents a displacement of 100 inches. It is obvious that in spite of its wide-band response (sometimes 0.1 —15 kH ), it is severely limited at the low end by a poor S/N ratio. 

Averaging is a teehnique to improve the S/N ratio. Two or more sueeessive speetra made up of both periodie and random (noise) signals are added together and then averaged. This eombination results in a speetrum with a periodie eomponent that is mueh the same as when viewed in the instantaneous signals but with random peaks of mueh less amplitude. This result oeeurs beeause the period peak stays at a fixed frequeney in the speetrum, while the noise peak is fluetuating in frequeney over the speetrum. 

Figure 1.34 (a) Reduced S/N ratio resulting from noise folding. If the Rf carrier frequency is placed outside the spectral width, then the noise lying beyond the carrier frequency can fold over, (b) Better S/N ratio is achieved by quadrature detection. The Rf carrier frequency in quadrature detection is placed in the center of the spectrum. Due to the reduced spectral width, noise cannot fold back on to the spectrum. 

What is signal-tonoise (S/N) ratio, and how can it be improved by acquiring a large number of FIDs ... 

If phase-sensitive spectra are not required, then magnitude-mode Pico) (or absolute-mode ) spectra may be recorded by combining the real and imaginary data points. These produce only positive signals and do not require phase correction. Since this procedure gives the best signal-to-noise ratio, it has found wide use. In heteronuclear experiments, in which the dynamic range tends to be low, the power-mode spectrum maybe preferred, since the S/N ratio is squared and a better line shape is obtained so that wider window functions can be applied. 

Increasing the number of dimensions from two to three may result in a reduction in the signal/noise (S/N) ratio. This may be due either to the distribution of the intensity of the multiplet lines over three dimensions or to some of the coherence transfer steps being inefficient, resulting in weak 3D cross-peaks. 

The applicability of these techniques depends on the type and form of the power spectra of both signal and noise. The only method that can be universally applied, is signal averaging. If the signal function is measured n times, the S/N ratio increases by Jn. 

ADEQUATE,49 can afford a significant gain in s/n ratios as shown in... 

It is logical to assume that the enhanced s/n ratio for co-processed INADEQUATE data, coupled with the diagonally symmetric nature of the HSQC-1,/1-ADEQUATE spectrum will allow the acquisition of useful spectral data for smaller samples or in shorter periods of time than would otherwise be possible. Indeed, useable HSQC-l,n-ADEQUATE spectra were obtained for l,n-ADEQUATE data recorded in just over 9 h. 

In most cases, you will use the first harmonic and the normal first-derivative of the absorption spectrum will be presented. If your spectrum has very good S/N and has some regions where you would like better resolution, a second-derivative presentation may help. However, second derivatives from second harmonic detection are very costly in terms of S/N ratio and so you really do have to have a strong signal ... 

For optimum S/N ratio, but decreased resolution Modulation amplitude = 2 x line-width. 

To improve the S/N ratio, the modulation signal is processed by amplification with a tuned amplifier using phase-sensitive detection. This means that the detected signal must not only be at the modulation frequency, but must also be in phase with the modulation. Since the amplifier itself can introduce a bit of phase shift, there is a phase control which, in principle, should be adjusted to maximize the signal amplitude. In practice, this control needs to be adjusted only rarely and in most cases the best approach is to leave it alone. 

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