Signal averaging


Effect of signal averaging on a spectrum s signal-to-noise ratio (a) spectrum for a single scan  [c.391]

In a Fourier transform, infrared spectrometer, or FT-IR, the monochromator is replaced with an interferometer (see Figure 10.13). Because an FT-IR includes only a single optical path, it is necessary to collect a separate spectrum to compensate for the absorbance of atmospheric CO2 and H2O vapor. This is done by collecting a background spectrum without the sample and storing the result in the instrument s computer memory. The background spectrum is removed from the sample s spectrum by ratioing the two signals. In comparison to other IR instruments, an FT-IR provides for rapid data acquisition, allowing an enhancement in signal-to-noise ratio through signal averaging.  [c.393]

Precision The precision of a chemical kinetic method is limited by the signal-to-noise ratio of the instrumental method used to monitor the reaction s progress. With integral methods, precisions of 1-2% are routinely possible. The precision for differential methods may be somewhat poorer, particularly for noisy signals, due to the difficulty in measuring the slope of a noisy rate curve.It may be possible to improve the precision in this case by using a combination of signal averaging and smoothing of the data before its analysis.  [c.640]

The perturbation methods described induce change as a step function, where the transition is much faster than the kinetics to be measured. There is no necessity for such a restriction. If an oscillatory temperature or pressure variation can be appHed to a sample, reactant concentrations attempt to foUow the oscillatory perturbation. If the perturbation oscillates slowly, the concentrations have a maximum ampHtude and remain in phase with the perturbation. If the perturbation is introduced at a higher frequency, faster than the chemical kinetics can foUow, there is a phase lag and a reduced ampHtude for the oscillatory concentration changes occurs. Methods based on this approach faciHtate extensive signal averaging. Rapid temperature changes of an oscillatory nature are difficult to impose, but there has been some demonstration of kinetic measurements using the lag of concentrations behind a monotonic increasing temperature change. This amounts to the same principle, restricted to less than one cycle, but is not particularly useful.  [c.511]

The unique virtue of flash photolysis is that is can be extended another 11 orders of magnitude toward shorter times. Down to times of a few nanoseconds, the most common procedures employ essentially the same principles used for millisecond experiments. The same excitation energy is dehvered in a shorter flash, and faster electronics are used to monitor changes in a continuous probe of concentrations. The probe is modified as necessary to permit faster measurements, using a brighter lamp to maintain an adequate signal-to-noise ratio while measuring faster transmittance changes. In the pre-laser era, flash photolysis technique developed in the direction of generating very energetic excitation flashes capable of making substantial concentration changes throughout a large volume so that kinetic changes could be monitored in a single flash. Signal averaging was rarely employed, except when using certain luminescence methods. It was difficult to make bright excitation flashes shorter than a microsecond, except for luminescence. Once pulsed lasers became available, pulse durations of 10 ns were easily attained. Several technologies, such as Q-switching, pulsed electrical excitation of gas discharge lasers, including the important uv-emitting excimer lasers, cavity-dumping, and even pulsed excitation of semiconductor lasers, conveniently generate pulses having durations near 10 ns. The first two methods can produce pulses with hundreds of millijoiiles of energy at rates of 1 to 1000 Uz the last two generate smaller energy pulses at repetition rates ranging from 1 to 1000 kH2.  [c.512]

Combined Methods. Combinations of the procedures described herein are usehil, especially for the study of unstable reagents. Double stopped-flow is increasingly common. Two reagents are mixed in the usual way and allowed to incubate for, eg, one second, to produce the reactive but unstable reagent of interest. Then the mixture is combined in the measurement cell with the contents of a third syringe to initiate the reaction of interest, which is studied in the usual way. In a similar vein, components may be combined using stopped-flow and the resulting mixture investigated by a T-jump timed to occur soon after mixing is complete. Elash photolysis could be done in the same manner, but because flash photolysis lends itself to signal averaging at high repetition rates, a continuous flow mixer with a flash photolysis cell located downstream at the appropriate time delay works very well.  [c.513]

One advantage of a linear photodiode array is the speed of data acquisition, which makes it possible to collect several spectra for a single sample. Individual spectra are added and averaged to obtain tbe final spectrum. This process of signal averaging improves a spectrum s signal-to-noise ratio. When a series of spectra is added, the sum of the signal at any point increases as (mS ), where n is the number of spectra, and is the signal for the spectrum s x-th point. The propagation of noise, which is a random event, increases as fnNx), where is the noise level for the spectrum s x-th point. The si al-to-noise ratio (S/N) at the x-th data point, therefore, increases by a factor of V n  [c.391]

Effect of signal averaging on a spectrum s signal-to-noise ratio (a) spectrum for a single scan  [c.391]

Precision The precision of a chemical kinetic method is limited by the signal-to-noise ratio of the instrumental method used to monitor the reaction s progress. With integral methods, precisions of 1-2% are routinely possible. The precision for differential methods may be somewhat poorer, particularly for noisy signals, due to the difficulty in measuring the slope of a noisy rate curve.It may be possible to improve the precision in this case by using a combination of signal averaging and smoothing of the data before its analysis.  [c.640]

Since typical NMR signals are quite weak, extensive signal averaging by repetitive scanning is generally necessary. The pulsing rate at which this can occur depends on the time it takes for the spin system to return into its initial state after the 90° pulse, with Mq along the magnetic field direction This process can generally be described by first-order kinetics. The associated time constant Tj, the spin-lattice relaxation time, can vary ftom a few ms to several hours in solids.  [c.463]

Z. Fiu and J. B. Phillips, Farge-volume sample introduction into narrow-hore gas chromatography columns using thermal desorption modulation and signal averaging , /. Microcolumn Sep. 2 33-40 (1990).  [c.431]

The principle of the lA architecture is to quickly and badly digitize the signal, and then to average the points to achieve the 16 bit precision. For example, the Harmonic 2000 digitizer samples the signal 128 times faster than the output frequency. Averaging is done on 128 samples thus achieving a very good spectral purity.  [c.280]

As the geometry of the sample is cylindrieal, the pump and the probe beams, foeused at the same side, are loeated on the generating line of the eylinder, as shown in figure 4. Thus, the control of the weld depths penetration and artifieial slots is achieved by seanning the laser system along this generating line. In faet, only the sample moves in one direction. Then, ultrasonic images such as B-scan views are obtained by this scanning, which is suitable for the evaluation of the defects. The analysis of the ultrasonic images, therefore the visualization of the lacks of weld penetration and slots, depends on several parameters linked to the operating system. First, in order to ensure a non-destructive testing, the incident energy deposited on the surface sample is very weak (about 1 mJ). So that, each measurement has to be averaged from few laser shots because of the signal to noise ratio induced by the laser system. The scanning step is optimized to obtain a sufficient spatial resolution of the ultrasonic image and an efficient flaw detection. Nevertheless, these parameters (average, scanning step and length) involve testing duration which have to be reduced within the field of industrial applications. We can also notice that all the following ultrasonic images are presented without any signal or image processing.  [c.697]

In our case F, (t) K the obtained in TO sound area and stored in memory averaged signal (the reference signal), F (t) is the current signal. Reference signal parameters we shall mark with subscript r , current signal parameters with subscript e .  [c.828]

Here 0(l) is additional noise in current signal. The noise in reference signal is assumed to be compensated in the process of its averaging.  [c.828]

After magnetizing the disk it is placed on a turntable to support and move it during measurements. Fig. 3 shows an example measurement of the remanent magnetization of artifical inclusions with masses of 1 mg, 10 mg, and 15 mg at a depth of 70 mm. The signal to noise ratio is very high. A test which is nearer to the resolution limit is shown in fig. 4. An iron particle with a mass of approximately 10 mg was magnetized and than placed 4 mm below the sensor, i. e. the bottom of the cryostat containing the SQUIDs. The single scan signal to noise ratio is still much larger than 10. Averaging will give further improvments.  [c.989]

Fig. 7b shows the detailed view of fig. 7a but with averaging over four cycles. The random signals have largely disappeared, the signal over the segregation remains and is more clearly to be seen. At this point of the examination, the signal can be caused by a susceptibility variation or by a ferromagnetic inclusion which is not fully demagnetized. In order to distinguish between them, the disk was also measured from the flat side with the direction of rotation being changed in order to have the measuring plots better comparable. Fig. 7c shows the result. The sign of the signal, which is marked with the arrow is the same. If the origin would be ferromagnetic remanence, the sign would have changed. Comparing fig. 7b and c shows that the origin of the signal is a susceptibility variation, either paramagnetic or diamagnetic.  [c.991]

Because of the large acoustic impedance mismatch between air and solid materials the through-transmission teclmique with separate transmitter and receiver transducers on opposite sides of the component is used [3]. However, many applications require a one side access, therefore experiences with a normal incidence echo technique were carried out. A CFRP fabric sandwich laminate with Nomex cores (100x350 mm% total thickness 16 mm) was used. The transducer C 95-137 (IzfP) with a centre frequency of 825 kHz enables a focal length of 40 mm in air. The DLR ultrasonic imaging system HFUS 2000 [4] used for this purpose had to be adapted with a special designed pulser/preamplifier to the air-coupled transducer. The rectangular pulser with an optimized pulse width provides a higher acoustic energy than a common spike pulser. The specially designed preamplifier with a band-pass filter (800-850 kHz, - 3dB) enables a very low-noise amplification so that no signal averaging was necessary for the C-scan recording.  [c.842]

A similar result holds for phase encoding where the gradient strength appearing in the expression for the resolution is the maximum gradient strengtii available. According to these results the resolution can be improved by raising the gradient strength. However, for any spectrometer system, there is a maximum gradient strength which can be switched within a given rise time due to teclnhcal limitations. Although modem gradient coil sets are actively shielded to avoid eddy currents in the magnet, in reality systems with pulsed gradients in excess of 1 T are rare. Since the of many connnonly imaged, more mobile, samples is of the order of 10 ms, resolution limits in imaging are generally in the range one to ten micrometres. At this resolution, considerable signal averaging is generally required in order to obtain a sufficient signal to noise ratio and the imaging time may extend to hours. Moreover, a slice thickness of typically 500 pm, which is significantly greater than the lateral resolution, is frequently used to improve signal to noise. In solids, is generally very much shorter than in soft matter and high resolution imagmg is not possible without recourse either to sophisticated line narrowing teclmiques [H], to magic-echo refocusing variants [2] or to very high gradient methods such as stray field imaging [13].  [c.1529]

If the rate of chemical decay of the RP is desired, the task is complex because the majority of the CIDEP signal decays via relaxation pathways on the 1-10 ps time scale, as opposed to chemical reaction rates which are nominally about an order of magnitude longer than this. There are two ways around this problem. The first is to use a transient digitizer or FT-EPR and signal average many times to improve the signal-to-noise ratio at long delay times where chemical reaction dominates the decay trace. The second is to return to the steady-state method described above and run what is called a kinetic EPR experiment, where the light source is suddenly interrupted and the EPR signal decay is collected over a very long time scale. The begiiming of the trace may contain both relaxation of CIDEP intensity as well as chemical decay however, the tail end of this trace should be dominated by the chemical reaction rates. Much use has been made of kinetic EPR in measuring free radical addition rates in polymerization reactions [65, 66].  [c.1617]

The infrared light passing through the collision cell impinges on a second InSb-solid-state detector (cooled to 77 K) producing both DC and AC signals at the detector output. Since most of the IR light is not absorbed by the sample, the DC signal that measures the continuous output level of laser light is much larger than the AC signal. It is in fact fluctuations in the DC light level that constitute one of the main noise sources in the experiment. Both AC and DC signals are fed to a high-speed transient recorder with at least two channels where the time-resolved ratio of the AC and DC currents is recorded and stored in memory. Single-collision data are obtained from this time-dependent absorjDtion data. Signals from a series of ultraviolet laser pulses can be added in memory with subsequent signal averaging and noise reduction. The ratio of the AC and DC infrared light levels. A///, is related to the pressure of absorbing molecules, P, the molecular absorjDtion coefficient, a and the cell path length, L.  [c.3002]

One advantage of a linear photodiode array is the speed of data acquisition, which makes it possible to collect several spectra for a single sample. Individual spectra are added and averaged to obtain the final spectrum. This process of signal averaging improves a spectrum s signal-to-noise ratio. When a series of spectra is added, the sum of the signal at any point increases as (nSx), where n is the number of spectra, and Sx is the signal for the spectrum s x-th point. The propagation of noise, which is a random event, increases as jnNx), where is the noise level for the spectrum s x-th point. The signal-to-noise ratio (S/N) at the x-th data point, therefore, increases by a factor of V  [c.391]

In addition to producing short pulses of spectrally well-defined light, lasers have the advantage that their optical energy can be dehvered to the sample efficiently and focused to illuminate a small volume. In order to produce large transmittance changes, the energy should be dehvered so as to create a long optical pathlength for the probe with a small cross-sectional area. The geometry of Figure 3b shows longitudinal excitation, which accomplishes this goal. In some cases, truly microscopic methods can be used. An energy of 1 pj deposited in a volume less than 0.01 mm yields an energy density greater than 0.1 J/cm3, which can be sufficient to produce a millimolar concentration of chemical transients. Such microjoule energies are compatible with high repetition rates and extensive signal averaging to attain impressive sensitivity. Repeated excitation of a small volume, however, often leads to sample degradation, which can be avoided by using flow systems for gas or hquid solutions, and spinning or translating samples for sohds.  [c.512]

In a Fourier transform, infrared spectrometer, or FT-IR, the monochromator is replaced with an interferometer (see Figure 10.13). Because an FT-IR includes only a single optical path, it is necessary to collect a separate spectrum to compensate for the absorbance of atmospheric CO2 and H2O vapor. This is done by collecting a background spectrum without the sample and storing the result in the instrument s computer memory. The background spectrum is removed from the sample s spectrum by ratioing the two signals. In comparison to other IR instruments, an FT-IR provides for rapid data acquisition, allowing an enhancement in signaTto-noise ratio through signal averaging.  [c.393]

Molecules found in petroleum products giving rise to absorption in the UV are niosfly aromatics and to a lesser degree conjugated diolefins and olefins. The saturated hydrocarbons, alkanes or naphthenes, give no signal for wavelengths greater that 180 nm. This particularity might seem restrictive but in fact is an advantage because knowing the aromatic content is very often necessary in refining. UV absorption is of interest also because for aromatics, ring condensation causes absorption towards the higher wavelengths, as well as large variations in the sensitivity coefficients (called extinction or absorption coefficients). Table 3.8 gives the average molar absorptivities for different wavelengths as a function of ring condensation for an optical path of 1 cm and a concentration of 1 mol/1.  [c.54]

An importcmt advantage of NMR over other spectral analysis methods comes from the signal s area being directly proportional to the number of protons. Therefore, in a spectrum, the area percentages for the different signals are related to the percentages of atoms. From that, it is easy to obtain the percentage of hydrogen for each of the above species. Furthermore, using the hypothesis that the average number of hydrogen atoms attached to the carbon atoms is two and using the resuits from elemental analysis of carbon and hydrogen, it is possible to deduce the following parameters  [c.66]

The features needed for the training of the neuronal networks are obtained for both kind of ROIs by a very similar procedure. First all local maxima and minima are determined in a y-scan. Afterwards the largest difference between two adjacent local mini- and maxima is determined. Then the corresponding surrounding of one local minimum in the middle and another two local minima on the sides of the local maxima is interpreted as the representation of the crack in the scan. From this representation several features are determined. Starting with the half-power width of the signal, the average depth of the central minimum, the depths and the width of the side minima etc. These features are averaged in y-direction. The denoising effect of the wavelet transform allows to detect the minimum in a scan according to the flaw with a precision of nearly 100% if there is any. Returning with this special information to the non denoised ROI and detecting the flaw in the noise of the original ROI heads to further features.  [c.463]

Calibration of the liquid crystal cell was carried out using a method described by Wu et al [16] in which the cell was placed between two polarising sheets, with the optical axis of the cell at 45 ° to the initial polarisation angle. As the ac signal voltage was increased, a large area photodiode was used to monitor the intensities of the ordinary and extraordinary beams passing through the cell. A simple calculation from this data provided an accurate measure of the birefringence for the cell, specific to the wavelength of light used, over a range of voltages. Using an Argon ion laser, two sets of readings were taken with 3mm diameter spots, in a comer and along an edge of the square cell. A third set was taken using a beam that covered the entire 1 Ommx 10mm area of liquid crystal in the cell such that an average birefringence was calculated.  [c.681]

In aecordanee with equation 4 the amplitude of the backscattering signal followed a exponential function over x. (see fig. 2). To reduce the stochastic fluctuation of the scattering and for a high precision of the results the rectified backscattering signal is averaged for about 100 single echo signals. The stochastic distribution of the very small bubbles prevents a high scattering signal. It is therefore necessary to operate with a high gain at the US- device and the thin plastic window will produce a high echo signal. This prevents also a direct measurement of Ps(Xo). The software determines the parameters of the exponential function and calculates the expected value at Xq using a fitting and extrapolation routine.  [c.868]

The many Raman spectroscopies are found as well defined subgroups among the electric field spectroscopies in general. (In this chapter, the magnetic field spectroscopies are mentioned only m passing.) First, except for very high intensities, the energy of interaction of light with matter is sufficiently weak to regard modem spectroscopies as classifiable according to perturbative orders in the electric field of the light. Thus any given spectroscopy is regarded as being linear or nonlinear with respect to the incident light fields. In another major classification, a given spectroscopy (linear or nonlinear) is said to be active or passive [6, 7]. The active spectroscopies are those in which the principal event is a change of state population in the material. In order to conserve energy this must be accompanied by an appropriate change of photon numbers in the light field. Thus net energy is transferred between light and matter in a maimer that survives averaging over many cycles of the perturbing light waves. We call these the Class I spectroscopies. They constitute all of the well known absorption and emission spectroscopies—whether they are one-photon or multi-photon. The passive spectroscopies, called Class II, arise from the momentary exchange of energy between light and matter diat induces a macroscopically coherent, oscillating, electrical polarization (an oscillating electric dipole density wave) in the material. As long as this coherence is sustained, such polarization can serve as a source tenn in the wave equation for the electric field. A new (EM) field (the signal field) is produced at the frequency of the oscillating polarization in the sample. Provided the polarization wave retains some coherence, and matches the signal field in direction and wavelength (a condition called phase matching ), die new EM field can build up and escape the sample and ultimately be measured in quadrature (as photons). In their extreme fomi, when no material resonances are operative, the Class II events ( spectroscopies is a misnomer in the absence of resonances ) will alter only the states of the EM radiation and none of the material. In this case, the material acts passively while catalysing alterations in the radiation. When resonances are present. Class II events become spectroscopies some net energy may be transferred between light and matter, even as one focuses experimentally not on population changes in the material, but on alterations of the radiation. Class II events include all of the resonant and nom-esonant, linear and nonlinear dispersions. Examples of Class II spectroscopies are classical diffraction and reflection (strongest at the linear level) and a whole array of light-scattering phenomena such as frequency simnning (liamionic generation).  [c.1179]

Eor SR, a Class I spectroscopy, there must be a net transfer of energy between light and matter which survives averaging over many cycles of the optical field. Thus, the material must undergo a state population change such that the overall energy (light and matter) may be conserved. In Stokes vibrational Raman scattering (figure B 1.3.3(a)), the cluomophore is assumed to be in the ground vibrational state g). The launching of the Stokes signal field creates a population shift from the ground state g) to an excited vibrational state 1). Conversely, in anti-Stokes vibrational Raman scattering (figure B1.3.3(b)), the cluomophore is assumed to be  [c.1197]


See pages that mention the term Signal averaging : [c.723]    [c.1607]    [c.2956]    [c.391]    [c.391]    [c.446]    [c.778]    [c.511]    [c.391]    [c.391]    [c.446]    [c.778]    [c.470]    [c.125]    [c.595]    [c.723]    [c.804]    [c.1182]   
Modern analytical chemistry (2000) -- [ c.391 , c.391 ]

Modern Analytical Chemistry (2000) -- [ c.391 , c.391 ]