Time-domain high-frequency

Figure Bl.2.7. Time domain and frequency domain representations of several interferograms. (a) Single frequency, (b) two frequencies, one of which is 1.2 times greater than the other, (c) same as (b), except the high frequency component has only half the amplitude and (d) Gaussian distribution of frequencies.

HRV assesses the modulation of autonomic tone on the sinus node, or simply put, the irregularity of sinus rhythm. Methods of measuring HRV fall under broad categories of being either time domain or frequency domain analyses. Time domain measurements involve statistical analyses of the variability in the R-R interval, while frequency domain measurements use spectral analysis of a series of R-R intervals to classify HRV into ultra-low frequency, very low frequency, low frequency, high frequency, and total power. One method is not better than another as there is no gold standard (65). 

For future work it is eligible to analyze the dependencies like high currents and low temperatures, to integrate them into the models and to carry out quantitative evaluations of the accuracy of the models. Furthermore, the technique should be applied the other way around to derive impedance spectra from voltage step response data. This enables comparisons of model parameter sets from time domain and frequency domain data. In order to prove state determination and tracking of the aging process, the techniques will be applied to a large number of recorded data from real operation. 

It is not a trivial problem to obtain a complete characterization of a material responding over many decades of time. The brute force method would be to carry out experiments over many decades of time. More efficient is to employ more than one instrument, and cover a time span that includes high frequencies. This is now possible with broad dielectric spectroscopy, with which the frequency reuige from 10 to 10 can be attained by using different techniques - time domain spectroscopy, frequency response analysis using AC-bridges, and coaxial line reflectrometry. Of course, each isothermal experiment has to be repeated at various temperatures in order to determine the temperature dependence. 

For radiofrequency and microwave radiation there are detectors which can respond sufficiently quickly to the low frequencies (<100 GHz) involved and record the time domain specttum directly. For infrared, visible and ultraviolet radiation the frequencies involved are so high (>600 GHz) that this is no longer possible. Instead, an interferometer is used and the specttum is recorded in the length domain rather than the frequency domain. Because the technique has been used mostly in the far-, mid- and near-infrared regions of the spectmm the instmment used is usually called a Fourier transform infrared (FTIR) spectrometer although it can be modified to operate in the visible and ultraviolet regions. 

In the time-domain detection of the vibrational coherence, the high-wavenumber limit of the spectral range is determined by the time width of the pump and probe pulses. Actually, the highest-wavenumber band identified in the time-domain fourth-order coherent Raman spectrum is the phonon band of Ti02 at 826 cm. Direct observation of a frequency-domain spectrum is free from the high-wavenum-ber limit. On the other hand, the narrow-bandwidth, picosecond light pulse will be less intense than the femtosecond pulse that is used in the time-domain method and may cause a problem in detecting weak fourth-order responses. 

Bar, G., M. Bennati et al. (2001). High-frequency (140-GHz) time domain EPR and ENDOR spectroscopy The tyrosyl radical-diiron cofactor in ribonucleotide reductase from yeast. J. Am. Chem. Soc. 123 3569-3576. 

The optimum gate width AT for a specific lifetime amounts to 2.5t. In Fig. 3.10 a typical F-x curve is shown for time domain lifetime detection with a variable number of time bins and a total detection window (sum of all the time bins/gate widths) of 10 ns (de Grauw and Gerritsen, 2001). The curves are representative for both TCSPC and TG operating in a high excitation frequency mode of... 

Ion detection is carried out using image current detection with subsequent Fourier transform of the time-domain signal in the same way as for the Fourier transform ion cyclotron resonance (FTICR) analyzer (see Section 2.2.6). Because frequency can be measured very precisely, high m/z separation can be attained. Here, the axial frequency is measured, since it is independent to the first order on energy and spatial spread of the ions. Since the orbitrap, contrary to the other mass analyzers described, is a recent invention, not many variations of the instrument exist. Apart from Thermo Fischer Scientific s commercial instrument, there is the earlier setup described in References 245 to 247. 

To summarize, current state of NIR research consists of two major groups of instrumentation (f) continuous wave and (2) time-resolved and frequency domain and two major groups of parameters assessed (f) slow responding hemodynamic (HbO and Hb) parameters and (2) fast response neuronal parameter. Assessing the fast response parameter requires high temporal resolution provided by NIR equipment. Such temporal resolutions are currently not possible using fMRI modality. Thus, a natural complementary relationship exists between fMRI and NIR methods, where fMRI can provide better spatial localization and NIR better temporal resolution. 

As shown in Section 11.2.1.1, more details can be obtained by confocal fluorescence microscopy than by conventional fluorescence microscopy. In principle, the extension of conventional FLIM to confocal FLIM using either time- or frequency-domain methods is possible. However, the time-domain method based on singlephoton timing requires expensive lasers with high repetition rates to acquire an image in a reasonable time, because each pixel requires many photon events to generate a decay curve. In contrast, the frequency-domain method using an inexpensive CW laser coupled with an acoustooptic modulator is well suited to confocal FLIM. 

Fig. 4.53. Simulated time-domain ICR signals (left) and frequency-domain spectra (right) for (a) low pressure, (b) medium pressure, and (c) high pressure. Reproduced from Ref. [201] by permission. John Wiley Sons, 1998.

Fig. 2. H, H-2Q-HoMQC (top) and 3Q-HoMQC (bottom) spectra of rabbit uteroglobin in D2O [16]. Spectral windows for the direct and remote dimensions are the same in each case. For the 3Q spectrum the frequency window in the remote dimension was shifted by half inverting every other complex points in time domain [10] in order to bring all high-field correlations together for easier analysis.

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